Collection of multiple articles in one palce....

Composite (resin)

2008-07-04T10:01:00.000+05:30

Composite is a mixture of acrylic resin and
powdered glass-like particles that produce a
tooth-colored flling. This type of material may be
self-hardening or may be hardened by exposure to
blue light. Composite is used for fllings, inlays and
veneers. Sometimes it is used to replace a portion
of a broken or chipped tooth.
A DvAnTAges :
   Color and shading can be matched to the
existing tooth;
   Composite is a relatively strong material,
providing good durability in small to mid sized
restorations that need to withstand moderate
chewing pressure;
   Composite may generally be used on either
front or back teeth;
   Fillings are usually completed in a single
visit (with exceptions noted below);
   Moderately resistant to breakage;
   Often permits preservation of as much of the
tooth as possible;
   Low risk of leakage if bonded only to enamel;
   Does not corrode;
   Generally holds up well to biting force (dependent
on the material used);
   Moderately resistant to further decay, new
decay is easy to fnd;
   Frequency of repair or replacement is low to
moderate.
DI sA DvAnTAges :
   This type of flling can break and wear out more
easily than metal fllings, especially in areas of
heavy biting force. Therefore, composite fllings
may need to be replaced more often than metal
fllings;
   Compared to other fllings, composites are
sometimes diffcult and time-consuming to place.
They can not be used in all situations;

   Composite generally is more expensive than
amalgam;
   May require more than one visit for inlays,
veneers and crowns;
   May wear faster than natural dental enamel;
   May leak over time when bonded beneath the
layer of enamel;
   In rare cases, a localized, allergic reaction such
as infammation or rash may occur.

critical care

2008-07-04T09:01:00.000+05:30

Critical Care

The goals of critical care medicine are to save the lives of patients with life-threatening but reversible medical or surgical conditions and to offer the dying a peaceful and dignified death. Open discussions between physicians and patients and their family members ensure that critical care is provided in a manner that is most consistent with the patient's wishes.

Respiratory Failure

General Considerations

Hypercapnic respiratory failure occurs with acute carbon dioxide retention [arterial carbon dioxide tension (PaCO₂) >45â€"55 mm Hg], producing a respiratory acidosis (pH <7.35).
Hypoxic respiratory failure occurs when normal gas exchange is seriously impaired, resulting in hypoxemia [arterial oxygen tension (PaO₂) <60 mm Hg or arterial oxygen saturation (SaO₂) <90%]. Usually, this type of respiratory failure is associated with tachypnea and hypocapnia; however, its progression can lead to hypercapnia as well. Hypoxic respiratory failure can result from a variety of insults, as shown in Table 8-1.
- The acute respiratory distress syndrome (ARDS) is a form of hypoxic respiratory failure caused by acute lung injury. The common end result is disruption of the alveolar capillary membrane, leading to increased vascular permeability and accumulation of inflammatory cells and protein-rich edema fluid within the alveolar space.
- The American-European Consensus Conference has defined ARDS as follows: (a) acute bilateral pulmonary infiltrates, (b) ratio of PaO₂ to inspired oxygen concentration (FIO₂) <200, and (c) no evidence for heart failure or volume overload as the principal cause of the pulmonary infiltrates.¹

nuclear deal 3

2008-06-20T06:22:00.000+05:30

New Delhi's nuclear planners can never be accused of thinking small. Even at the very beginning of India's nuclear efforts, Homi Bhabha proposed an ambitious three-stage plan for Indian nuclear development that sought to develop original technology that would allow the country to compensate for its insufficient uranium reserves.

Thermal reactorstoday's typical power reactorsrepresented the first part of Bhabha's vision. Thermal reactors use slow or thermal energy neutrons to fission uranium-235, a naturally occurring fissile isotope of uranium.

Bhabha envisioned that, in a second stage, spent fuel from these thermal reactors would be reprocessed to separate plutonium for fueling breeder reactors, which would "breed" more plutonium.

In the third and final stage, this plutonium would fuel reactors that would irradiate thorium to make uranium-233. India has about one-third of the world's known supply of thorium, which is not useful by itself but can transform into the fissile material U-233. U-233 can power nuclear reactors and provide the fissile material for nuclear weapons. This material could therefore provide additional fuel for India's electrical power production reactors and additional material for nuclear weapons.

If India were able to develop the thorium fuel cycle, it could have available as much as 155,502 gigawatt-years of electrical energy (GWe-yr), in comparison to the potential for 328 GWe-yr from indigenously fueled thermal reactors; 10,660 GWe-yr from indigenous coal (which now provides 69 percent of Indian electricity); and 42,231 GWe-yr from plutonium breeder reactors.[1] Currently, India has an installed electrical generating capacity of about 140 GWe, and the rate of electricity demand is expected to increase by 6 to 8 percent per year through 2020 during this period of projected ambitious economic growth.[2] Thus, the thorium cycle holds out the potential to provide a huge portion of India's projected electricity needs for several hundred years.

Indian engineers have recognized, however, that significant hurdles block the way toward commercializing the thorium fuel cycle. High costs and major technical problems are likely to delay full commercialization of the thorium cycle until at least 2050, according to Indian energy experts.

To fully realize the thorium cycle, Indian engineers first face the mainly financial challenge of proving the commercial viability of the plutonium breeder program. India has operated a small 40-megawatt pilot-scale breeder reactor since 1985.Although India is building a commercial-scale breeder reactor, which is projected to be completed in 2011, and is planning to build four more of these reactors by 2020, ramping up to a fleet of breeder reactors will likely take decades, and it is uncertain if this program will succeed commercially. Thus, full realization of India's civilian nuclear energy vision appears blurry, and this program could remain stuck at a low level for the next few decades.

Indeed, after nearly half a century of investment, nuclear energy provides only about 4,000 megawatts of electricity, or 3 percent of India's electricity needs. That compares to about 20 percent in the United States. Even if the nuclear deal were to go through and India were to meet all of its goals for nuclear power generation, nuclear-generated electricity would only account for about 5 percent of India's projected electricity demands in 2020. CHARLES D. FERGUSON

ENDNOTES

1. Subhinder Thakur, Interview with author, Mumbai, January 4, 2008. Similar estimates appear in R. B. Grover and Subhash Chandra, "Scenario for Growth of Electricity in India," Energy Policy, November 2006, pp. 2834-2847. For data on coal use, see World Coal Institute, www.worldcoal.org/pages/content/index.asp?PageID=402.

2. John Stephenson and Peter Tynan, "Will the U.S.-India Civil Nuclear Cooperation Initiative Light India?" in Gauging U.S. Indian Strategic Cooperation, Henry Sokolski, editor (Strategic Studies Institute, 2007), p. 24.

India's Planned Nuclear Triad: Seeking a "Credible Deterrent"

Charles D. Ferguson

If the U.S.-Indian nuclear deal were to move forward without any conditions, it would allow India to achieve its goal of deploying a triad of land-, sea-, and air-based nuclear weapons without hampering its nuclear energy ambitions.

India's desire for a nuclear triad arises out of its stated need for a "credible minimal deterrent." Exactly what that means is still being debated within the country, although the emphasis is clearly on "credibility" not minimalism. "Minimal" has been dropped at times from government pronouncements, but Indian analysts have consistently underscored the notion of credibility.[1] Even those who are strong supporters of eventual nuclear disarmament generally agree that credibility requires a second-strike capability.

Second-strike capability demands survivable nuclear forces. To achieve this, Indian analysts have borrowed from the U.S.-Soviet experience during the Cold War and have sought to acquire nuclear-armed submarines. In late February, India took a decisive step toward a sea-based nuclear capability by conducting a test of the K-15 ballistic missile from a submerged pontoon. The K-15 has a reported top range of 700 kilometers, allowing it to strike many targets in Pakistan. Deployed K-15 missiles on submarines could also target high-value sites in China.

The Indian military has been less successful in building nuclear submarines from which to launch such missiles. India's nuclear-powered submarine program has limped along since 1985, although the Indian navy is trying to ready its first nuclear submarine for sea trials next year. India also received some experience in nuclear submarine operations from 1988 to 1991 when it leased a nuclear-powered attack submarine from the Soviet Union. A Russian crew manned this submarine while training Indian sailors. Presently, Russia is building an Akula-class nuclear submarine for lease to India.

Despite the substantial delays in deploying nuclear-powered submarines, these types of warships are not essential for deploying nuclear-armed forces at sea. India could use conventionally powered submarines as missile carriers, surface ships carrying nuclear-armed cruise missiles, or aircraft carriers with nuclear-capable bombers. Russia is refitting an aircraft carrier for India. Having fallen behind schedule, Moscow will likely complete the refit by late 2010. India has renamed the Admiral Gorshkov carrier as the Vikramaditya, which would be capable of helping protect India's submarine fleet as well as launching fighter-bomber aircraft.[2] Of these platforms, Indian defense planners prefer the submarine force, whether nuclear or conventionally powered, to optimize survivability of this leg of the envisioned triad.

At this stage, India has not indicated how large its nuclear-armed submarine force could become. Submarines are least vulnerable to a pre-emptive attack when deployed; in port, a submarine is more exposed to attack. Even when deployed, a small submarine force could be vulnerable to anti-submarine warfare. If Pakistan develops effective anti-submarine capabilities, Indian defense planners would feel pressure to build a larger fleet of submarines, thereby increasing the perceived need for more weapons-usable fissile material and more nuclear weapons.

The other two legs of the triad would also require ready-to-deploy nuclear weapons. In the absence of clarifying information from the Indian government, there has been considerable debate about the deployment status of India's nuclear weapons. Estimates of weapons that are fully assembled or can be fully assembled within days to weeks vary from a few to up to 100 with many analysts settling on about 30 to 50.[3]

There is even more certainty about the numbers of aircraft India has. India has more than 300 nuclear-capable planes, but it is uncertain how many are devoted to the nuclear mission. The most likely nuclear delivery systems are the Jaguar IS and Mirage 2000H fighter-bombers. Russian-acquired older MiG-27 and newer Su-30MKI fighter-bombers might also have a nuclear role.[4] India plans to upgrade its military aircraft within the next few years by purchasing 126 multipurpose planes for up to $12 billion. During a late February 2008 official visit to India, Secretary of Defense Robert Gates reportedly promoted sales of U.S.-made aircraft.[5] It is uncertain how many aircraft India has armed or would consider arming with nuclear weapons.

