A B S T R A C T Regional myocardial blood flow was 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 ventricular myocardial flow was 0.94±0.09 ml/min/g and endocardial flow (endo) exceeded epicardial flow (epi) in all regions (endo/epi = 1.12-1.33). When treadmill exercise 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 significantly 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, exercise resulted in increased myocardial blood flow to all regions of the left and right ventricles with maintenance of subendocardial flow equal to subepicardial flow.

SUMMARY The effect of a proximal coronary artery stenosis on transmural myocardial blood flow during exercise was studied in nine dogs with electromagnetic flowmeter probes and hydraulic occluders on the left circumflex coronary artery. Regional myocardial blood flow at rest and during treadmill exercise was estimated with radioactive microspheres 7-10 j»m in diameter. Exercise studies were performed during unrestricted coronary artery inflow (control exercise) and during partial inflation of the occluder to a level which did not reduce flow at rest but which limited the increase in flow during exercise to 66 ± 6% (mild restriction) or 44 ± 3% (severe restriction) of the value during control exercise. Mean myocardial blood flow at rest was 0.94 ± 0.06 rnl/min per g of myocardium and increased to 2.45 ± 0.15 ml/min per g during control exercise, with uniform distribution across the wall of the left ventricle. Flow to the subepicardial myocardium was significantly greater during exercise in the presence of a mild restriction than during control exercise, whereas flow to deeper layers of myocardium was progressively decreased below the control level. A similar pattern of redistribution of flow occurred during exercise in the presence of a severe restriction, but flow to all transmural layers was below that during mild restriction, resulting in more marked subendocardial underperfusion. Thus, exercise in the presence of stenosis resulted in transmural redistribution of myocardial blood flow with subendocardial underperfusion in proportion to the degree of restriction of coronary artery inflow.IN THE presence of occlusive coronary artery disease patients who have normal electrocardiograms at rest may develop S-T segment depression during exercise, often in association with chest pain. This electrocardiographic change suggests that in these patients exercise may result in selective ischemia of the subendocardial myocardium.1 Previous experimental studies in animals have demonstrated that in contrast to the uniform left ventricular perfusion that exists during unimpeded coronary artery inflow, transmural redistribution of flow may occur during restricted inflow. Thus, in open-chest dogs reduction of coronary inflow below the metabolic requirements of the myocardium resulted in transmural redistribution of myocardial blood flow with preferential underperfusion of the subendocardial myocardium.2 " 6In patients with occlusive coronary artery disease, arterial inflow at rest may be adequate and transmural distribution uniform, but because arterial inflow is unable to increase adequately during exercise a similar redistribution of myocardial blood flow may occur with preferential underperfusion of the subendocardial myocardium. The present study was an attempt to determine whether, in an experimental model with a flow-limiting proximal coronary artery stenosis, exercise could result in the proposed redistribution of myocardial blood flow. MethodsNine adult mongrel dogs weighing 22-34 kg were anesthetized with sodium thiamyl...

AB STRA CT This study was designed to determine the effect of nitroglycerin upon transmural distribution of myocardial blood flow in the awake dog during normal conditions and in the presence of ischemia-induced coronary vasodilation. Studies were performed in chronically prepared dogs with electromagnetic flowmeters and hydraulic occluders on the left circumflex coronary artery. Regional myocardial blood flow was estimated by using radionuclide-labeled microspheres, 7-10 im in diameter, injected into the left atrium. During control conditions endocardial flow (0.86±SEMI 0.05 ml/min per g) slightly exceeded epicardial flow (0.72±0.03 ml/min per g, P < 0.05), and this distribution of flow was not significantly altered by nitroglycerin. After a 5-s coronary artery occlusion, reactive hyperemia occurred with excess inflow of arterial blood effecting 360+15 15% repayment of the blood flow debt incurred during occlusion. When arterial inflow was limited to the preocclusion rate during coronary vasodilation after a 5-s total coronary artery occlusion, flow to the subepicardial myocardium was increased at the expense of underperfusion of the subendocardial myocardium, and the delayed reactive hyperemia was markedly augmented (mean blood flow debt repayment = 775±105%, P < 0.01). These data suggested that subendocardial underperfusion during the interval of coronary vasodilation in the presence of a flow-limiting

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