LOAD INDEPENDENCE OF EARLY DIASTOLIC FILLING PARAMETERS IN THE ANESTHETIZED CANINE MODEL

To evaluate radionuclide diastolic filling indices during acute pharma cologic changes in ventricular loading, 11 artrially paced dogs underw ent simultaneous micromanometer left atrial and left ventricular press ure measurements. During phenylephrine infusion, systolic blood pressu re increased from 110 +/- 16 (s.d.) to 147 +/- 19 mmHg (p < 0.01), cau sing the atrioventricular gradient to increase from 5 +/- 2 to 8 +/- 5 mmHg (p < 0.03) with no change in the time constant of isovolumic rel axation (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 f illing fraction. During dobutamine infusion, Tau shortened from 43 +/- 13 msec to 33 +/- 5 msec (p < 0.01) and first-half filling fraction i ncreased from 39% +/- 19% to 56% +/- 18% (p < 0.05), with no change in atrioventricular gradient or absolute peak filling rate. Absolute cha nges from baseline for the first-half filling fraction were inversely proportional to absolute changes in Tau (r = -0.76, p < 0.05). We conc lude that the left ventricular absolute peak filling rate is a load de pendent index of diastolic function. In contrast, the radionuclide fir st-half filling fraction is independent of loading conditions, but is sensitive to substantial alterations in the rate of left ventricular i sovolumic myocardial relaxation.