Animal experiments suggest a model of coronary blood flow in which left ventricular subendocardial muscle is perfused only in diastole, while more superficial muscle is perfused throughout the cardiac cycle. Once maximal coronary vasodilatation has occured, subendocardial flow per minute depends on the area between aortic and left ventricular diastolic pressure time heart rate [DPTI per minute]. Since left ventricular oxygen needs are related to the area under the left ventricular pressure curve in systole [SPTI], the ratio DPTI/SPTI reflects balance between myocardial oxygen supply and demand. Marked reduction in the ratio causes relative subendocardial ischemia which can be predicted from teh ratio. There is evidence that outflow tract obstructions in man also cause subendocardial ischemia by similar mechanisms, and that this ischemia occurs despite normal coronary arteries.