Although the number of nuclear-armed land-based missiles is also uncertain, tests of these missiles are easier to track. The Prithvi I, with a range of 150 kilometers and a payload of 1,000 kilograms, has been approved for the Indian army. The Dhanush is the naval version of the Prithvi II, which is under development and has a range of approximately 350 kilometers. In addition, India has been developing longer-range Agni missiles. Although the Agni I with a 700-kilometer range and the Agni II with a range greater than 2,000 kilometers have reportedly been "inducted" into the army's missile groups, their operational status is uncertain. In addition, the Agni III with a range greater than 3,000 kilometers is still under development and was test-launched on April 12, 2007. The Natural Resources Defense Council estimates that the Agni I and II will become fully operational in the next two years. Both can be deployed on road or rail launchers.[6] Once operational, these missile systems would significantly enhance India's nuclear strike capabilities and could strike parts of China. India is estimated to have up to 100 ballistic missiles with more than half of those in the longer-range Agni class, but it is uncertain how many of these could be armed with nuclear warheads.[7]

Perceived pressures to deter China as well as Pakistan could increase the numbers of deployed and reserve Indian nuclear weapons. Although the actual size of the Indian arsenal is unknown, accounting for even modestly sized bomber, land-based missile, and submarine legs in a triad can give a rough estimate of the potential future size. For aircraft, India may choose to have a few dozen nuclear bombs. Presently, for example, India has about 48 Mirage 2000H planes and about 70 Jaguar ISs, but probably only a portion would have nuclear bombs devoted to them. In the missile leg, a few dozen Prithvi and Agni missiles could be devoted to nuclear missions. In the submarine leg, to ensure survivable forces, India would likely plan at a minimum for one submarine in the shipyard, one in port readying for deployment, and one or two at sea. Assuming up to a dozen missiles per submarine, India may have at least a few dozen warheads for the submarine force. If multiple warheads are placed on the missiles, the warhead numbers could expand by three or more times.

In sum, India's triad including a single-warhead missile force based on land and underwater and a bomber fleet could exceed more than 100 operational weapons in the coming years. In addition, this warhead amount could increase by a factor of two or more depending on the size of a reserve fissile material stockpile

neuclear deal 2

2008-06-20T06:21:00.000+05:30

Plutonium Production

To be sure, Indian officials I interviewed, as well as some deal supporters in the United States, contend that whether or not the deal goes through will not significantly affect India's weapons-grade plutonium production.[4] Given New Delhi's dedication to maintaining such production at full capacity, the deal's potential impact in this regard is indeed murky.

New Delhi has neither published its weapons-usable fissile material holdings nor indicated how large a nuclear arsenal it intends to make. Unofficial estimates by the Institute for Science and International Security (ISIS) indicate that India may have amassed 575 kilograms of weapons-grade plutonium as of the end of 2004.[5] ISIS has also estimated that India may have consumed about 131 kilograms of this plutonium in nuclear weapons tests, as reactor fuel, and in processing losses. The CIRUS reactor could produce about 9 kilograms of weapons-grade plutonium annually, and Dhruva could make about 23 kilograms annually. If these estimates are accurate, India may have had available 540 kilograms of weapons-grade plutonium as of the end of 2007. Using the conservative International Atomic Energy Agency (IAEA) estimates that 8 kilograms of plutonium are needed to make a nuclear bomb, the stockpiled Indian plutonium could fuel a minimum of 67 first-generation fission bombs. Some analysts have argued that more advanced designs could use as little as a few kilograms of plutonium.[6] Therefore, the upper bound estimate for India's current warhead capacity is somewhat more than 100 nuclear weapons.

It does appear that, in at least one respect, the deal could stimulate near-term growth in weapons-grade plutonium production. Under the deal, India has pledged to shut down the aging CIRUS reactor by 2010. CIRUS is contentious because India obtained it from Canada in the late 1950s and gave assurances "that the reactor would be used only for peaceful uses." The United States had provided the heavy water for the reactor. This reactor, however, produced plutonium for India's 1974 "peaceful" nuclear test, which spurred the United States and other countries to form the NSG. India has considered replacing this 40-megawatt thermal (MWth) reactor with a larger capacity 100 MWth reactor.[7] This replacement reactor could produce about two-and-a-half times the amount of plutonium produced annually by CIRUS, or about 23 kilograms compared to 9 kilograms.

In addition to its weapons-grade plutonium stockpile, with or without the deal, India can make hundreds of nuclear weapons from several tons of unsafeguarded reactor-grade plutonium in spent nuclear fuel it has already accumulated, although the deal could somewhat affect future production. It is unknown how much reactor-grade plutonium India has separated from spent fuel, but the unsafeguarded reactors have produced more than 20 times the amount of plutonium that India has obtained from the two weapons-plutonium-production reactors. The deal did not place any of this past production under safeguards.

The most direct and immediate means of using this material would be as fissile material in nuclear weapons. Although weapons-grade plutonium is ideal for weapons use, reactor-grade plutonium can also serve this purpose.[8] Reportedly, India may have used reactor-grade plutonium in one of its May 1998 tests.[9]

Moreover, this feedstock of unsafeguarded plutonium could fuel India's planned breeder reactor program (the second stage of Bhabha's three-stage plan), which will remain outside of safeguards. The five planned breeder reactors by 2020 would require two initial cores of plutonium before recycling of plutonium would make the breeders more than self-sufficient. If only the first 500-megawatt electric Prototype Fast Breeder Reactor were dedicated to weapons production, it could produce up to 140 kilograms of weapons-grade plutonium each year, more than four times the current rate of production from CIRUS and Dhruva.[10]

It should be noted that, in a few years, the deal might lower the future rate of production of reactor-grade plutonium. Without the deal, India would have only six reactors under safeguards: the U.S.-built Tarapur 1 and 2, the Canadian-built Rajasthan 1 and 2, and the two Russian reactors under construction at Kudankulam. With the deal, India has agreed to place eight additional indigenously made reactors under safeguards, meaning that eight pressurized heavy-water reactors and their produced plutonium would remain outside of safeguards. Over the course of the next seven years, the net result would be that the annual production rate of unsafeguarded plutonium would be set to peak at about 2,000 kilograms per year in the next two years and fall to about 1,250 kilograms per year by 2015, when safeguards would be applied to all of the reactors subject to the deal.

Therefore, the deal would serve to lower India's future unsafeguarded plutonium production rate by about one-third.[11] In that respect, the deal is arguably positive for nonproliferation as long as permanent safeguards are applied. Nonetheless, existing and future stocks of spent fuel would be more than sufficient to fuel the breeder program or to provide direct fissile material for nuclear weapons. Furthermore, the deal as structured has given implicit U.S. approval to India's nuclear weapons program under the guise of bringing India into "the nonproliferation mainstream."

Directing India Onto a More Responsible Path

To truly bring India into the nonproliferation mainstream, the NSG and Congress must insist on certain conditions. These conditions are minimal in the sense that they would not roll back India's nuclear weapons program and would not significantly curtail India's weapons-usable fissile material production capabilities. In that sense, India will have won what it has most sought, recognition of its nuclear weapons program. Even if the deal dies, the United States in effect has already bestowed that recognition. Nonetheless, as a price for that acknowledgement, India should be willing to accept more responsible behavior that would lessen the damage to the nonproliferation regime.

Nuclear trade should be contingent on India refraining from nuclear testing. Also, such commerce should depend on maintenance of permanent safeguards on all designated nuclear facilities. Moreover, the NSG should hold back on transferring enrichment, reprocessing, and heavy-water technologies that could further enhance India's weapons production capabilities. In addition, the United States should press for India to sign the CTBT and adhere to a weapons-usable fissile material cap. Fully implementing these measures, however, will depend on Chinese and Pakistani actions.

Although most Indian policymakers and analysts have supported the country's unilateral testing moratorium since 1998, all interviewees agreed that India's accession to the CTBT has become increasingly tied to the U.S. position on the treaty. India will not ratify the treaty unless the United States does so. Although there is no direct nuclear threat between India and the United States, Indian analysts have made a direct connection between U.S. nuclear actions and India's place in the world. Summing up this view, Professor Pratap Mehta, the executive director of the Center for Policy Research, based in New Delhi, said India "cannot support a world order that gives into the U.S. maintaining its nuclear primacy." Moreover, he said that "as long as the U.S. holds out on modernizing its arsenal, India will not sign the FMCT [fissile material cutoff treaty] or the CTBT."

Acknowledging U.S. influence, top defense expert K. Santhanam, who had a leadership role during the 1998 tests, drew a more direct connection to China and Pakistan. He expressed willingness for India to continue indefinitely the testing moratorium as long as China and Pakistan refrain from testing.

All of the five original nuclear-weapon states, including China, have signed the CTBT. Even if ratification by the United States remains out of reach for the time being, India should be encouraged in tandem with Pakistan to take a step beyond the moratorium and sign the treaty.

Similarly, fissile material production depends crucially on Chinese and Pakistani production. All of the five legally recognized nuclear-weapon states but China have committed to stop making fissile material for weapons. China is believed to have stopped weapons-usable fissile material production, but Beijing has never officially said so. If China would make a public pledge not to make fissile material for weapons, it would put added pressure on India to specify when it would stop stockpiling nuclear weapons material. To bring Pakistan into this arrangement, India could offer a series of alternating unilateral moves. For example, India could verifiably shut down one of its plutonium-production reactors for a period of time. Pakistan could take a similar step with one of its production reactors. Verification could be achieved through third-party commercial satellite monitoring of the status of the reactors.

Although turning back the growth in India's nuclear arsenal appears unlikely for the foreseeable future, the NSG and the United States have opportunities to shape the future direction of India's strategic weapons program. They should take it.

India's Nuclear Energy Program: Ambitious Dreams, Sober Realities

Neuclear Deal For India

2008-06-20T06:20:00.001+05:30

Arms Control Today April 2008

Reshaping the U.S.-Indian Nuclear Deal to Lessen the Nonproliferation Losses

Charles D. Ferguson

India's Nuclear Energy Program: Ambitious Dreams, Sober Realities

India's Planned Nuclear Triad: Seeking a "Credible Deterrent"

For decades, India's nuclear programs have been defined by two contradictory forces: the country's vast ambitions and its limited uranium reserves. Its ambitions have led New Delhi to establish a significant civilian nuclear enterprise, to refuse to sign the nuclear Nonproliferation Treaty (NPT), and to develop and test nuclear weapons. Its limited uranium reserves, on the other hand, have clearly slowed India's nuclear energy development, most likely hampered its nuclear weapons program, and intertwined the two efforts to a high degree.

The tension between India's goals and resources has grown much stronger in the past decade. By bringing India's nuclear weapons programs into the open, the country's 1998 nuclear tests fueled calls to develop the full panoply of nuclear capabilities, including a nuclear triad. India's recent impressive economic growth has strained the country's energy system, increasing interest in nuclear energy. In particular, India would like to quintuple the production of electricity through nuclear energy by 2020.

