Ventricular Systolic Dysfunction
The effects of a loss of intrinsic inotropy on stroke volume, and end-diastolic and end-systolic volumes, are best depicted using ventricular pressure-volume loops (Figure 2). Loss of intrinsic inotropy decreases the slope of the end-systolic pressure-volume relationship (ESPVR). This leads to an increase in end-systolic volume. There is also an increase in end-diastolic volume (compensatory increase in preload), but this increase is not as great as the increase in end-systolic volume.
The force-velocity relationship provides insight as to why a loss of contractility causes a reduction in stroke volume (Figure 3). Briefly, at any given preload and afterload, a loss of inotropy results in a decrease in the shortening velocity of cardiac fibers. Because there is only a finite period of time available for ejection, a reduced velocity of ejection results in less blood ejected per stroke. The residual volume of blood within the ventricle is increased (increased end-systolic volume) because less blood is ejected.
The reason for preload rising as inotropy declines acutely is that the increased end-systolic volume is added to the normal venous return filling the ventricle. For example, if end-systolic volume is normally 50 ml of blood and it is increased to 80 ml in failure, this extra residual volume is added to the incoming venous return leading to an increase in end-diastolic volume and pressure.
Treatment for systolic dysfunction involves the use of inotropic drugs, afterload reducing drugs, venous dilators, and diuretics. Inotropic drugs include digitalis and drugs that stimulate the heart via beta-adrenoceptor activation or inhibition of cAMP-dependent phosphodiesterase. Afterload reducing drugs (e.g., arterial vasodilators) augment ventricular ejection by increasing the velocity of fiber shortening (see force-velocity relationship). Venous dilators and diuretics are used to reduce ventricular preload and venous pressures (pulmonary and systemic) rather than augmenting systolic function directly.