Hypoxia is a reduction in oxygen supply to tissue despite adequate perfusion. Ischemia is oxygen deprivation accompanied by inadequate removal of metabolites due to reduced perfusion. Both ischemia and hypoxia must be discussed in relative terms, since conditions that result in ischemia in one patient may not result in ischemia in another. Ischemia occurs when there is an imbalance between oxygen demand and oxygen supply. Oxygen supply is influenced by the oxygen-carrying capacity of the blood and the coronary arterial blood flow. The oxygen-carrying capacity of the blood is determined by the amount of hemoglobin present. Coronary arterial blood flow is determined by diastolic relaxation of the heart and the vascular resistance. Humoral, neural, metabolic, and extravascular compressive forces and local autoregulation mechanisms determine the coronary vascular resistance.
Myocardial ischemia and its sequelae usually occur as a result of fixed atheroscerotic lesions or secondary reduction in myocardial blood flow due to coronary arterial spasm, disruption of atherosclerotic plaques, and platelet aggregation or thrombus formation. Nonatherosclerotic etiologies of acute myocardial infarction are considerably less common (TabjeiZ-l).
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