Using visual inspection of phasic flow patterns and understanding their physical determinants, intraoperative decisions regarding flow distribution, function of revascularized myocardium, and collateral communications can be made. Mean flow measurements cannot provide this information. Systolic compressive forces across most of the normally contracting left ventricle limit systolic myocardial perfusion. Consequently, normal flow through the left anterior descending, left circumflex, and dominant right coronary artery (supplying the inferior left ventricle) is predominantly diastolic (greater than 60%) and remains so during reactive hyperemia. Representative examples from 100 consecutive revascularizations are presented showing that when more than 40% of flow is systolic in the right coronary artery, high mean flows (greater than 100 ml/min) may go predominantly to the right rather than the left ventricle; in the case of the left coronary artery, high mean flow may supply myocardium undergoing infarction or replaced by scar rather than normally contracting muscle. When more than 60% of flow is diastolic without reactive hyperemia, borderline mean flow (40 to 60 ml/min) may indicate lack of distal ischemia rather than fixed distal resistance.