More than 2 million Americans have congestive heart failure and of these, approximately 60,000 meet criteria for some type of cardiac support or transplant. Despite expansion of criteria in the donor pool, there remains a limited number of donors. This number has essentially plateaued over the last decade between 2,000 and 3,000 annually. Because heart transplantation is not a viable option for many patients, there has been significant effort and research in the development of alternative medical and mechanical therapies. Mechanical ventricular assist devices (VADs) are currently used as a temporizing measure in the setting of acute heart failure, a bridge to transplantation, or as destination therapy. These devices are reported to improve the quality of life in these patients who are otherwise crippled by their heart failure despite maximal medical therapy.

VADs can be categorized in many ways: short term or long term; pulsatile or nonpulsatile; internal or external (i.e., intra- or extra-corporeal). Short-term devices include cardiopulmonary bypass, extracorporeal membrane oxygenation (ECMO), and the Abiomed BVS. Long-term devices include the Novacor, the HeartMate I and II, Thoratec, and the newer Abiomed AB5000 Ventricle. The nonpulsatile devices are cardiopulmonary bypass and ECMO in addition to the HeartMate II. The permanent devices have an internal chamber. No device has a totally implantable energy source.

All VADs except the HeartMate II, which generates nonpulsatile flow using a rotarylike motor, generate pulsatile flow because of their design with unidirectional flow valves. They operate in a fill-to-empty cycle, similar to the action of the native ventricle (i.e., diastole and systole). The determinants of the stroke volume are the pre-ejection chamber volume (preload) and the outflow resistance, either pulmonary or systemic, depending which side is mechanically supported. Emptying is initiated when the control unit senses that the chamber is nearly full. The rate of the ejection is volume, or preload, dependent. It is important to note that the rate of output is independent of the native cardiac rhythm. The goal is to maintain unloading

of the ventricle, especially in those patients who have had this placed with planned removal after recovery of the myocardium. This typically is limited to the postcardiac surgery patient in which a temporary device is placed.