The Ventricle As A Pump

Normal pumping of the ventricles requires that they deliver appropriate amounts of blood to the tissues at acceptably low filling pressures (FPs). Thus the most physiologically relevant means of characterizing the pump is to construct a function curve relating FP to a measure of mechanical output (SV, minute volume, work, power). Ventricular function curves display a prominent Frank-Starling effect:, manifest as a curvilinear relationship between FP and output (once again, there is no descending limb in the normal ventricle) (Fig. 3-16). As discussed earlier, at the myocyte level, the Frank-Starling effect is mainly caused by increased myofilament Ca sensitivity at longer sarcomere lengths. Thus a function curve relating ED volume (ventricular preload) to mechanical output is a more accurate representation of the ventricular Frank-Starling effect. However, in the clinical setting, FP (pulmonary capillary wedge or RA pressure) is usually more readily available than volume. Whether FP or volume is employed, changes in intrinsic contractile performance result in upward or downward shifts of the ventricular function curve. However, characterization of ventricular performance in terms of function curves relating FP to output is a "black box" approach; alterations in diastolic compliance (see below) produce effects that are indistinguishable from alterations in contractile performance.

Coronary Wedge Pressure

Figure 3-16: LV function curves relating SV to ED pressure (see text). A. Normal function. B,C. Augmented and depressed contractility, respectively, as occur with increases or decreases in adrenergic stimulation. Because ED pressure is plotted, identical shifts could be observed with altered diastolic compliance.

Figure 3-16: LV function curves relating SV to ED pressure (see text). A. Normal function. B,C. Augmented and depressed contractility, respectively, as occur with increases or decreases in adrenergic stimulation. Because ED pressure is plotted, identical shifts could be observed with altered diastolic compliance.

The normal heart can pump adequate amounts of blood to meet the needs of the body under the most stressful conditions. Indeed, maximal CO normally is not limited by pumping capacity but by the ability of the systemic circulation, via venoconstriction and the systemic venous system of valves and muscular pumps, to return blood to the heart.111 Under pathologic conditions, pumping capacity may limit CO.

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