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

Abstract

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 less than 0.001, respectively). This finding was also evident when data analysis was confined to the initial 10 seconds of the loading interval. Nonlinear regression analysis of pooled data from both preloading protocols revealed curvilinear characteristics for each relationship. In contrast, the slope of the preload recruitable stroke work relationship was lower as a result of volume unloading (16.8 +/- 2.0 versus 22.8 +/- 1.7 mJ/beat/100 gm left ventricle/ml, p less than 0.01) and was independent of loading protocol when data analysis was confined to the initial 10-second acquisition period. Furthermore, second-order regression analysis of the pooled data showed no curvilinearity. These findings emphasize the importance of both the timing and method of varying cardiac preload in the determination of intrinsic myocardial contractility and suggest that the preload recruitable stroke work relationship may be a more linear contractile index that is less affected by preloading history over brief intervals.