To the Indian government, the civil nuclear cooperation agreement it signed with the United States last year looks like a way for New Delhi to escape this dilemma, giving it access to global uranium reserves without imposing limits on its nuclear weapons program. India's right and left wings may claim the Congress-led government has somehow shortchanged their country. The truth is that, without the deal, New Delhi will be forced to confront painful trade-offs between its energy and national security goals, as a series of January interviews I conducted in India of nuclear scientists, policy experts, and energy and defense analysts made clear.

For the deal to go forward, the 45 members of the voluntary Nuclear Suppliers Group (NSG) must first agree to carve out an exception for India to its guidelines. These currently require a non-nuclear-weapon state, as India is legally defined under the NPT, to have comprehensive safeguards on all nuclear facilities before receiving civilian nuclear assistance from NSG countries.

The U.S. Congress too must sign off on the final nuclear cooperation agreement, meaning that it and the NSG will retain considerable leverage over India. They should use this power to condition the agreement in a way that does less damage to the nuclear nonproliferation regime.

The NSG has an opportunity to condition this exception on India's behaviors, including continuing to refrain from testing nuclear explosives and placing permanent safeguards on any foreign technologies and fuel, as well as designated indigenous facilities. Moreover, the NSG should hold back on transferring enrichment and reprocessing technologies, which could further enhance India's weapons production capabilities, and only supply as much reserve fuel as needed for reasonable power plant requirements. U.S. leadership could also influence India to become a more responsible nuclear-armed state through signing the Comprehensive Test Ban Treaty (CTBT) and committing to a cutoff of weapons-usable fissile material in addition to adhering to conditions on civilian nuclear commerce.

Two Intertwined Visions

The roots of the current controversy over the nuclear deal go back to India's emergence as an independent nation in the late 1940s. At that time, Dr. Homi Bhabha, widely viewed as a father of India's nuclear programs, sought to develop these efforts in a way that exploited indigenous resources. He was well aware that India's uranium resources were only sufficient to power a modest nuclear energy program of about 10,000 megawatts per year and even less would be available if some were used for weapons. To compensate, Bhabha laid out a three-stage plan for India to hoard these limited indigenous uranium deposits and to leverage its abundant thorium deposits to bootstrap itself to a massive production of electricity through nuclear energy and to produce weapons-grade plutonium.

This vision of self-sufficiency, which arose in part from India's desire to escape its colonial heritage, has remained a guiding vision for India's nuclear establishment even as its practical fulfillment has receded further into the future. India's positions in the discussions on a nuclear cooperation agreement with the United States in many ways reflect a compromise between those who want to be self-reliant and stick almost exclusively with Bhabha's three-stage plan, which one interviewee called "a sacred cow," and those who are willing to bring in outside foreign suppliers. India's preference for autarky was reinforced by its isolation from international nuclear trade after a 1974 nuclear test, which relied on U.S. and Canadian technology and nuclear materials. This is also reflected in India's current negotiating posture, which seeks to ensure that foreign suppliers cannot shut off access to fuel and reactors if New Delhi tests nuclear explosives or commits some other proliferation transgression, such as transferring nuclear technologies to states of concern.

Moreover, while Bhabha sought to ensure that fissile materials would be available for a nuclear weapons program, India in recent years has fleshed out what it means when it says that it seeks a "credible minimal deterrent." In its draft nuclear doctrine published soon after the 1998 tests, New Delhi explicitly stated its objective was to deploy a triad of nuclear forces. The triad would consist of land-based ballistic missiles, nuclear-capable aircraft, and nuclear-armed submarines. As with the U.S.-Soviet experience during the Cold War, such a triad is designed to provide India with survivable nuclear forces and a second-strike capability. It would also mean that India's arsenal would increase from an estimated few dozen operational warheads today to as many as 200 or more, a level akin to China and the United Kingdom. The nuclear deal would not prevent India from building up to these projected operational and reserve capacities within several years.

Complications during Peritoneal Dialysis

2008-06-13T01:15:00.000+05:30

Complications during Peritoneal Dialysis

The major complications of peritoneal dialysis are peritonitis, catheter-associated nonperitonitis infections, weight gain a disturbances, and residual uremia (especially among patients with no residual kidney function).

Peritonitis typically develops when there has been a break in sterile technique during one or more of the exchange proc defined by an elevated peritoneal fluid leukocyte count (100/mm3, of which at least 50% are polymorphonuclear neutro presentation typically consists of pain and cloudy dialysate, often with fever and other constitutional symptoms. The mo organisms are gram-positive cocci, including Staphylococcus, reflecting the origin from the skin. Gram-negative rod infe fungal and mycobacterial infections can be seen in selected patients, particularly after antibacterial therapy. Most cases managed either with intraperitoneal or oral antibiotics, depending on the organism; many patients with peritonitis do no cases where peritonitis is due to hydrophilic gram negative rods (e.g., Pseudomonas sp.) or yeast, antimicrobial therapy and catheter removal is required to ensure complete eradication of infection. Nonperitonitis catheter-associated infection infections) vary widely in severity. Some cases can be managed with local antibiotic or silver nitrate administration, whi enough to require parenteral antibiotic therapy and catheter removal.

Peritoneal dialysis is associated with a variety of metabolic complications. As noted above, albumin and other proteins c peritoneal membrane in concert with the loss of metabolic wastes. The hypoproteinemia induced by peritoneal dialysis o protein intake in order to maintain nitrogen balance. Hyperglycemia and weight gain are also common complications of hundred calories in the form of dextrose are absorbed each day, depending on the concentration employed. Peritoneal d those with type II diabetes mellitus, are then prone to other complications of insulin resistance, including hypertriglycer side, the continuous nature of peritoneal dialysis usually allows for a more liberal diet, due to continuous removal of pot two major dietary components whose accumulation can be hazardous in ESRD.

GLOBAL PERSPECTIVE

The incidence of ESRD is increasing worldwide with longer life expectancies and improved care of infectious and cardiov management of ESRD varies widely by country and within country by region, and it is influenced by economic and other peritoneal dialysis is more commonly performed in poorer countries owing to its lower expense and the high cost of esta hemodialysis units

PERITONEAL DIALYSIS

2008-06-13T01:14:00.000+05:30

PERITONEAL DIALYSIS

In peritoneal dialysis, 1.5-3 L of a dextrose-containing solution is infused into the peritoneal cavity and allowed to dwel usually 2-4 h. As with hemodialysis, toxic materials are removed through a combination of convective clearance genera and diffusive clearance down a concentration gradient. The clearance of solutes and water during a peritoneal dialysis e balance between the movement of solute and water into the peritoneal cavity versus absorption from the peritoneal cav diminishes with time and eventually stops when equilibration between plasma and dialysate is reached. Absorption of so peritoneal cavity occurs across the peritoneal membrane into the peritoneal capillary circulation and via peritoneal lymp circulation. The rate of peritoneal solute transport varies from patient to patient and may be altered by the presence of drugs, and physical factors such as position and exercise.

Forms of Peritoneal Dialysis

Peritoneal dialysis may be carried out as continuous ambulatory peritoneal dialysis (CAPD), continuous cyclic peritoneal combination of both. In CAPD, dialysis solution is manually infused into the peritoneal cavity during the day and exchan daily. A nighttime dwell is frequently instilled at bedtime and remains in the peritoneal cavity through the night. The dra performed manually with the assistance of gravity to move fluid out of the abdomen. In CCPD, exchanges are performe usually at night; the patient is connected to an automated cycler that performs a series of exchange cycles while the pa exchange cycles required to optimize peritoneal solute clearance varies by the peritoneal membrane characteristics; as suggest careful tracking of solute clearances to ensure dialysis "adequacy."

Peritoneal dialysis solutions are available in volumes typically ranging from 1.5 to 3.0 L. Lactate is the preferred buffer i solutions. The most common additives to peritoneal dialysis solutions are heparin to prevent obstruction of the dialysis c and antibiotics during an episode of acute peritonitis. Insulin may also be added in patients with diabetes mellitus.

Access to the Peritoneal Cavity

Access to the peritoneal cavity is obtained through a peritoneal catheter. Catheters used for maintenance peritoneal dia made of silicon rubber with numerous side holes at the distal end. These catheters usually have two Dacron cuffs to pro proliferation, granulation, and invasion of the cuff. The scarring that occurs around the cuffs anchors the catheter and s tracking from the skin surface into the peritoneal cavity; it also prevents the external leakage of fluid from the peritonea placed in the preperitoneal plane and ~2 cm from the skin surface.

The peritoneal equilibrium test is a formal evaluation of peritoneal membrane characteristics that measures the transfer glucose across the peritoneal membrane. Patients are classified as low, low-average, high-average, and high "transpor equilibration (i.e., high transporters) tend to absorb more glucose and lose efficiency of ultrafiltration with long daytime also tend to lose larger quantities of albumin and other proteins across the peritoneal membrane. In general, patients w characteristics require more frequent, shorter dwell time exchanges, nearly always obligating use of a cycler for feasibil average) transporters tend to do well with fewer exchanges. The efficiency of solute clearance also depends on the volu Larger volumes allow for greater solute clearance, particularly with CAPD in patients with low and low-average transpor Interestingly, solute clearance also increases with physical activity, presumably related to more efficient flow dynamics

As with hemodialysis, the optimal dose of peritoneal dialysis is unknown. Several observational studies have suggested and creatinine clearance (the latter generally measured in L/week) are associated with lower mortality rates and fewer u However, a randomized clinical trial (ADEMEX) failed to show a significant reduction in mortality or complications with a in urea clearance. In general, patients on peritoneal dialysis do well when they retain residual kidney function. The rates increase with years on dialysis and have been correlated with loss of residual function to a greater extent than loss of p capacity. Recently, a nonabsorbable carbohydrate (icodextrin) has been introduced as an alternative osmotic agent. Stu more efficient ultrafiltration with icodextrin than with dextrose-containing solutions. Icodextrin is typically used as the "l CCPD or for the longest dwell in patients on CAPD. For some patients in whom CCPD does not provide sufficient solute c approach can be adopted where one or more daytime exchanges are added to the CCPD regimen. While this approach c clearance and prolong a patient's capacity to remain on peritoneal dialysis, the burden of the hybrid approach can be ov

Complications during Hemodialysis

2008-06-13T01:13:00.002+05:30

Complications during Hemodialysis

Hypotension is the most common acute complication of hemodialysis, particularly among diabetics. Numerous factors ap hypotension, including excessive ultrafiltration with inadequate compensatory vascular filling, impaired vasoactive or au osmolar shifts, overzealous use of antihypertensive agents, and reduced cardiac reserve. Patients with arteriovenous fis develop high output cardiac failure due to shunting of blood through the dialysis access; on rare occasions, this may nec fistula or graft. Because of the vasodilatory and cardiodepressive effects of acetate, its use as the buffer in dialysate wa hypotension. Since the introduction of bicarbonate-containing dialysate, dialysis-associated hypotension has become les management of hypotension during dialysis consists of discontinuing ultrafiltration, the administration of 100 23% saturated hypertonic saline, and administration of salt-poor albumin. Hypotension during dialysis can frequently be evaluation of the dry weight and by ultrafiltration modeling, such that more fluid is removed at the beginning rather tha procedure. Additional maneuvers include the performance of sequential ultrafiltration followed by dialysis; the use of mi adrenergic pressor agent; cooling of the dialysate during dialysis treatment; and avoiding heavy meals during dialysis. Muscle cramps during dialysis are also a common complication of the procedure. The etiology of dialysis-associated cram Changes in muscle perfusion because of excessively aggressive volume removal, particularly below the estimated dry w sodium-containing dialysate, have been proposed as precipitants of dialysis-associated cramps. Strategies that may be include reducing volume removal during dialysis, ultrafiltration profiling, and the use of higher concentrations of sodium modeling (see above).

