Every patient having a heart attack should receive supplemental oxygen, even when blood oxygen is normal.

Supplemental oxygen became one of the standard sights of heart-attack care. The rationale seemed almost self-evident: a coronary artery was starving heart muscle of oxygen, so giving the patient more oxygen should help threatened tissue. The practice became routine long before strong randomized evidence showed that normoxic patients benefited.

That intuition overlooked an important distinction between oxygen in the lungs and blood and blood flow through the blocked coronary circulation. Once hemoglobin is already well saturated, extra oxygen adds relatively little to the blood’s oxygen content. Hyperoxia can also constrict coronary vessels and alter circulation in ways that might be unhelpful.

A controlled trial published in the 1970s failed to show benefit in uncomplicated myocardial infarction, but routine oxygen persisted for decades. Larger modern trials and reviews eventually reinforced the absence of benefit in patients whose oxygen saturation is already normal, and guidelines narrowed accordingly. Oxygen remains essential when a heart-attack patient is hypoxemic or has respiratory compromise; what disappeared was the assumption that every infarction automatically requires it.