Myocardial ischemia and reperfusion

Myocardial infarction is a dynamic process that begins with the transition from reversible to irreversible ischemic injury and culminates in the replacement of dead myocardium by a fibrous scar. Many biochemical and metabolic changes have been observed early after the onset of ischemia, but the precise cause of the transition to irreversibility has not been elucidated. However, disruption of the plasmalemma of the sarcolemma is an early event, the presence of which indicates that the ischemic myocytes are dead. Not all ischemic myocytes become irreversibly injured simultaneously in experimental infarction in the canine heart; rather, myocytes die in a transmural wavefront of cell death proceeding from the subendocardial to the subepicardial myocardium with the subendocardial layer dying first and the subepicardial layer last. About 6 hours of ischemia are required to complete the wave-front. During the reversible phase of ischemic injury, reperfusion salvages all ischemic myocytes in all layers, but once lethal injury begins to develop, reperfusion salvages reversibly injured myocytes that are located chiefly in the subepicardial and midmyocardial layers and thereby limits the transmural extent of infarction. The gradual evolution of cell death in experimental acute ischemia provides a basis for limitation of infarct size by reperfusion with arterial blood in man. Many functions of myocardium subjected to reversible episodes of ischemia return to the control condition a few seconds or minutes after the onset of reperfusion. Others, such as repletion of the adenine nucleotide pool, require hours to days to repair. Reversibly injured myocardium exhibits reduced contractile efficiency, termed stunning, which is a form of reperfusion injury. Stunning is reversible; it disappears after hours or days of reperfusion. Finally, reversibly injured myocardium develops adaptive changes that protect it against subsequent episodes of ischemia. One such change, termed ischemic preconditioning, persists for 1-2 hours and serves to delay the development of cell death if the tissue is subjected to a new prolonged episode of ischemia. Another, heat shock protein synthesis, does not appear until the tissue has been reperfused for 12-24 hours; it also protects the myocardium against subsequent ischemic injury. The molecular mechanisms underlying stunning, ischemic preconditioning, and heat shock protein synthesis remain to be established.