Anaphylactoid reactions to the dialyzer, particularly on its first use, have been reported most frequently with the bioinco containing membranes. With the gradual phasing out of cuprophane membranes in the United States, dialyzer reactions uncommon. Dialyzer reactions can be divided into two types, A and B. Type A reactions are attributed to an IgE hypersensitivity reaction to ethylene oxide used in the sterilization of new dialyzers. This reaction typically occurs soon a treatment (within the first few minutes) and can progress to full-blown anaphylaxis if the therapy is not promptly discon steroids or epinephrine may be needed if symptoms are severe. The type B reaction consists of a symptom complex of n pain, which appears to result from complement activation and cytokine release. These symptoms typically occur several run and typically resolve over time with continued dialysis.

diseases constitute the major causes of death in patients with ESRD. Cardiovascular mortality and event patients than in patients posttransplantation, although rates are extraordinarily high in both populations. The underlying disease is unclear but may be related to shared risk factors (e.g., diabetes mellitus), chronic inflammation, massive cha volume (especially with high interdialytic weight gains), inadequate treatment of hypertension, dyslipidemia, anemia, dy calcification, hyperhomocysteinemia, and, perhaps, alterations in cardiovascular dynamics during the dialysis treatment cardiovascular risk reduction in ESRD patients; none have demonstrated consistent benefit. Nevertheless, most experts cardioprotective strategies (e.g., lipid-lowering agents, aspirin, -adrenergic antagonists) in dialysis patients based on t risk profile, which appears to be increased by more than an order of magnitude relative to persons unaffected by kidney

DIALYSIS ACCESS

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DIALYSIS ACCESS

The fistula, graft, or catheter through which blood is obtained for hemodialysis is often referred to as a dialysis access the anastomosis of an artery to a vein (e.g., the Brescia-Cimino fistula, in which the cephalic vein is anastomosed end results in arterialization of the vein. This facilitates its subsequent use in the placement of large needles (typically 15 ga circulation. Although fistulas have the highest long-term patency rate of all dialysis access options, fistulas are created i the United States. Many patients undergo placement of an arteriovenous graft (i.e., the interposition of prosthetic mate polytetrafluoroethylene, between an artery and a vein) or a tunneled dialysis catheter. In recent years, nephrologists, v health care policy makers in the United States have encouraged creation of arteriovenous fistulas in a larger fraction of initiative). Unfortunately, even when created, arteriovenous fistulas may not mature sufficiently to provide reliable acce they may thrombose early in their development. Novel surgical approaches (e.g., brachiobasilic fistula creation with tran fistula to the arm surface) have increased options for "native" vascular access.

Grafts and catheters tend to be used among persons with smaller-caliber veins or persons whose veins have been dama venipuncture, or after prolonged hospitalization. The most important complication of arteriovenous grafts is thrombosis failure, due principally to intimal hyperplasia at the anastomosis between the graft and recipient vein. When grafts (or f guided angioplasty can be used to dilate stenoses; monitoring of venous pressures on dialysis and of access flow, thoug may assist in the early recognition of impending vascular access failure. In addition to an increased rate of access failur catheters are associated with much higher rates of infection than fistulas.

Intravenous large-bore catheters are often used in patients with acute and chronic kidney disease. For persons on main tunneled catheters (either two separate catheters or a single catheter with two lumens) are often used when arterioven failed or are not feasible due to anatomical considerations. These catheters are tunneled under the skin; the tunnel redu from the skin, resulting in a lower infection rate than with nontunneled temporary catheters. Most tunneled catheters ar

jugular veins; the external jugular, femoral, and subclavian veins may also be used. Nephrologists, interventional radiol surgeons generally prefer to avoid placement of catheters into the subclavian veins; while flow rates are usually excelle frequent complication and, if present, will likely prohibit permanent vascular access (i.e., a fistula or graft) in the ipsilat rates may be higher with femoral catheters. For patients with multiple vascular access complications and no other optio access, tunneled catheters may be the last "lifeline" for hemodialysis. Translumbar or transhepatic approaches into the required if the superior vena cava or other central veins draining the upper extremities are stenosed or thrombosed.

Goals of Dialysis

The hemodialysis procedure is targeted at removing both low- and high-molecular-weight solutes. The procedure consis blood through the dialyzer at a flow rate of 300-500 mL/min, while dialysate flows in an opposite counter-current The efficiency of dialysis is determined by blood and dialysate flow through the dialyzer as well as dialyzer characteristic removing solute). The dose of dialysis, which is currently defined as a derivation of the fractional urea clearance during is further governed by patient size, residual kidney function, dietary protein intake, the degree of anabolism or catabolis comorbid conditions.

Since the landmark studies of Sargent and Gotch relating the measurement of the dose of dialysis using urea concentra National Cooperative Dialysis Study, the delivered dose of dialysis has been measured and considered as a quality assur tool. While the fractional removal of urea nitrogen and derivations thereof are considered to be the standard methods b dialysis" is measured, a large multicenter randomized clinical trial (the HEMO Study) failed to show a difference in morta difference in urea clearance. Still, multiple observational studies and widespread expert opinion have suggested that hig warranted; current targets include a urea reduction ratio (the fractional reduction in blood urea nitrogen per hemodialys and a body water-indexed clearance x time product (KT/V) above 1.3 or 1.05, depending on whether urea concentration For the majority of patients with ESRD, between 9 and 12 h of dialysis are required each week, usually divided into thre studies have suggested that longer hemodialysis session lengths may be beneficial, although these studies are confound characteristics, including body size and nutritional status. Hemodialysis "dose" should be individualized, and factors othe should be considered, including the adequacy of ultrafiltration or fluid removal. Several authors have highlighted improv associated with more frequent hemodialysis (i.e., more than three times a week), although these studies are also confo A randomized clinical trial is currently underway to test whether more frequent dialysis results in differences in a variety functional markers.

The Dialyzer

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The Dialyzer

There are three essential components to hemodialysis: the dialyzer, the composition and delivery of the dialysate, and The dialyzer consists of a plastic device with the facility to perfuse blood and dialysate compartments at ve surface area of modern dialysis membranes in adult patients is usually in the range of 1.5-2.0 m2. The hollow use in the United States. These dialyzers are composed of bundles of capillary tubes through which blood circulates whil outside of the fiber bundle.

Recent advances have led to the development of many different types of membrane material. Broadly, there are four ca membranes: cellulose, substituted cellulose, cellulosynthetic, and synthetic. Over the past three decades, there has bee cellulose-derived to synthetic membranes, because the latter are more "biocompatible." Bioincompatibility is generally d membrane to activate the complement cascade. Cellulosic membranes are bioincompatible because of the presence of f membrane surface. In contrast, with the substituted cellulose membranes (e.g., cellulose acetate) or the cellulosyntheti groups are chemically bound to either acetate or tertiary amino groups, resulting in limited complement activation. Syn polysulfone, polymethylmethacrylate, and polyacrylonitrile membranes, are even more biocompatible because of the ab groups. Polysulfone membranes are now used in >60% of the dialysis treatments in the United States. Reprocessing and reuse of hemodialyzers are often employed for patients on maintenance hemodialysis in the United St manufacturing costs for disposable dialyzers have declined, more and more outpatient dialysis facilities are no longer re most centers employing reuse, only the dialyzer unit is reprocessed and reused, whereas in the developing world blood reused. The reprocessing procedure can be either manual or automated. It consists of the sequential rinsing of the bloo compartments with water, a chemical cleansing step with reverse ultrafiltration from the dialysate to the blood compart patency of the dialyzer, and, finally, disinfection of the dialyzer. Formaldehyde, peracetic acid-hydrogen peroxide, gluta all been used as reprocessing agents.

Dialysate

The potassium concentration of dialysate may be varied from 0 to 4 mmol/L depending on the predialysis plasma potass usual dialysate calcium concentration in U.S. hemodialysis centers is 1.25 mmol/L (2.5 meq/L), although modification m settings (e.g., higher dialysate calcium concentrations may be used in patients with hypocalcemia associated with secon or following parathyroidectomy). The usual dialysate sodium concentration is 140 mmol/L. Lower dialysate sodium conc with a higher frequency of hypotension, cramping, nausea, vomiting, fatigue, and dizziness. In patients who frequently their dialysis run, "sodium modeling" to counterbalance urea-related osmolar gradients is often used. When sodium mod concentration is gradually lowered from the range of 145-155 meq/L to isotonic concentrations (140 meq/L) near the e treatment, typically declining either in steps or in a linear or exponential fashion. Because patients are exposed to appro during each dialysis treatment, water used for the dialysate is subjected to filtration, softening, deionization, and, ultim During the reverse osmosis process, water is forced through a semipermeable membrane at very high pressure to remo contaminants and >90% of dissolved ions.

Blood Delivery System

The blood delivery system is composed of the extracorporeal circuit in the dialysis machine and the dialysis access. The of a blood pump, dialysis solution delivery system, and various safety monitors. The blood pump moves blood from the dialyzer, and back to the patient. The blood flow rate may range from 250-500 mL/min, depending largely on the type vascular access. Negative hydrostatic pressure on the dialysate side can be manipulated to achieve desirable fluid remo Dialysis membranes have different ultrafiltration coefficients (i.e., mL removed/min per mmHg) so that along with hydro removal can be varied. The dialysis solution delivery system dilutes the concentrated dialysate with water and monitors conductivity, and flow of dialysate.

