Methods for the metabolic quantification of regional myocardial ischemia

An adequate balance between oxygen supply and demand is a basic requirement for normal cardiac function. When oxygen supply does not meet the demand, progressive cellular damage occurs leading to cardiac dysfunction and, ultimately, tissue death. While traditionally "ischemia" has been defined as decreased oxygen supply secondary to a decrease in blood flow, and "hypoxia" as decreased oxygen supply secondary to a decrease in oxygen tension, this review defines ischemia in its broader sense, namely as a pathophysiologic state in which there is a lack of oxygen relative to the demand for it. In a large number of experimental studies involving the heart, there is need to promptly recognize the ischemic state, to monitor its course in vivo, and to quantify it. Because of cardiac autoregulatory mechanisms, research methods which attempt to quantify supply (e.g., measurement of myocardial blood flow) and/or demand (e.g., measurement of myocardial oxygen consumption) do not necessarily reflect the status of the balance between supply and demand. An imbalance between myocardial supply and demand is more likely to be reflected by metabolic fluxes and by the accumulation of products specific to the ischemic state. Thus, the purpose of this review is to summarize the various methods available to the cardiac surgical investigator today for the metabolic quantification of myocardial ischemia. Due to the complexity of the heart and its inherent regional differences, myocardial ischemic changes are frequently regional in nature. Thus, this review will address metabolic methods for the regional quantification of myocardial ischemia.