Abstract
The term myocardial ischemia describes a condition that exists when fractional uptake of oxygen in the heart is not sufficient to maintain the rate of cellular oxidation. This leads to extremely complex situations that have been extensively studied in recent years. Experimental research has been directed toward establishing the precise sequence of biochemical events leading to myocyte necrosis, as such knowledge could lead to rational treatments designed to delay myocardial cell death. At the present time, there is no simple answer to the question of what determines cell death and the failure to recover cell function after reperfusion. Problems arise because: (1) ischemic damage is not homogeneous and many factors may combine to cause cell death; (2) severity of biochemical changes and development of necrosis are usually linked (both the processes being dependent on the duration of ischemia) and it is impossible to establish a causal relation; and (3) the inevitability of necrosis can only be assessed by reperfusion of the ischemic myocardium. Restoration of flow, however, might result in numerous other negative consequences, thus directly influencing the degree of recovery. From the clinical point of view, we have recently learned that there are several potential manifestations and outcomes associated with myocardial ischemia and reperfusion. Without a doubt, ventricular dysfunction (either systolic or diastolic) of the ischemic zone is the most reliable clinical sign of ischemia, since electrocardiographic changes and symptoms are often absent. The ischemia-induced ventricular dysfunction, at least initially, is reversible, as early reperfusion of the myocardium results in restoration of normal metabolism and contraction. In the ischemic zone, recovery of contraction may occur instantaneously or, more frequently, with a considerable delay, thus yielding the condition recently recognized as the "stunned" myocardium. On the other hand, when ischemia is severe and prolonged, cell death may occur. Reperfusion at this stage is associated with the release of intracellular enzymes, damage of cell membranes, influx of calcium, persistent reduction of contractility, and eventual necrosis of at least a portion of the tissue. This entity has been called "reperfusion damage" by those who believe that much of the injury is the consequence of events occurring at the moment of reperfusion rather than a result of changes occurring during the period of ischemia. The existence of reperfusion damage, however, has been questioned, and it has been argued that, with the exception of induction of arrhythmias, it is difficult to be certain that reperfusion causes further injury. The existence of such an entity has clinical relevance, as it would imply the possibility of improving recovery with specific interventions applied at the time of reperfusion. In 1985, Rahimtoola described another possible outcome of myocardial ischemia. He demonstrated that late reperfusion (after months or even years) of an ischemic area showing ventricular wall-motion abnormalities might restore normal metabolism and function. He was the first to introduce the term "hibernating myocardium," referring to ischemic myocardium wherein the myocytes remain viable but in which contraction is chronically depressed. In this article we review our data on metabolic changes occurring during ischemia followed by reperfusion, obtained either in the isolated and perfused rabbit hearts or in ischemic heart disease patients undergoing intracoronary thrombolysis or aortocoronary bypass grafting.
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