Substrate utilization in the myocardium.

Substrate extraction is the disappearance of a substance from arterial blood into the myocardium, substrate utilization being the combustion of that substrate. The terms used for extraction and utilization are defined. There are many problems of accurately measuring glucose and free fatty acid oxidation. Lactate is the preferred substrate of the normal, blood-perfused heart. If the substrate oxidized affects the myocardial oxygen consumption, this could be of considerable importance under conditions of limited oxygen supply. At present there is conflicting evidence. It may be beneficial to switch the heart from one substrate to another, e.g., from fat to carbohydrate fuels. The conditions of ischaemia and hypoxaemia are not the same, and future investigations should be made under carefully controlled conditions.