ABSTRA CT Acute aortic valvular insufficiency was induced in open chest dogs by employing a special intravascular cannula, or by rupturing an aortic valve leaflet. Phasic and mean coronary flow were assessed in some animals, while in others data were obtained on arterial and coronary sinus blood lactate, pyruvate, Po2, Pco2, and pH, and on myocardial tissue lactate, pyruvate, and water content in the outer and inner halves of the free wall of the left ventricle. Results showed that in acute aortic insufficiency diastolic coronary flow decreased as a function of aortic diastolic pressure, but systolic coronary flow increased in such proportion that mean coronary flow did not decrease. With moderate reductions in aortic diastolic pressure due to aortic insufficiency, myocardial blood flow was judged to be nutritionally adequate in both the outer and inner regions of the left ventricle. With more severe reductions in aortic diastolic pressure, the inner region exihibited biochemical signs of anaerobic metabolism. The presence of these metabolic changes could be correlated with either of two previously described pressure indexes. These findings suggest that the reduced coronary perfusion pressure and the intramyocardial tissue pressure gradient can be compensated for by autoregulation in some cases of aortic insufficiency, but in others such compensation may be incomplete, in which case oxygen delivery to the subendocardium will be inadequate to meet local tissue oxygen needs. INTRODUCTIONIt is apparent from the high incidence of angina pectoris in patients with severe aortic insufficiency (1) December 1973. this is related to the low perfusion pressure in the coronary circulation during diastole-the time when coronary flow is normally greatest-and the greater than normal energy requirements of the hyperfunctioning left ventricle. Recently, Buckberg, Fixler, Archie, and Hoffman (2) demonstrated that when aortic diastolic pressure was lowered in the dog by opening a large arteriovenous fistula, blood flow in the wall of the left ventricle became uneven and lower in the subendocardium than the subepicardium. They correlated the regional flow changes with a special pressure index they developed to assess the balance between oxygen supply and demand in the subendocardium. It was not possible, however, to determine from the flow data or the pressure index to what extent, if any, the metabolic needs of the subendocardium were being under met when aortic diastolic pressure was reduced.In previous animal studies from our laboratory (3, 4) we showed that when pressure in the coronary circulation was independently reduced by coronary constriction, the subendocardium developed metabolic signs of inadequate oxygen delivery. This finding correlated well with a pressure index developed by us to predict the presence of uneven myocardial blood flow (5). It (Fig. 1) into the aorta via the ligated left subclavian artery and passing it retrograde across the aortic valve. This tube, henceforth referred to as an "aortic insuffi...