Xenotransplantation - Nuffield Council on Bioethics

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Animal-to-Human Transplants : the ethics of xenotransplantation Heart 2.14 Of all implantable mechanical organs, most progress has been made with artificial heart devices. 21 This is because the operation of the heart is fairly simple (it is basically a pump) and, unlike many other organs, it performs only limited biochemical or metabolic functions. Two types of artificial hearts are being developed: ventricular assist devices and total artificial hearts. 2.15 Ventricular assist devices (VAD) are at a more advanced stage of development than totally artificial hearts. Ventricular assist devices support but do not replace the heart, which remains in place and continues to perform some functions. Left ventricular assist devices (LVADs) are the most common type and help the left ventricle of the heart pump blood around the body. Since 80 per cent of the work of the heart is done by the left ventricle, LVADs are the most important type of artificial heart. Right ventricular assist devices (RVADs) help pump blood to the lungs for oxygenation, and can be used in conjunction with LVADs. 2.16 Most LVADs are implanted in the abdomen. A small lead through the skin attaches the LVAD to batteries, which are carried in a pack on a belt. The LVAD can keep time with the natural heartbeat and a computer adjusts the pumping action to cope with changes in activity. Patients have lived with the aid of an LVAD for up to two years. Currently, LVADs are used on a short-term basis, to keep a patient alive until a human organ is available. Supported by an LVAD, the heart can rest and, in some cases may recover sufficiently to allow the device to be removed after which the patient’s condition can be managed with drugs. Thus, LVADs could in principle be used as an early form of treatment for heart disease before the condition becomes lifethreatening. Also being developed are intra-ventricular artificial hearts which are so small they can be implanted into the heart itself. These devices have the potential to be used on a long-term basis as an alternative to human organ transplantation. 2.17 Total artificial hearts are designed to carry out all of the main functions of the heart. Several designs have been tried, but all follow the same basic principles, in which chambers pump blood to the lungs for oxygenation and then around the rest of the body. In 1982, a patient was kept alive for 112 days with a total artificial heart. Development continues and the devices have potential for short-term use in patients suffering from biventricular heart failure who are waiting for a human organ. 2.18 Artificial hearts may eventually provide an alternative for patients waiting for a human organ and they do not require the use of immunosuppressive drugs. But there are various problems to be overcome with the use of artificial hearts: 21 Reviewed in Westaby S (1996) The need for artificial hearts. British Heart Journal, in press. 18
Alternatives to xenotransplantation since blood tends to react with nearly every artificial substance, clots can form inside the devices and may cause strokes if they dislodge and travel through the circulation to the brain. Researchers are attempting to produce materials that reduce the tendency of blood to clot. One approach is to use the patient’s own endothelial cells to line the surfaces of the device that are in contact with blood. In the meantime, patients are treated with anticoagulant drugs to reduce the tendency of the blood to clot; artificial hearts tend to damage blood cells, causing anaemia and sometimes liver and kidney problems. Such damage is reduced by designing devices which allow a less turbulent blood flow; the power supplies are bulky and inconvenient, and need frequent recharging; often the person must be connected to a recharging unit at night. Because of their size, and the heat they generate, the power units cannot be implanted. The lead connecting the heart to a power supply outside the body can cause problems with infections; the devices are very expensive, costing about £30,000 each. Lungs Thus, for the foreseeable future at least, mechanical hearts are unlikely to satisfy the demand for heart transplants. 2.19 The first artificial lung capable of oxygenating blood and eliminating carbon dioxide was developed in the 1950s. 22 These devices were located outside the body and were used for periods of up to five hours to support patients during heart surgery when it was necessary to bypass the function of the heart and lungs. In 1992, a patient survived for 45 days with the aid of extracorporeal membrane oxygenation in which blood is removed from the body, oxygenated and returned to the body. This technique is now in clinical use for treating lung failure, especially in infants, but it remains a short-term measure. 2.20 Trials of implantable oxygenating devices are now in progress. The intravascular oxygenator is a miniature membrane lung which can be placed inside the body. The heart pumps blood through the device (unlike external devices which require external pumps) and gas is exchanged through microporous polypropylene fibres. The efficiency of the gas exchange depends greatly upon the surface area of the membrane, and therefore on the size of the devices. Thus, small implantable devices are not as efficient as external artificial lungs. 22 Goldsmith M (1993) After two decades Penn State researchers may be near perfecting replacement hearts and lungs. Journal of the American Medical Association, 269:966-973. 19
Page 1 and 2: NUFFIELD COUNCIL ON BIOETHICS Anima
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Page 6 and 7: Preface Preface Since the N
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Xenotransplantation - Nuffield Council on Bioethics

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