Preloading history influences pressure-volume-derived indices of myocardial contractility in the ejecting canine left ventricle.

The dependency of indices of myocardial contractility on the immediate preloading protocol was assessed in 14 pentobarbital anesthetized open-chest dogs. An intracavitary micromanometer and epicardially placed ultrasonic dimension transducers allowed acquisition of dynamic left ventricular pressure-volume relationships while varying preload on right heart bypass. A resting cardiac output of 75 ml/kg/min was either increased twofold or reduced to zero flow over a 20-second interval. Linear regression analysis of the mechanical parameters permitted construction of the end-systolic pressure-volume relationship, rate of pressure rise-end-diastolic volume relationship, and the preload recruitable stroke work relationship. The slopes of the end-systolic pressure-volume relationship and rate of pressure rate-end-diastolic volume relationship were significantly higher when measured during volume depletion than during volume expansion (17.2 ± 1.3 versus 8.5 ± 0.8 mm Hg/ml and 205 ± 14 versus 126 ± 11 mm Hg/sec/ml, p