Radionuclide assessment of myocardial metabolism

Metabolism is the link between myocardial blood flow and mechanical performance of the heart. Blood supplies oxygen and energy substrates to the myocardium where oxidative metabolism liberates from these fuel substrates energy, which is then primarily expended as mechanical work. Blood flow, mechanical function, and metabolism of the heart are thus highly interdependent. For example, as the demand for mechanical work increases with exercise, the rate of oxidative metabolism and, thus, production of energy increases. In response, more fuel must be supplied to the heart, which is usually accomplished by an increase in blood flow. Conversely, if the supply of oxygen and fuel substrates is restricted or inadequate, as in ischemia or myocardial infarction, oxidative metabolism and production of chemical energy declines, and, consequently, mechanical function falls. The severity of ischemia is determined by the degree of the imbalance between the supply and demand; in other words, it is a function not only of the degree of flow reduction but also of the demand for mechanical work. Most conventional radionuclide techniques evaluate only one aspect of ischemia, i.e., the degree of regional flow reduction or the effects on regional mechanical performance. Because the severity of ischemia is the result of both inadequate blood flow and the demand for mechanical work, it can be evaluated most accurately by assessing the resulting metabolic alteration. Similarly, intrinsic disturbances of myocardial metabolism, as in primary myocardiopathies for example, will affect both perfusion and mechanical performance