Abstract

measured in nine dogs at rest and during three levels of treadmill exercise by using left atrial injections of 7-10-,um radioactive microspheres. At rest, heart rate was 76±3 beats/min (mean+SEM), mean left ventricu-lar myocardial flow was 0.94±0.09 ml/min/g and endo-cardial flow (endo) exceeded epicardial flow (epi) in all regions (endo/epi = 1.12-1.33). When treadmill ex-ercise was regulated to increase heart rates from 152±3 to 190±3 to 240±6 beats/min, myocardial blood flow (MBF) to all regions of the left ventricle increased linearly with heart rate (HR) from 1.83±f-0.11 to 2.75± 0.22 to 3.90±0.26 ml/min/g (MBF = 0.0175HR- 0.523 ±0.614, r = 0.87). Exercise abolished the gradient of blood flow favoring the left ventricular endocardium at rest, so that the endo/epi flow ratios were not signifi-cantly different from 1.00. Right ventricular flows were consistently less than corresponding left ventricular flows, but showed a similar linear increase with heart rate. Right ventricular endo/epi ratios were not different from 1.00 either at rest or during exercise. Thus, exer-cise resulted in increased myocardial blood flow to all regions of the left and right ventricles with maintenance of subendocardial flow equal to subepicardial flow.