Oxygen therapy for acute myocardial infarction-then and now. A century of uncertainty.

For about 100 years, inhaled oxygen has been administered to all patients suspected of having an acute myocardial infarction. The basis for this practice was the belief that oxygen supplementation raised often-deficient arterial oxygen content to improve myocardial oxygenation, thereby reducing infarct size. This assumption is conditional and not evidence-based. While such physiological changes may pertain in some patients who are hypoxemic, considerable data suggest that oxygen therapy may be detrimental in others. Acute oxygen therapy may raise blood pressure and lower cardiac index, heart rate, cardiac oxygen consumption, and blood flow in the cerebral and renal beds. Oxygen also may lower capillary density and redistribute blood in the microcirculation. Several reports now confirm that these changes occur in humans. In patients with both acute coronary syndromes and stable coronary disease, oxygen administration may constrict the coronary vessels, lower myocardial oxygen delivery, and may actually worsen ischemia. There are no large, contemporary, randomized studies that examine clinical outcomes after this intervention. Hence, this long-accepted but potentially harmful tradition urgently needs reevaluation. Clinical guidelines appear to be changing, favoring use of oxygen only in hypoxemic patients, and then cautiously titrating to individual oxygen tensions.