## An Ejection Loop Can Be Plotted On The Pressure Versus Volume Graph

When Frank tried to calculate how much blood the frog heart would pump, he found that the relationship was quite complex because the frog heart begins to relax before the end of ejection. It was assumed that the human heart behaved in a similar fashion until the studies of Kichi Sagawa in the 1970s. Sagawa found that the mammalian heart, unlike the frog heart, does completely eject before it begins to relax and, as a result, a relatively simple analysis, the ejection loop, can be employed to calculate the heart's output. Figure 5 depicts a pressure-volume curve for the human heart in both systole (upper curve) and diastole (lower curve). The ventricle fills with blood passively during diastole. The filling pressure for the left ventricle is the pulmonary venous pressure and for the right ventricle the systemic venous pressure. These pressures typically will be from 3-7 mm Hg. Note in Fig. 5 that a filling pressure of 5 mm Hg causes the ventricle to fill to an end-diastolic volume of 150 mL (point A). The filling pressure is often termed the preload because this is the load on the muscle fibers before contraction.

When the heart is activated, it moves from the diastolic compliance curve to the systolic curve. Because the pressure rises isovolumetrically in the ventricle, the trajectory is a vertical line drawn between point A and B. At a volume of 150 mL, the ventricle shown would be

FIGURE 5 A pressure-volume plot for a human left ventricle during both systole and diastole. Note that the ventricle is very compliant during diastole (shallow slope) and becomes very stiff during systole (steep slope). The four points A, B, C, and D describe an ejection loop. See text for details.

capable of developing 200 mm Hg of pressure (point E), but as soon as the ventricular pressure exceeds the pressure in the aorta, the aortic valve opens and the ventricle will begin to eject blood into the aorta. At point B in Fig. 5, the mode of muscle contraction changes abruptly from isometric (A to B) to isotonic (B to C). The afterload for the contraction is the aortic pressure, which for simplicity is depicted in these examples as being constant through ejection. Actually, aortic pressure rises and falls during ejection (see Fig. 3) so that the contraction is not strictly isotonic from B to C but is actually auxotonic, as discussed later in this chapter.

Because of the Frank-Starling relationship, as the ventricular volume gets smaller during ejection the potential for the ventricle to develop pressure also falls. At point C, a stable equilibrium has been reached. Note that any further ejection into the aorta (moving to the left) would put the fibers at a length that could not support the aortic pressure. The heart then stays at point C, the end-diastolic volume, until the action potential subsides and the heart begins to relax.

At the end of ejection, the aortic valve closes and the heart relaxes isovolumetrically so the plot moves vertically from C to D, where it rejoins the diastolic compliance curve. As pressure in the ventricle falls below atrial pressure, the mitral valve opens, and the ventricle refills with venous blood and returns to the starting point A.

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