TREATMENT OPTIONS FOR ESRD PATIENTS

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TREATMENT OPTIONS FOR ESRD PATIENTS

Commonly accepted criteria for initiating patients on maintenance dialysis include the presence of uremic symptoms, th unresponsive to conservative measures, persistent extracellular volume expansion despite diuretic therapy, acidosis ref a bleeding diathesis, and a creatinine clearance or estimated glomerular filtration rate (GFR) below 10 mL/min per 1.73 estimating equations). Timely referral to a nephrologist for advanced planning and creation of a dialysis access, educati options, and management of the complications of advanced chronic kidney disease, including hypertension, anemia, aci hyperparathyroidism, is advisable.

In ESRD, treatment options include hemodialysis (in center or at home); peritoneal dialysis, as either continuous ambul (CAPD) or continuous cyclic peritoneal dialysis (CCPD); or transplantation (Chap. 276). Although there are geographic v remains the most common therapeutic modality for ESRD (>90% of patients) in the United States. In contrast to hemo is continuous, but much less efficient, in terms of solute clearance. While no large-scale clinical trials have been comple among patients randomized to either hemodialysis or peritoneal dialysis, outcomes associated with both therapies are s the decision of which modality to select is often based on personal preferences and quality-of-life considerations.

HEMODIALYSIS

Hemodialysis relies on the principles of solute diffusion across a semipermeable membrane. Movement of metabolic was down a concentration gradient from the circulation into the dialysate. The rate of diffusive transport increases in respon including the magnitude of the concentration gradient, the membrane surface area, and the mass transfer coefficient of is a function of the porosity and thickness of the membrane, the size of the solute molecule, and the conditions of flow o membrane. According to the laws of diffusion, the larger the molecule, the slower its rate of transfer across the membra as urea (60 Da), undergoes substantial clearance, whereas a larger molecule, such as creatinine (113 Da), is cleared les diffusive clearance, movement of waste products from the circulation into the dialysate may occur as a result of ultrafilt clearance occurs because of solvent drag, with solutes being swept along with water across the semipermeable dialysis .

DIALYSIS IN THE TREATMENT OF RENAL FAILURE: INTRODUCTION

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DIALYSIS IN THE TREATMENT OF RENAL FAILURE: INTRODUCTION

Dialysis may be required for the treatment of either acute or chronic kidney disease. The use of continuous renal replac and slow, low-efficiency dialysis (SLED) is specific to the management of acute renal failure and is discussed in Chap. 2 performed continuously (CRRT) or over 6-12 hours per session (SLED), in contrast to the 3-4 hours of an intermittent Advantages and disadvantages of CRRT and SLED.

Peritoneal dialysis is rarely used in developed countries for the treatment of acute renal failure because of the increased will be discussed in more detail below) less efficient clearance per unit of time. The focus of the majority of this chapter dialysis for end-stage renal disease (ESRD).

With the widespread availability of dialysis, the lives of hundreds of thousands of patients with ESRD have been prolong alone, there are now approximately 450,000 patients with ESRD, the vast majority of whom require dialysis. The incide cases per million population per year. The incidence of ESRD is disproportionately higher in African Americans (approxim population per year) as compared with white Americans (259 per million population per year). In the United States, the diabetes mellitus, currently accounting for nearly 45% of newly diagnosed cases of ESRD. Over one-quarter (27%) of p been attributed to hypertension, although it is unclear whether in these cases hypertension is the cause or a consequen other unknown causes of kidney failure. Other important causes of ESRD include glomerulonephritis, polycystic kidney d uropathy.

Globally, mortality rates for patients with ESRD are lowest in Europe and Japan but very high in the developing world b availability of dialysis. In the United States, the mortality rate of patients on dialysis is approximately 18-20% per year of approximately 30-35%. Deaths are due mainly to cardiovascular diseases and infections (approximately 50 and 15% Older age, male sex, nonblack race, diabetes mellitus, malnutrition, and underlying heart disease are important predict

Noninvasive Cardiac Imaging: Echocardiography, Nuclear Cardiology, and MRI/CT Imaging:

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Noninvasive Cardiac Imaging: Echocardiography, Nuclear Cardiology, and MRI/CT Imaging:

Cardiovascular imaging has significantly enhanced the practice of cardiology over the past few decades. Two

(2D) echocardiography is able to visualize the heart directly in real time using ultrasound, providing instantaneous

assessment of the myocardium, cardiac chambers, valves, pericardium, and great vessels. Doppler echocardiography

measures the velocity of moving red blood cells and has become a noninvasive alternative to cardiac catheterization for assessment of hemodynamics. Transesophageal echocardiography (TEE) provides a unique window for high
imaging of posterior structures of the heart, particularly the left atrium, mitral valve, and aorta. Nuclear cardiology uses isotopes to assess myocardial perfusion and ventricular function and has contributed greatly to the evaluation of patient with ischemic heart disease. Cardiac MRI and CT can delineate cardiac structure and function with high resolution. They particularly useful in the examination of cardiac masses, the pericardium, and the great vessels. MRI stress testing is no possible examining both ventricular function and perfusion. Detection of coronary calcification by CT as well as direct
visualization of coronary arteries by CT angiography (CTA) are of growing utility in patients with suspected coronary art disease (CAD). This chapter provides an overview of the basic concepts of these cardiac imaging modalities, as well as t clinical indications for each procedure. The illustrations in this chapter are supplemented by "real time" and other static images in Chap. e20, "The Atlas of Noninvasive Cardiac Imaging."

Two-Dimensional Echocardiography

BASIC PRINCIPLES

2D echocardiography uses the principle of ultrasound reflection off cardiac structures to produce images of the heart (T 222-1). For a transthoracic echocardiogram (TTE), the imaging is performed with a handheld transducer placed directly the chest wall. In selected patients, a TEE may be performed, in which an ultrasound transducer is mounted on the tip o endoscope placed in the esophagus and directed toward the cardiac structures.

Table 1 Clinical Uses of Echocardiography

Two-Dimensional Echocardiography Doppler Echocardiography

Cardiac chambers Valve stenosis

Chamber size Gradient

Left ventricular Valve area

Hypertrophy Valve regurgitation

Regional wall motion abnormalities Semiquantitation

Valve Intracardiac pressures

Morphology and motion Volumetric flow

Pericardium Diastolic filling

Effusion Intracardiac shunts

Tamponade Transesophageal Echocardiography

Masses Inadequate transthoracic images

Great vessels Aortic disease

Stress Echocardiography Infective endocarditis

Two-dimensional Source of embolism

Myocardial ischemia Valve prosthesis

Viable myocardium Intraoperative

Doppler

Valve disease

Current echocardiographic machines are portable and can be wheeled directly to the patient's bedside. Thus, a major

advantage of echocardiography over other imaging modalities is the ability to obtain instantaneous images of the cardia structures for immediate interpretation. Handheld echocardiographic units weighing 6 lb (<2.7 kg) have now become available, further enhancing the ease and portability of echocardiography. They are becoming an essential initial diagno modality for the critically ill patient in the emergency room and critical care setting.

A limitation of TTE is the inability to obtain high-quality images in all patients, especially those with a thick chest wall o severe lung disease, as ultrasound waves are poorly transmitted through lung parenchyma. New technology such as harmonic imaging and IV contrast agents (which traverse the pulmonary circulation) can now be used to enhance
endocardial borders in patients with poor acoustic windows.

CHAMBER SIZE AND FUNCTION

2D echocardiography is an ideal imaging modality for assessing left ventricular (LV) size and function (Fig. 1

qualitative assessment of the cavity sizes of the ventricles and systolic function can be made directly from the 2D imag experienced observers. 2D echocardiography is useful in the diagnosis of LV hypertrophy and is the imaging modality o choice for the diagnosis of hypertrophic cardiomyopathy. Other chamber sizes are assessed by visual analysis, including left atrium and right-sided chambers.

Figure 1

Two-dimensional echocardiographic still-frame images from a normal patient with a normal heart. Top: Parasternal l axis view during systole and diastole (left) and systole (right). During systole, there is thickening of the myocardium and reducti the size of the left ventricle (LV). The valve leaflets are thin and open widely. Bottom: Parasternal short axis view during diastol (left) and systole (right) demonstrating a decrease in the left ventricular cavity size during systole as well as an increase in wall thickening. LA, left atrium; RV, right ventricle; Ao, aorta.

photography tip

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Shoot photos for a

PANORAMA

As long as photographs have been taken, it has always been a challenge for photographers to capture the beauty found in wide-sweeping scenes. A wide-angle lens can capture more of a scene than a shorter focal length lens, but wide-angle lenses tend to add unwanted distortion to the photos, and they still do not capture as much of a scene as you often want. Using one of the digital stitching applications or a feature such as Adobe Photoshop Elements Photomerge, you can shoot and later combine multiple photos into a single, long vertical or horizontal panoramic photo.

When you shoot photos that you will later combine using a digital stitching application, you need to overlap each photo by ¹/3 to ¹/2 so that you can match and blend the images seamlessly. You also need to be careful to maintain the same exposure throughout your photos. Avoid shooting moving subjects such as clouds or ocean waves that make photos too different to be combined. Finally, you should always use a tripod.

These four photographs of a country landscape were taken with a camera mounted on a tripod with a head that allows panning.

Did You Know?

You can use the Adobe Photoshop

Elements Photomerge feature to combine multiple photos into a single, large photo for making large prints. If your digital camera does not have enough pixels to make a quality print in the size that you want, you can shoot several photos and combine them with Photomerge.

Did You Know?

You can take multiple photos of vertical subjects and create vertical panoramas as easily as you can create horizontal panoramas. Good subjects for vertical panoramas include tall trees and buildings. Shooting from a distance with a telephoto lens can help minimize unwanted perspective distortion caused by using a lens with a shorter focal length.

artificial organs

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Artificial Organs
Cardiovascular
[valves] [assist] [total heart]

The human heart beats about 35 million in order to pump millions of gallons of blood through an individual's circulatory system in just one year. The machinery of the heart and the circulatory system takes a tremendous amount of abuse, abuse which sometimes leads to dangerous wear and tear. Over 700,000 deaths a year, in the US, are attributed to heart failure. Many devices have been created to help people with heart problems. These devices range from artificial valves, to assist devices, to total heart replacement units.

Valves

Artificial heart valves are very difficult to make properly because of the tremendous performance requirements associated with such a crucial replacement. The materials must be extremely durable, the valve must resist extensive wear, it must be completely impervious when sealed, it should be easy to implant surgically, and there should be little or no tendency of blood to clot on the valve.

