Ventricular volume (mL)
FIGURE 10 An ejection loop drawn to reflect the auxotonic conditions of an actual ejection. The inset shows the aortic pressure tracing for that cardiac cycle.
cardiac failure, contractility may be reduced to the point at which an adequate cardiac output cannot be achieved. Because of the heart's inaccessible position in the chest, it has been surprisingly difficult to measure the heart's contractility in the clinical setting. Over the years, several indices of cardiac contractility have been introduced; the student should have some understanding of the major ones, including the ventricular function curve, the velocity index dP/dt, the ejection fraction, and the end-systolic pressure-volume plot. Each of these is discussed in the following subsections.
The most obvious approach to assessing contractility is to find a way to relate the heart's pump performance to its contractility. The problem with that approach is that two other factors, venous pressure and aortic pressure, also affect stroke volume. That problem was cleverly solved by Stanley Sarnoff in the 1960s. To isolate the effect of venous pressure, he proposed simply to plot stroke volume as a function of venous pressure. If one examines the ejection loop shown in Fig. 5, it can be seen easily that if contractility and aortic pressure are held constant, then a simple relationship exists between stroke volume and venous pressure. That relationship is depicted in Fig. 11A. Similarly, it can easily be appreciated that increasing the contractility of the heart will shift that line upward as indicated in the figure and decreasing contractility will shift it downward.
The problem with the preceding approach is that the stroke volume-venous pressure curve also depends on the aortic pressure. As shown in Fig. 11B, increasing
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