Volume Loading Slows Left Ventricular Isovolumic Relaxation Rate

We studied the effects of volume loading on left ventricular isovolumic relaxation rate in 16 intact anesthetized dogs. End-diastolic pressure, mean aortic systolic pressure, dp/dtmnx, and heart rate were measured at end expiration and end inspiration. Volume loading to approximately 5, 10, 15, and 20 mm Hg above initial end-diastolic pressure was performed. In nine dogs, simultaneous ventricular dimensions were measured with previously implanted tantalum screws using biplane cineangiography. Similar volume loading was done in open-chest and open-pericardium states. Relaxation rate was measured in 3413 beats using T, the time constant of exponential isovolumic pressure fall. T was calculated as reported previously by others and also from a linear regression of dp/dt against p, to eliminate the effects of extracavity pressure changes. T always increased significantly with volume loading, indicating slower relaxation. (For example, with the chest intact, mean T increased from 26 ± 2 (SEM) msec before volume loading to 41.5 ± 4 msec after volume loading.) Using multiple linear regression analysis, we found, in agreement with previous reports, that T decreased significantly as dp/dtMAX, and heart rate increased. In contrast to previous reports, we also found that T increased significantly as end-diastolic and mean aortic systolic pressure increased. These four variables taken together accurately predicted T [SEE (standard error of estimate) = 3.2 msec, R = 0.94, P < 0.001]. Geometric variables, including ventricular dimensions and ejection fraction, did not have a statistically significant effect on T independent of the hemodynamic variables. Opening the chest or pericardium did not have a consistent effect on T. Volume loading slows isovolumic relaxation rate in the intact canine heart. This effect appears to be a reflection of the dependence of relaxation on both end-diastolic and mean aortic systolic pressures.