Indicators can be used to measure myocardial blood flow if the myocardial circulation can be isolated from other segments of the systemic circulation.Methods of measuring myocardial blood flow can be divided into groups according to the technic employed to provide “isolation” of the myocardium.There is no practical method of proved accuracy whereby coronary blood flow can be measured in man in absolute terms of ml./min.All the so-called clearance methods measure myocardial blood flow in flow/volume units or ml./min./100 Gm.Myocardial blood flow measurements in these flow/volume units have not proved useful in separation of normal individuals from those with ischemic heart disease at rest, but have been useful in the study of acute intervention such as exercise or the administration of drugs.