Mechanical circulatory assistance: established (IABP) and evolving (LVAD). A narrative summary.

Comparisons of the physiologic bases of intraaortic balloon and extracorporeal or implantable left ventricular assist device or partial artificial heart pumping in experimental and clinical setting are made. The concepts and first principles common and unique to both of these forms of mechanical circulatory support are presented with emphasis on the similarities and important differences. Intraaortic balloon counterpulsation is examined as an intravascular, volume displacement device in series with the systemic circulations. Left ventricular assist devices are analyzed as extravascular in-series or parallel volume-capturing/ejecting devices and as true blood pumps which can be implanted. The interrelated mechanisms of synchronous, diversion/counterpulsation/diastolic augmentation are discussed in relation to quantitative indices of myocardial ischemia, myocardial oxygen supply/demand ratios, the Sarnoff theorem and the Laplace relationship, vis à vis ventricular unloading/impedance reductions. Some of the many clinical settings of IABP are mentioned, along with hydraulic considerations which allow non-invasive determination of stroke volume during clinical IABP mechanical circulatory support. Cardiogenic shock/left ventricular failure/low cardiac output are defined in terms of failure to generate pressure and displace volume and deficits of ejection fraction and stroke volume. Vasodilator therapy (nitroprusside), IABP, and LVAD are then viewed as escalating methods of reversing these deficits. Finally, pressure volume loops of the human left ventricle are compared with those of LVADs, experimentally and clinically, to indicate that LVADs can function and support the circulation during all low output states including ventricular fibrillation and standstill. Representative hemodynamic traces (Fig. 5 and 6) obtained during clinical LVAD trials in man are included to illustrate these theoretical and practical considerations. The use of LVADs in any instance of IABP inadequacy is implicit and inferred.