At present, four TAH developers are just beyond the mid-point of NHLBI contracts for continuing R&D. If NHLBI funding continues, the next phase of extensive device readiness testing might require up to five years; this would be followed by another five or more years of clinical trials. TAHs are not likely to be a candidate for wide use until at least 2005.

This report of an Institute of Medicine (IOM) study is primarily intended to help NHLBI determine the extent of its support of the next step of TAH development after the current contracts end in September 1993. The report also considers the need to continue developing additional VAD models and examines issues related to both types of devices, such as appropriateness of technology use and access by patients to this technology. Finally, the report suggests several methods that can assist NHLBI in allocating resources among R&D programs and examines related R&D policy issues. (Temporary-use devices are not considered, except as their experience provides a basis for projecting the performance of long-term devices.)

END-STAGE HEART DISEASE

Consideration of the future role of TAHs and VADs—referred to collectively as mechanical circulatory support systems (MCSSs)—is helped by a basic understanding of the heart's functioning. As Figure 1.1 illustrates, the heart is a double pump. Oxygenated blood from the lungs flows into the left atrium and from there into the left ventricle. The left ventricle pumps the blood via the aorta into arteries throughout the body. After oxygen has been removed in organs and capillaries, the blood flows through veins back to the heart's right atrium and then to the right ventricle. This chamber sends the venous blood to the lungs to be oxygenated, completing a pumping cycle that normally occurs 70 to 90 times per minute.

Because considerably more pumping force is needed to move arterial blood throughout the body than to move venous blood through the lungs, the left ventricle's muscle strength is greater than the right ventricle 's and it is also more likely to fail than is the right ventricle. Heart failure is identified as left-sided, right-sided, or both (biventricular); end-stage heart disease occurs when one ventricle (or both) is unable to perform the necessary pumping function. As the name indicates, the typical case of end-stage heart disease becomes steadily more severe, until death occurs.

End-stage heart disease may result from a variety of cardiovascular causes; “heart failure”—sometimes called congestive heart failure—is, technically speaking, not itself a disease but a condition that is caused by many different disease processes. The most common causes of end-stage heart disease and heart failure are hypertension and coronary atherosclerosis (also called coronary artery disease or coronary heart disease, a constriction of the arteries that convey blood to the heart muscle itself). Viral



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