REGIONAL MYOCARDIAL CAPILLARY ERYTHROCYTE TRANSIT-TIME IN THE NORMAL RESTING HEART

A major determinant of oxygen transport to the myocardium is the time spent by the erythrocytes (red blood cells [RBCs]) traversing the microcirculation. Although it has been shown that the myocardium has regional differences in blood volume, blood How, metabolism, and sensitivity to ischemic injury, the regional distribution of RBC transit times through the myocardial capillaries has not been previously measured. The present study was designed to measure the regional myocardial capillary RBC transit time by a new technique to determine whether there are regional differences in the capillary RBC transit time in the normal resting heart. Anesthetized open-chest male New Zealand White rabbits (3.0-3.7 kg, n=8) were studied. Regional myocardial blood volume was determined using chromium-51-labeled RBCs, and regional blood flow was measured using a reference How technique and a left atrial injection of 15-mum-radiolabeled (gadolinium- 153, 10-20 muCi) microspheres. Capillary blood volume was determined by multiplying the regional blood volume by the histologically determined fraction of the total blood volume that was in the capillaries. Capillary RBC transit time was calculated as the quotient of capillary blood volume and blood flow. The myocardial capillary blood volume was the same in the endocardium and the epicardium (4.67+/-0.67 ml/100 g for endocardium versus 4.52+/-0.70 ml/100 g for epicardium, p=NS), whereas myocardial blood flow tended to be greater in the endocardium (6.09+/-0.73 ml/sec per 100 g for endocardium versus 5.47+/-0.75 ml/sec per 100 g for epicardium), although this was not statistically significant. Myocardial capillary RBC transit times ranged from 0.22 to 2.58 seconds with a mean of 0.89+/-0.13 and 0.83+/-0.13 seconds and a median of 0.72 and 0.70 seconds in the epicardium and endocardium, respectively. We conclude that there are no regional differences in the myocardial capillary RBC transit time in the normal heart at rest.