Response of the left ventricular end-systolic pressure-volume relation in conscious dogs to a wide range of contractile states

Abstract

We assessed the linearity and slope of the left ventricular end-systolic pressure (P(ES))-volume (V(ES)) relation over a wide range of contractile states in conscious dogs. The animals were instrumented to determine left ventricular volume from ultrasonic left ventricular internal dimensions and measure left ventricular pressure with a micromanometer. Studies were performed 1-2 weeks after instrumentation while the animals were conscious. Contractile state was increased by incremental infusion of dobutamine (0, 2, 4, 6, and 8 μg/kg/min i.v.) and decreased by verapamil (10 mg i.v.) given after autonomic blockade. The 44 ± 11 mm Hg (mean ± SD) portion of the P(ES)-V(ES) relation generated by bicaval occlusion demonstrated a slight but consistent nonlinearity, apparent as a concavity toward the volume axis. This nonlinearity, present at all inotropic states, did not prevent the P(ES)-V(ES) relation from being well approximated by a straight line (r = 0.984 ± 0.020, SEE = 2.1 ± 1.4 mm Hg); furthermore, the slope of the P(ES)-V(ES) line provided a sensitive index of contractile state, progressively increasing with incremental doses of dobutamine and decreasing in response to verapamil. The volume-axis intercept of the linear approximation of the P(ES)-V(ES) relation was 2.9 ± 3.3 ml less (p < 0.05) than the volume-axis intercept of the nonlinear quadratic fit. Thus, the linear P(ES)-V(ES) relation, whose slope is sensitive to a wide variety of inotropic states, is a reasonable and useful description of the left ventricle in the range of P(ES)-V(ES) points that can be produced by bicaval occlusion in the conscious dog. However, linear extrapolation of the relation beyond the range of data points may not be accurate.