Myocardial Hypoxia and Ischemia

Abstract

Myocardial hypoxia is the result of disproportion between oxygen supply and demand. Owing to the high coronary arteriovenous difference, the myocardium is not able to bring about a substantial improvement in oxygen supply by the increased extraction of oxygen from the blood; thus the only way of meeting the higher oxygen demand is through an increased blood supply (Fig. 1). Theoretically, any of the known mechanisms leading to tissue hypoxia can be responsible for a reduced oxygen supply in the myocardium, but the most common causes are undoubtedly (i) ischemic hypoxia (often described as “cardiac ischemia”) induced by reduction or interruption of the coronary blood flow and (ii) systemic (hypoxic) hypoxia (“cardiac hypoxia”) characterized by a drop in PO₂ in the arterial blood but adequate perfusion. For the sake of completeness we would add (iii) anemic hypoxia in which the arterial PO₂ is normal but the oxygen transport capacity of the blood is decreased and (iv) histotoxic hypoxia resulting from reduced intracellular utilization of oxygen in the presence of adequate saturation and an adequate blood flow (e.g. by inhibition of oxidative enzymes as a result of cyanide poisoning).