Anatomic and Physiologic Considerations

THE CLINICAL importance of ischemic heart disease has provided a powerful stimulus for the development of new and better methods for the study of the coronary circulation. In the animal laboratory, the electromagnetic flowmeter has replaced the rotameter as the standard method of measuring coronary blood flow. Flowmeter probes can be implanted around a dog's coronary artery and utilized to study the coronary circulation during exercise in an intact unanesthetized animal.' These excellent methods cannot, for obvious reasons, be applied to the study of patients, and therefore clinical investigators have been forced to develop other methods for the study of patients with ischemic heart disease. The coronary circulation can be defined as a peripheral circulatory bed, and therefore any discussion of the method of measuring myocardial blood flow should logically begin with a consideration of the simplest kind of peripheral circulatory bed as diagrammed in figure 1. The artery and vein are both accessible, and an indicator can be injected into the blood flowing in through the artery and its concentration measured in venous blood leaving the bed, and there is no pathway whereby it can recirculate. The indicator substance may be identified by virtue of its color, its chemical properties, or its radioactivity. If a radioactive indicator is used, and a scintillation probe is placed over the vascular bed, a concentration time plot can be obtained by means