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5 10

mmHg.mL

FIGURE 11 Evolution of the ventricular function curve. When venous filling pressure is plotted against stroke volume it is sensitive to both changes in (A) contractility and (B) aortic pressure. (C) If stroke work is chosen for the vertical axis, however, only changes in contractility can shift the curve.

aortic pressure shifts the curve downward and decreasing aortic pressure shifts it upward. Thus, it is not possible to tell if a shift in the stroke volume-venous pressure curve resulted from a change in contractility or aortic pressure. This ambiguity can be eliminated by plotting stroke work rather than stroke volume on the vertical axis. In a fluid system, physical work is given by the volume pumped times the pressure it is pumped against, in this case the product of stroke volume and aortic pressure. Stroke work for the ventricle is fairly independent of the aortic pressure because aortic pressure and stroke volume are reciprocally related. As a result, their product is surprisingly constant over a wide range of pressures. Thus, the stroke work-venous pressure plot shown in Fig. 11C, usually called the ventricular function curve, is affected only by changes in contractility. In practice, the venous pressure is changed over several beats by rapid infusion of intravenous fluids or suddenly lifting the patient's legs. Stroke volume and aortic pressure are monitored and from that data the plot is made. Any factor that shifts that plot is said to have changed contractility. The shortcomings are that very invasive procedures are required to obtain such data, which limits the clinical application.

Velocity-Related Indices of Contractility

As contractility increases, two factors change in cardiac muscle: the isometric force and the velocity of isotonic shortening. The velocity of shortening can be assessed by measuring the rate at which ventricular pressure rises (dP/dt) during the isovolumetric period of contraction. In practice, the ventricular pressure recording from a catheter is differentiated electronically so that a continuous recording of ventricular dP/dt can be monitored. Actually, ventricular filling pressure also affects the dP/dt signal. As ventricular filling rises so does the dP/dt. As will be seen in a subsequent chapter, however, a rise in contractility usually evokes a fall in venous pressure. Thus, an increase in dP/dt can be interpreted unambiguously as an increase in contractility if the end-diastolic pressure is either unchanged or falls. The major shortcoming of dP/dt is that it is imprecise and requires insertion of a ventricular catheter. It is often used in animal experiments in which a simple index of contractility is to be continuously monitored.

Ejection Fraction

The ejection fraction, as mentioned earlier, is calculated by dividing the stroke volume by the end-diastolic volume. It is literally the fraction of the ventricular volume that is ejected with each beat. Ejection fraction

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