The first type of artificial heart valve, a development which was the result of the work Dr. Charles Hufnagel and Dr. Dwight Harken, was composed of a cage and a ball. Originally, the valves were made with bulky cages and hollow balls made of acrylic. More modern versions have cages made of titanium and balls made of silicone. Ball valves worked well because they did not wear out easily, however their hemodynamic (blood flow) functionality did not even remotely match that of a natural heart valve, and the shape and size of the cage made it difficult to implant and sometimes caused problems with other parts of the heart.

In an attempt to solve the problem of the size of the valve, lower-profile valves were introduced in which a disc was substituted for the ball. Although this decreased profile did make the more fit for implantation, the performance of the valve was not increased any, and artificial valves still not nearly as good as real ones.

In 1969, the Bjork-Shiley and the Lillehei-Kaster tilting disc valves increased the use of prosthetic heart valves tremendously. Tens of thousands of these valves were implanted in the United States alone. Unfortunately, attempts to improve the hemodynamics of the once successful valve led to disaster. Certain models developed strut fractures which often resulted in heart failure. Although most models of the Bjork-Shiley valve are extremely durable and have very low (nearly zero) rates of structural failure, all Bjork-Shily heart valves were removed from the US valve market in 1992. Modification on this so-called "leaflet" valve continued for years, some of which are still used today.

The next step in artificial heart valve technology was the integration of actual, living tissue into the design. Either porcine valves (from pig valve tissue) or bovine pericardial tissue (cow heart tissue) is sewn onto a metal wire stent. These types of implant have been quite successful and have excellent hemodynamics.

Click here* for extensive information on artificial heart valves.

Assist Devices

Assist devices are devices which do not replace the heart completely, but aid a weak or damaged heart in pumping blood through the circulatory system (most often as a temporary solution until a heart transplant can be performed. There following are two examples of current cardiac assist devices:

The HeartMate is made by Thermo* Cardiosystems.

The HeartMate LVAS is an implantable cardiac-assist device that takes over the pumping function of the natural heart. There are two versions of the HeartMate system: the air-driven version, which is powered by an external console, and the electric version, which is powered by batteries that can be worn discreetly to allow the patient complete mobility. The air-driven version, which is sold commercially in the U.S. and Europe, is used to sustain patients awaiting heart transplants. The electric version is sold commercially in Europe, both as a "bridge to transplant" and as a long-term alternative to transplant. The electric version is also being evaluated in the U.S. as a bridge to transplant and as a long-term alternative to medical therapy under clinical studies. Each HeartMate device is designed to assist the main pumping chamber, or left ventricle, of the natural heart, which is responsible for pumping oxygen-rich blood from the lungs throughout the body. The device, which is implanted below the diaphragm, is attached between the natural heart and the aorta (the main artery for feeding blood to the entire body), leaving the natural circulation undisturbed while providing all of the energy necessary to propel blood throughout the body. The air-driven HeartMate pump weighs about one and a half pounds, and is approximately four inches in diameter and less than two inches thick. The electric version is slightly heavier and thicker than the air-driven pump, due to the size and weight of the motor. Unlike a total artificial heart, the LVAS allows a patient's natural heart to be left in place where it can still perform certain biological functions such as regulating blood flow.

(From Thermo* web-page)

The Thoratec* pump is another heart assist device. It uses the previously described leaflet valves. This unit is very versatile. Blood can be taken from the left atrium or the left ventricle. It is then pumped into the aorta. Furthermore, right heart support can be achieved by installing the pump receive blood from the right atrium and pump it into the pulmonary artery. This is the only system that offers total circulatory support - left, right or biventricular. As of December 1, 1997, this system has been used in more than 879 patients and is currently being used in heart centers worldwide

Total Artificial Heart
(Stats. from Guy, 1998)

Cardiac failure is a tremendously common occurence. The Institute of Medicine estimates that, by the year 2010, 35,000 to 70,000 patients will be candidates for permanent cardiac replacement or support. As real hearts for transplantation are tremendously difficult to come by, the invention of total artificial hearts is one which is in tremendous demand. As of now, however, the FDA has approved only two such devices for humans in the United States: The Penn State and the Jarvik-7 (now CardioWest)pneumatic total artificial hearts. However, these hearts are nowhere near ready for permanant use, as the best transplant survivor lasted only 620 days after transplantation with a Jarvik-7.

In 1988, the National Heart, Lung, and Blood Institute (NHLBI)provided five-year contracts to companies to work on permenant, tether-free artificial hearts. Having ended the contracts without success in 1993, the NHLBI awarded further contracts to three of the orginal groups. In 1996, two of these groups, Penn State/3M and Texas Heart Institute* were selected to continue their studies. If experiments are successful, human studies should be ready to be performed by the year 2000. There are, of course, many other groups around the world which are trying to create a similar system, but none of these appear to be as close to human trials (Guy, 1998).

heart valves

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Table of Content

Abstract

Introduction
The Heart
Heart Valves
Heart Valve problems
Treatment Options

Mechanical Heart Valves
Evolution
Materials
Advantages and Disadavantages
The Future

Prosthetic Tissue Valves
Human Tissue Valves
Animal Tissue Valves

Conclusion

Home

Abstract

The heart is a vital part of the human anatomy because it functions as a pump to circulate blood throughout the body. Heart valves allow the heart to pump blood to specific locations efficiently. These valves are prone to disease and malfunction, and can be replaced by prosthetic heart valves. The two main types of prosthetic heart valves are mechanical and bioprosthetic. The mechanical valves are excellent in terms of durability, but are hindered by their tendency to coagulate the blood. Bioprosthetic valves are less durable and must be replaced periodically. All valve types must be durable, because the body is an extremely hostile environment for a foreign object, including prosthetic heart valves. Today, chemical engineers are researching new designs of prostheticheart valves. Many engineers believe the future lies within the regime of tissue engineering.

Introduction

The Heart

The heart consists of four chambers: the right atrium, the right ventricle, the left atrium, and the left ventricle. It's function is to pump oxygen-rich blood to the arteries where the blood can flow to the cells of the body to provide them with oxygen. The deoxygenated blood from the cells is circulated back to the heart to regain the oxygen that was lost. The deoxygenated blood from the body enters the right atrium, and once this chamber fills with blood, the atrium contracts, forcing the blood down through the tricuspid valve into right ventricle. Next, the ventricle contracts, pushing the blood to the lungs through the pulmonary valve to receive oxygen. The oxygen-rich blood returns to the left atrium of the heart and then it travels to the left ventricle through the mitral valve. From the left ventricle, the blood travels through the aortic valve to the large blood vessel called the aorta. The aorta then distributes blood to the rest of the body.

Heart Valves

Heart valves are very important, as they prevent the backflow of blood, which ensures the proper direction of blood flow through the circulatory system. Without these valves, the heart would have to work much harder to push blood into adjacent chambers. The heart is composed of 4 valves:tricuspid, pulmonary, mitral, and aortic.

Heart Valve Problems

There are numerous complications and diseases of the heart valves that prevent the proper flow of blood. Heart valve diseases fall into two categories, Stenosis and Incompetence. The stenotic heart valve prevents the valve from opening fully, due to stiffened valve tissue. Hence, there is more work required to push blood through the valve. Whereas, the incompetent valves cause inefficient blood circulation by permitting backflow of blood in the heart.

Treatment Options

On a large scale, medication is the best alternative, although in some cases defective valves have to be replaced with a prosthetic valve in order for the patient to live a normal life. An enormous amount of research and development has proven to be most beneficial, as prosthetic heart valve technology has saved hundreds of thousands of lives. Engineers and scientists have done much work to design a valve that can withstand millions, if not billions, of cardiac cycles.

The two main prosthetic valve designs include mechanical and bioprosthetic(tissue) heart valves, some of which are shown below.

MECHANICAL HEART VALVES

Evolution of Mechanical Heart Valves

The first mechanical prosthetic heart valve was implanted in 1952. Over the years, 30 different mechanical designs have originated worldwide. These valves have progressed from simple caged ball valves, to modern bileaflet valves. Heart valves are designed to fit the peculiar requirements of blood flow through the specific chambers of the heart, with emphasis on producing more central flow and reducing blood clots.

The caged ball design is one of the early mechanical heart valves, that uses a small ball that is held in place by a welded metal cage. The ball in cage design was modeled after ball valves used in industry to limit the flow of fluids to a single direction. Natural heart valves allow blood to flow straight through the center of the valve. This property is known as central flow, which keeps the amount of work done by the heart to a minimum. With non-central flow, the heart must work harder to compensate for the momentum lost to the change of direction of the fluid. Caged-ball valves completely block central flow, therefore the blood requires more energy to flow around the central ball. In addition, the ball is notorious for causing damage to blood cells due to collisions. Damaged blood cells release blood clotting ingredients, hence the patients are required to take lifelong prescriptions of anticoagulants.

For a decade and a half, the caged ball valve remained the best design. In the mid-1960s, a new class of prosthetic valves were designed that used a tilting disc to better mimic the natural patterns of blood flow. The tilting-disc valves have a polymer disc held in place by two welded struts. The disc floats between the two struts in such a way, as to close when the blood begins to travel backward and then reopens when blood begins to travel forward again. The tilting-disc valves are vastly superior to the ball-cage design. The titling-disc valves open at an angle of 60° and close shut completely at a rate of 70 times/minute. This tilting pattern provides improved central flow while still preventing backflow. The tilting-disc valves reduce mechanical damage to blood cells. This improved flow pattern reduced blood clotting and infection. However, the only problem with this design is its tendency for the outlet struts to fracture as a result of fatigue from the repeated ramming of the struts by the disc.

In 1979, a new mechanical heart valve was introduced. These valves were known as bileaflet valves, and consisted of two semicircular leaflets that pivot on hinges. The carbon leaflets exhibit high strength and excellent biocompatibility. The leaflets swing open completely, parallel to the direction of the blood flow. They do not close completely, which allows some backflow. Since backflow is one of the properties of defective valves, the bileaflet valves are still not ideal valves. The bileaflet valve constitutes the majority of modern valve designs. These valves are distinguished mainly for providing the closest approximation to central flow achieved in a natural heart valve.

Materials

Current research has been able to produce materials that do not cause clotting in the blood stream. However, they have yet to design an entire valve that will not induce coagulation.

    Most commonly used materials include:
    - stainless steel alloys
    - molybdenum alloys
    - pyrolitic carbon for the valve housings and leaflets
    - silicone, teflon®
    - polyester (Dacron®) for sewing rings

A new generation of mechanical valves made of materials with improved blood contact properties, better wear characteristics and resistance to infection are under development.

Advantages

The main advantages of mechanical valves are their high durability. Mechanical heart valvesare placed in young patients because they typically last for the lifetime of the patient.

