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This study was designed to quantitate transmural myocardial blood flow when coronary arterial inflow was limited to systole and during the subsequent reactive hyperemic response. Studies were performed in 10 awake dogs chronically prepared with electromagnetic flowmeters and pneumatic occluders on the left circumflex coronary artery. Intermittent coronary perfusion, confined to the interval of left ventricular systole or an equivalent period during diastole, was effected by an R wave-triggered solenoid valve connected to the occluder. To measure regional myocardial blood flow we injected radionuclidelabeled microspheres, 7-10 fim in diameter, into the left atrium. When arterial inflow was limited to systole, flow was normal in the subepicardial layers and was decreased as a linear function of tissue depth in the subendocardial layers. When coronary arterial inflow was limited to an equivalent interval in diastole, the transmural distribution of flow was uniform. When coronary flow was confined to systole for more than 20 seconds, the blood flow debt incurred elicited a reactive hyperemia similar to that following a total occlusion of equivalent blood flow debt. However, regional myocardial blood flow during the peak of reactive hyperemia following systolic perfusion was preferentially directed to the subendocardium, where underperfusion was most marked, whereas reactive hyperemia flow following a total occlusion of equiva-

A B S T R A C T Since the ability of mature intercoronary collateral channels to increase myocardial blood flow in response to drug-induced coronary vasodilation has been questioned, the present study was undertaken to evaluate the response of coronary collateral circulation to the stress of exercise. Studies were performed at rest and during two levels of treadmill exercise in six dogs a minimum of 6 mo after placement of an Ameroid constrictor on the left circumflex coronary artery. Regional myocardial blood flow was estimated in normally perfused anterior and predominantly collateral-dependent posterior left ventricular wall with left atrial injections of radionuclide-labeled microspheres 7-10 lOtm in diameter. At rest, heart rate was 87+7 beats/min and mean myocardial blood flow was comparable in control and collateral-dependent regions (0.96+0.13 and 0.97 +0.14 ml/min g, respectively). During exercise, heart rates increased to 180+13 and 228+14 beats/min and myocardial blood flow (MBF) in the anterior control region increased linearly with heart rate (HR), (MBF = 0.133 HR -0.202, r = 0.88). MBF to the posterior collateral-dependent region was similarly augmented during exercise (MBF = 0.140 HR -0.252, r = 0.89), so that the linear correlation between HR and MBF was similar for the control and collateral-

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