Load independence of early diastolic filling parameters in the anesthetized canine model.

To evaluate radionuclide diastolic filling indices during acute pharmacologic changes in ventricular loading, 11 atrially paced dogs underwent simultaneous micromanometer left atrial and left ventricular pressure measurements. During phenylephrine infusion, systolic blood pressure increased from 110 +/- 16 (s.d.) to 147 +/- 19 mmHg (p < 0.01), causing the atrioventricular gradient to increase from 5 +/- 2 to 8 +/- 5 mmHg (p < 0.03) with no change in the time constant of isovolumic relaxation (Tau). Absolute peak filling rate increased from 40 +/- 21 to 59 +/- 44 kcts/sec (p < 0.05), but there was no change in first-half filling fraction. During dobutamine infusion, Tau shortened from 43 +/- 13 msec to 33 +/- 5 msec (p < 0.01) and first-half filling fraction increased from 39% +/- 19% to 56% +/- 18% (p < 0.05), with no change in atrioventricular gradient or absolute peak filling rate. Absolute changes from baseline for the first-half filling fraction were inversely proportional to absolute changes in Tau (r = -0.76, p < 0.05). We conclude that the left ventricular absolute peak filling rate is a load dependent index of diastolic function. In contrast, the radionuclide first-half filling fraction is independent of loading conditions, but is sensitive to substantial alterations in the rate of left ventricular isovolumic myocardial relaxation.