Disadvantages

The main problem with all mechanical valves is the increased risk of blood clotting. When blood clots of any kind occur in the heart, there is a high probability of a heart attack or stroke. As a result, to prevent blood clots, mechanical valve recipients must take anti-coagulant drugs (sodium warfarin) chronically, which effectively makes them borderline hemophiliacs. The anti-coagulant used causes birth defects in the first trimester of fetal development, rendering mechanical valves unsuitable for women of child-bearing age. Mechanical valves are suitable for people who do not want additional valve replacement surgery in the future.

The Future of Mechanical Heart Valves

The new age tools that are being used to improve mechanical valve design include accelerated wear testing, advanced blood contact property testing, computer assisted design and manufacturing, coatings to reduce the chance of infection and improve healing and advanced polymer chemistry to develop the next generation of medical materials.

PROSTHETIC TISSUE VALVES

Prosthetic tissue valves can be broken into two groups: human tissue valves, and animal tissue valves. Both types are often referred to as bioprosthetic valves, which hold many advantages over mechanical valves. The design of bioprosthetic valves are closer to the design of the natural valve. Bioprosthetic valves do not require long-term anticoagulats, have better hemodynamics, do not cause damage to blood cells, and do not suffer from many of the structural problems experienced by the mechanical heart valves.

Human Tissue Valves

Human tissue valves fall into two categories: Homografts, which are valves that are transplanted from another human being, and Autografts, which are valves that are transplanted from one position to another within the same person.

A homograft is a valve that is transplanted from a deceased person to a recipient. A recipient has minimal problems with valve rejection and they do not require immunosuppressive therapy. A homograft that has been donated must be cryopreserved in liquid nitrogen until it is needed. In cases where the valve implants fit the dimensions of the patient correctly, homografts tend to have good hemodynamics and good durability. However, it is not clear whether homografts have better hemodynamics or durability than animal tissue valves.

Autografts are valves taken from the same patient that they are implanted into. The most common autograft procedure is the Ross procedure, which is used in patients with diseased aortic valves. The dysfunctional aortic valve is removed and the patient's pulmonic valve is then transplanted to the aortic position. A homograft pulmonic valve is usually used to replace the patient's pulmonic valve. The Ross procedure allows the patient the advantage of receiving a living valve in the aortic position. The long term survival and freedom from complications for patients with aortic valve disease are better with the Ross Procedure than any other type of valve replacement. After 20 years, only 15% of patients require additional valve procedures. In cases where a human pulmonary artery homograft is used to replace the patients' pulmonary valve, freedom from failure has been 94% after 5 years time, and 83% at 20 years. The tissues of the patients' pulmonary valve have not shown a tendency to calcify, degenerate, perforate, or develop leakage.

The Ross procedure requires a high level of technical skill on the part of the surgeon. The pulmonic valve and the pulmonary homograft must be sculpted to fit the aortic root. Many patients have small amounts of aortic regurgitation, which in some cases is severe enough to merit a second operation for valve replacement. Other possible complications could include stenosis, right-sided endocarditis, as well as the usual complications of valve replacement.

Animal Tissue Valves

Animal tissue valves are often referred to as heterograft or xenograft valves. These valves are most often heart tissues recovered from animals at the time of commercial meat processing. The leaflet valve tissue of the animals is inspected, and the highest quality leaflet tissues are then preserved. They are then stiffened by a tanning solution, most often glutaraldehyde. The most commonly used animal tissues are: porcine, which is valve tissue from a pig, and bovine pericardial tissue, which is from a cow.

In Porcine valves, the valve tissue is sewn to a metal wire stent, often made from a cobalt-nickel alloy. The wire is bent to form three U-shaped prongs. A Dacron cloth sewing skirt is attached to the base of the wire stent, and then the stents themselves are also covered with cloth. Porcine valves have good durability and usually last for ten to fifteen years.

Bovine pericardial valves are similar to porcine valves in design. The major difference is the location of the small metal cylinder which joins the ends of the wire stents together. In the case of pericardial valves, the metal cylinder is located in the middle of one of the stent post loops. Pericardial valves have excellent hemodynamics and have durability equal to that of standard porcine valves after 10 years.

Both the porcine and bovine pericardial valves are stented valves. The metal stent in these valves takes up room which could be available for blood flow. Stentless valves are made by removing the entire aortic root and adjacent aorta as a block, usually from a pig. The coronary arteries are tied off, and the entire section is trimmed and then implanted into the patient. The St. Jude Toronto Stentless Porcine Valve (SPV) is one such valve. It appears to have excellent hemodynamics, and a significant decrease in the thickness of the heart has been observed after the valve is implanted. However, the valve is extremely difficult to implant, and it is still too new to have any valid data accounting for durability.

The most common cause of bioprosthesis failure is stiffening of the tissue due to the build up calcium. Calcification can cause a restriction of blood flow through the valve (stenosis) or cause tears in the valve leaflets. Since younger patients have a greater calcium metabolism, bioprostheses tend to last best in senior citizens. Once a bioprosthesis is implanted, the valve itself does not require any type of anti-coagulant drugs. Its degeneration is simply a gradual process, as it grows with the body.

The future for replacement heart valves lies in tissue engineering. The most ideal replacement would be formed from the patient's tissue, and tailored to the right shape and dimensions. Researchers have transplanted specifically tailored valves into sheep. The valves are made by growing tissue from the artery of a lamb on a matrix of the correct dimensions in an artificial culture medium.

Conclusion

When one of the valves of the heart becomes infected with a disease, it can be replaced with one of several different types of prosthetic valves. These prosthetic valves must create a non-return flow system and must meet certain standards with regard to inertia, strength, elasticity, and electrochemical properties. The replacement valve must also be durable, as the human body is a harsh place for foreign objects, which can corrode or break down.

Of the different prosthetic valve types, the ball and cage mechanical valve was first used. Other mechanical valve types include a tilting disc model and a bileaflet valve. Mechanical heart valves make satisfactory valves replacements, but they all encounter similar problems. Flow pattern disruptions can lead to thrombus formation, a fact that requires valve recipients to take anticoagulants on a long term basis. Also, mechanical stresses can damage blood cells and bacterial infections can lead to further damage.

Bioprosthetic tissue valves do not encounter as many problems as mechanical valves. Tissue valves can be made from human or animal tissue. Valves of human tissue are classified as homografts, which are valves transplanted from another human being or autografts, which are valves transplanted from one position to another within a patient. This is most often done using the Ross Procedure. Prosthetic valves made of animal tissue are often referred to as heterografts or xenografts and can be made of porcine (pig) tissue or bovine peacardial (cow) tissue.

Many people have benefited from prosthetic heart valves over the past 30 years. Chemical engineers believe that the future of prosthetic valves lies in the regime of tissue engineering. This would improve the biocompatibily factor, and increase the life expectancy of the heart valve.

heart valve sugery

2008-06-10T00:32:00.001+05:30

Heart valve surgery
Overview
Recovery
Risks
Definition:

Heart valve surgery is used to repair or replace diseased heart valves.
Alternative Names:
Valve replacement; Valve repair; Heart valve prosthesis
Description:

There are four valves in your heart:
Aortic valve
Mitral valve
Tricuspid valve
Pulmonary valve

The valves control the direction of blood flow through your heart. The opening and closing of the heart valves produce the sound of the heartbeat.

Heart valve surgery is open-heart surgery that is done while you are under general anesthesia. A cut is made through the breast bone (sternum). Your blood is routed away from your heart to a heart-lung bypass machine. This machine keeps the blood circulating while your heart is being operated on.

Valves may be repaired or replaced. Replacement heart valves are either natural (biologic) or artificial (mechanical):
Natural valves are from human donors (cadavers).
Modified natural valves come from animal donors. (Porcine valves are from pigs, bovine are from cows.) These are placed in synthetic rings.
Artificial valves are made of metal.

If you receive an artificial valve, you will need to take life-long medication to prevent blood clots . Natural valves rarely require life-long medication.
Indications:

Heart valve surgery may be recommended for the following conditions:
Narrowing of the heart valve (stenosis)
Leaking of the heart valve (regurgitation)

Valve problems may be caused by infections such as rheumatic fever , birth defects, calcification, or certain medications such as Fen-Phen. Defective valves may cause congestive heart failure and infections ( infective endocarditis ).

Artificial Heart Valves

2008-06-10T00:29:00.001+05:30

Artificial heart valves are either used as a replacement for human heart valves (prosthetic) or in cardiac assist systems (mechanical). The Biofluid Mechanics Lab is working mainly on heart valves for cardiac assist systems.

Clinical applications of cardiac assist systems continue to have a severe problem, namely thromboembolic complications. The problem originates mainly at the valves, which are usually made of an antithrombogenic material, such as bovine pericardium. However, the valve housing is made of a less suitable material, and wherever the blood flow is stagnant a thrombus is likely to form. Such stagnant blood flow is found in the space between the housing of the valve and the leaflets, called the sinuses. Consequently, thrombi often are generated in the sinuses.

The Biofluid Mechanics Lab is designing two new patented types of heart valves:
S-shape valve
Purge flow valve

In addition, the Biofluid Mechanics Lab is working on test methods for heart valves and a new method to measure the residence time of blood in heart valves called the fluorescent dye washout method.

S-shape valve
The S-shape valve (German patent no.: 196 04 881; EU patent no.: 971 02 039; US patent no.: 5 980 568) consists of a monoleaflet valve (grey) in a special designed duct (red). The duct is optimized according to optimum flow, which means that there is neither large flow acceleration nor stagnant areas.

S-shape valve

The flow in this valve was calculated by Computational Fluid Dynamics (CFD). You can find the results here. In addition, the flow was measured using a ten times enlarged model with Digital Particle Image Velocimetry (DPIV).These results you can find here.

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Purge flow valve
Principle:
The purge flow valve is intended for use in cardiac assist systems. The special design (German patent no.: 198 07 599) reduces the formation of the stagnation zone behind the leaflets by means of a purge flow during systole. This purge flow is separated from the valve's main flow through a flow divider, thus directing a part of the main flow into the sinuses behind the leaflets.

Principle of the purge flow valve

Parameter study and design:
The investigation and optimization of the purge flow effect was performed on a mono-leaflet valve due to the simple geometry. The devider's position and the geometry of the sinus and the devider were varied systematically. Details of the varied parameters are shown in the figure on the right. Theoretically, combining all possible parameter variations woul result in 200 models. Using a special factorial design technique known from quality management (Taguchi's method) the number of required models could be reduced to about 30. These models were designed using the 3D CAD Tool SolidWorks.

Varied parameters

Numerical investigation:
To narrow down the choice, stagnation areas in the sinuses were computed using methods from CFD. The models with the smallest integrated stagnation areas were preselected and manufactured on a scale of 1:1. The figure shows a comparison of two different parameters length of the leaflet and position of the flow divider based on the calculated wall shear stresses. Areas of stresses lower than 0.5 Pa are marked blacked thus indicating unwanted separation and stagnation areas.

