The myocardial vasculature during ischemia and reperfusion: a target for injury and protection.

Ischemia and reperfusion may result in injury to one or more of the cellular components of the heart. In addition to damaging myocytes and their contractile capability, ischemia and reperfusion may inflict early and severe injury on the vasculature which, in turn, may further jeopardize the survival of the myocytes. While ischemia is known to cause progressive injury to endothelium and vascular smooth muscle it now appears that reperfusion can inflict additional, possibly severe, injury on the microcirculation which may compromise the return of normal coronary perfusion. This post-ischemic/reperfusion microvascular incompetence ranges from a transient exacerbation of the increase in vascular resistance initiated during ischemia, to a sustained loss of competent capillaries and eventually to the "no reflow" phenomenon which is characterized by the total inability of the affected tissue to be reperfused. Whereas "no reflow" may be of little importance if it occurs in already infarcted tissue, post-ischemic microvascular incompetence in potentially salvable myocardium could be of considerable significance. Evidence is presented that the vascular endothelium, and its ability to regulate coronary vascular tone, play a central and early role in the pathogenesis of myocardial injury. Mechanisms underlying microvascular injury have been identified and pharmacological strategies now exist for the effective manipulation of this injury--a prospect that is of considerable importance in the light of the widespread use of thrombolytic procedures for the reperfusion of the human myocardium.