Clinical Note

A fall in contractility as is often seen in the failing heart shifts the cardiac function curve down and to the right. The equilibrium point would then be shifted to a reduced cardiac output and an increased venous pressure. The increased venous pressure is the cardinal sign of heart failure and can cause complications of its own. Venous pressure can rise so high that fluid is extruded from the capillaries into the tissue. The resulting peripheral or pulmonary edema can be more of a problem than the reduced cardiac output.

Note that the equilibrium point has also been shifted from A to B. Cardiac output will be increased as expected, but the increased cardiac output caused venous pressure to fall, thus blunting the increase in output.

Effect of Changing Blood Volume

It was noted previously that mean circulatory filling pressure is solely determined by the blood volume and the capacitance of the blood vessels. Mean circulatory filling pressure (cf. the venous function curve's x intercept in Fig. 6) would be increased by increasing

10 r

10 r

Venous Pressure (mm Hg)

FIGURE 6 Effect of altering blood volume on the vascular function curve.

the blood volume, resulting in hypervolemia. Similarly, mean circulatory filling pressure would be decreased by reducing the blood volume, resulting in hypovolemia. Assuming that TPR remains constant, the difference in arterial and venous pressure required to force any given cardiac output across the TPR will be unchanged regardless of the blood volume and, therefore, the slope of the curves remains the same. Changes in blood volume shift the vascular function curve in a parallel fashion, as indicated in Fig. 6.

An increase in blood volume resulting from a blood transfusion (or perhaps renal fluid retention) would shift the vascular function curve up and to the right, as shown in Fig. 7. Note the change in the equilibrium point from A to B in Fig. 7. Hypervolemia causes cardiac output to increase with a concomitant increase in venous pressure. Contrast the effect of hypervolemia with increasing contractility in Fig. 5, in which cardiac output increases but venous pressure falls.

Effect of Changing Venous Tone

Venoconstriction decreases the unstressed volume of the venous compartment and shifts the function curve in the same manner as transfusion. Whether one makes the container smaller (venoconstriction) or the contents larger (transfusion), the effect will be the same: a greater venous pressure for any given cardiac output. Venodilation increases the unstressed volume and shifts the vascular function curve in the same manner as

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