Left Ventricular–Arterial Coupling

Abstract

The performance of the cardiovascular system depends on the interaction of its components. The left ventricle pumps its stroke volume into the arterial system, which then delivers the flow to the tissues. Thus, optimal cardiovascular function requires appropriate coupling of the left ventricle and the arterial system. Functional analysis of this interaction requires that the left ventricle and arterial system be described in similar terms. The left ventricle can be evaluated by plotting left ventricular (LV) pressure versus volume through the cardiac cycle (Figure 1A). The upper left-hand corner of the LV pressure-volume loop is end-systole. This is the point at which the left ventricle reaches maximal stiffness. Connecting the end-systolic points of variably loaded beats determines the LV end-systolic pressure-volume relation (Figure 1B). The slope of this relation is the LV end-systolic elastance (EES). An increase in contractility shifts the relation to the left and increases EES. Sunagawa et al proposed that the arterial system can be evaluated in an analogous manner. In this analysis, the arterial system is described by the relation between the stroke volume and end-systolic arterial pressure (Figure 1C). The higher the stroke volume, the greater the LV and arterial end-systolic pressure. The slope of this relation represents the effective arterial end-systolic elastance (EA). If it is assumed that zero stroke volume will be associated with zero pressure, then EA can be calculated as the ratio of end-systolic pressure to stroke volume. Furthermore, it is approximately equal to the peripheral vascular resistance times the heart rate. EES indicates how much the LV end-systolic volume will increase (and stroke volume will decrease) in response to an elevation of end-systolic pressure. However, LVend-systolic pressure is also determined by the arterial pressure-stroke volume relation; the higher the stoke volume, the higher the end-systolic pressure. Thus, the stroke volume and the end-systolic pressure results from the balance between EES (describing the LV) and EA (describing the arterial system) (Figure 1D). The left ventricle and arterial system are optimally coupled to produce stroke work (SW) when EA = EES or EA/EES = 1.0. The SW is the product of systolic arterial pressure and the stroke volume and measures how much useful work is produced by the left ventricle. When EES exceeds EA (EA/EES 1.0), SW falls and the left ventricle becomes less efficient. Normally, the left ventricle and arterial system are optimally coupled both at rest and during exercise. In patients with systolic heart failure, EES is reduced and peripheral vascular resistance (and thus EA) is increased. In this situation, the left ventricle and arterial system are suboptimally coupled (EA/EES > 1.0). Any increase in heart rate will further increase EA, making the coupling even worse. Vasodilator therapy, which lowers EA, will