Numerical calculation (CFD)

Experimental investigation:
The main hydrodynamic parameters were measured with a computer controlled valve tester and the washout of a dye previously filled into the sinus was observed, digitally recorded and quantified. In the figures on the right, a wash-out sequence and the course of the normalized gray value are shown. Subsequently the same valve geometries were investigated in an enlarged model scale 2:1 with DPIV in order to verify the CFD results.
Both the numerical and the experimental investigation show that the best results are achieved with a short leaflet, a small sinus, a big flow divider and with the flow divider in symmetrical position. However, only one of the models with flow divider showed the expected large improvement of the washout process compared to the model without the flow divider (see the figure showing the course of the gray value). The DPIV investigation confirmed the results of the CFD and showed complex flow patterns in the sinus region. After further investigation the purge flow principle will be applied to the tri-leaflet valve.

Wash-out sequence

Course of normalized gray value

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Testing method for artificial heart valves bulk qualities

Numerous devices and mock-circulations have been described for the testing of artificial heart valves in regard to their pressure loss, closure time, closing and leakage volumes as well as energy losses. However, all devices have been troubled by the difficulty to generate and assess the precise flow through the valve and by the problem to define the arterial load, i.e. the artificial aorta.
The new test device (see figure) follows a radically different approach: there is no artificial ventricle with two valves, one of them being the test valve, instead, only a piston which forces the fluid through the test valve. Thus the movement of the piston defines the flow with great precision (0.3 %) and there is no influence from a second valve. As a result, there is no additional device needed to measure the flow. The piston is computer controlled and follows a physiological flow curve which is identical for all types of heart valves of the same size.

Valve tester

After the forward flow phase the controller switches over from flow control to pressure control, the piston moves slightly backwards, imitating the diastolic pressure difference between ventricle and aorta. This physiological pressure difference curve is mathematically defined and generated by the computer as well. Consequently there is no influence through an imprecisely defined after-load caused by a mechanically simulated elasticity of the aorta or peripheral resistance. Additionally the valve duct discharges into an open vessel. Since th transparent rigid aortic root is screwed in, this greatly simplifies the insertion and exchange of the test valve. This makes the tester suitable for production control.

The pressure difference across the valve is measured conventionally with two pressure transducers. The amplification of one transducer is changed according to the signal strength in order to achieve a higher resolution of the pressure signal during the systolic phase. The measurement of the piston displacement is done with a digital angular transducer.
The results flow, pressure difference and energy loss are printed out as curves, optionally as data lists. The output diagram includes the integrated data: closure time, closing volume, leakage volume, mean systolic pressure difference, closure time, energy loss during systole and diastole. It appears on the computer screen some seconds after the test.

Output diagram

The small size allows for the setup on a normal desk and as a result the preparation for a standard valve test is a question of some minutes.

Besides standard valve tests according to the ISO proposals, the device has also been used for the development of new valve designs and the investigation of failed explanted valves.

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Fluorescent dye washout method

For a long time, the hydraulic performance of artificial heart valves was the focus of interest. However, this does not really reflect the performance of the valve in the patient. Much more important are the thrombogenic qualities of the valve. How can these be assessed? While passing the valve, platelets are activated in areas of high shear stress and are likely to coagulate in stagnant areas. Flow separations behind the valve are an example of such areas of stagnant flow and the platelets may recirculate there many times. Given the critical shear rate and enough time, the coagulation starts and a thrombus is formed. The amount of time a platelet remains in the stagnant zone known as residence time is used as a measure of the thrombogenicity of the valve. The platelet stagnation zones of different valves have been investigated and their residence time calculated. In order to study the detailed flow behind the valve a 10:1 times enlarged model of the aortic valve was used. Dye was used as a model for the platelets. The flow was created in a pulsatile flow channel, the fluid used was water. To maintain Reynolds similarity the time scale was set to 1:254. In order to simulate a pulse rate of 70, one heart cycle lasted for 217 seconds. As a result, a common video camera was sufficient to study the flow. So that a realistic flow could be obtained a transparent model of the aortic root was placed in the flow channel. The area of interest was illuminated by a slide projector holding a slide with a narrow light slit. No intensive light sources were necessary, since the velocity was low and the video camera was very light sensitive. A fluorescent dye was mixed into the aorta. The higher the concentration of the dye the more light was reflected. The time dependant distribution of the concentration of the dye during the cycle was a measure of the platelets residence time behind the valve. The recorded cycle was then digitized to a PC and calculations were done using image processing software such as "Image" and custom made programs. The resolution of the pictures was 768 x 512 Pixel and used 256 grey levels. The following valves were investigated: Björk-Shiley Standard, Björk-Shiley Monostrut, Starr-Edwards Ball valve, St. Jude Medical, trileaflet PU valve, Jellyfish valve and a custom made ball valve.

We obtained video frames that showed areas of stagnant flow behind all the valves. In some valves the fluid was washed out better. There was a good correlation between the observed areas of flow stagnation and the thrombus formations found during post mortems. This proves, that the 10:1 flow channel using a fluorescent dye is a good method to use in order to obtain the residence time of different valve types and from that a measure of the thrombogenicity. One major advantage of the method is, that the individual cycle can be viewed and investigated and average velocity does not have to be calculated. In the future, evaluations will be done to decrease the thrombogenicity of artificial valves.

Two examples acquired with the described method are shown in the figures below.

Artificial Heart Valves

2008-06-10T00:27:00.001+05:30

Artificial heart valves are used to replace damaged or diseased heart valves that can't be repaired.

The American Heart Association does not conduct technical review of mechanical heart valves. The U.S. Food and Drug Administration is the agency empowered to make evaluations.

Operations to restore the function of heart valves are commonly performed. They're done to improve the health and vigor of people with heart valve diseases. The surgeon who'll perform the operation is the best person to talk to about specific questions or concerns. He or she can best explain the details of the surgical procedure and recovery period.

People with artificial heart valves are at higher risk for developing an infection of the valve (endocarditis). They are also in the highest risk category for having bad outcomes from endocarditis. The American Heart Association says that people in this highest risk category (others in this category include people who have previously had endocardits, people with certain types of congenital heart disease, and people with a heart transplant who develop heart valve problems) need to take antibiotics before most dental procedures. However, the association says that people undergoing gastrointestinal or genitourinary procedures do not need to take routine antibiotics solely to prevent endocarditis.

In Love

2008-02-21T08:18:00.001+05:30

The day you appeared I lost my heart
To you, to love.
And from that day I cannot part
From you, from love.

You hold me tight
To you, to love.
In my thoughts all day and night
Of you, of love.

I offer all that I have to give
To you, to love.
And all my days I want to live
With you, in love.

Inside Of Me

2008-02-21T08:16:00.000+05:30

If you could see inside my soul
see inside my heart
you would know how I long for you
whenever we're apart

If you could see inside my head
if thoughts were things to see
you would know how I cherish you
how much you mean to me

In all the ways you comfort me
the way you hold me near
the way you know just what to do
to chase away my fear

The sparkle in your beautiful eyes
your smile, laugh, your touch
are just a few of many reasons
I love you oh so much

Knowing I can talk to you
about any and everything
and knowing together we will get
through whatever life may bring

I could search the whole world over
and this I know is true
I would never find another love
like the love I found with you

Though with each new day, each sunrise
we can't know what's in store
there is one thing I know for sure
each day I love you more

So if you could see inside my head
if thoughts were things to see
you would know I blessed I feel
to have you here with me

Key To My Heart

2008-02-21T08:14:00.000+05:30

I had closed the door upon my heart
And wouldn't let anyone in,
I had trusted and loved only to be hurt
But, that would never happen again.

I had locked the door and tossed the key
As hard, and as far as I could,
Love would never enter there again,
My heart was closed for good.

Then you came into my life
And made me change my mind,
Just when I thought that tiny key
was impossible to find.

That's when you held out your hand
And proved to me I was wrong,
Inside your palm was the key to my heart...
You had it all along.

It's Been Years

2008-02-21T08:13:00.001+05:30

It’s been years since we first met
Since we looked into each other's eyes
Not realizing that we were meant to be together
It's been years since we shared our first laugh
Our first smile
Since we shared our first kiss
I remember how strong your arms embraced me
How soft you held me
How warm and gentle you were
I remember how passionate that first kiss was
Now my heart is dancing
Happy, knowing that it's been years
It's been years since we first met
And you're still here kissing me just the same
You're still here holding me just the same
What I felt for you is still the same
It's been years
And that feeling will never change

Whispers in the Wind

2008-02-21T08:10:00.002+05:30

Your whispers are gentle echoes
that sway ardent winds of harmony
and in the symphony of life
each word is wrapped in rhapsody.

We travel separate roads of life
gather flowers along the way and
share the music of their fragrance.

We exist between two winds,
I here under a pale moon
you, the other end of somewhere.

Sometimes when wind blows through trees
I pause to listen and in its passing
I hear the tenderness of your voice
that fills the spectrum of my soul.

You are the chime of warm rain,
the moon that glows through the trees
and within the luster of evening
your aura fills the scene.

I hear the whispers of the wind
see the stars shine in the sky,
but I hold the sunrise in my pocket.

Dance with me within the wind,
then just let me love you.

Believe

2008-02-21T08:10:00.001+05:30

What Is There,
To Believe In During Life,
Nothing Lasts,
Aside From Strife,
Life May Perish,
Life May End,
Our Soul We May Give,
Our Soul We May Spend,
But Our Love,
That Eternal Flame,
Will Never Dim,
Will Never Change,
It Will Forever Last,
When All Other Lights Leave,
For You Light My Life,
You Make Me Believe.

A Perfect Moment

2008-02-21T08:08:00.000+05:30

Beautiful eyes open across a pillow top,
It seems for this moment time will stop,
The earth slows and sounds wane,
A frozen moment without stain.

Your body close to mine,
Our hearts beat in time,
With a smile of simple pleasure,
This moment should last forever.

Some say perfection is unattainable
For most things that's explainable,
But in your arms and with your heart's affection
I have found a brief moment of perfection.

Freeze This Moment

2008-02-21T08:06:00.000+05:30

We were sitting on your rooftop
Staring at the skies
The sun was dipping lower
I looked into your eyes
You saw what I was feeling
I know you felt it too
We wanted time to just stand still
Then forever there'd be me and you
Why can't we freeze this moment?
Return to it in time
Stay together through the years
Proclaim I'm yours and you are mine
So let us freeze this moment
Store it safely away
Even if we leave this place
We'll return to it someday