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

We assessed the linearity and slope of the left ventricular end-systolic pressure (PES)-volume (VES) 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 micrograms/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 PES-VES 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 PES-VES 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 PES-VES 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 PES-VES relation was 2.9 +/- 3.3 ml less (p less than 0.05) than the volume-axis intercept of the nonlinear quadratic fit. Thus, the linear PES-VES 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 PES-VES 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.