Under which condition is myocardial oxygen consumption highest?

• A. Hypothermic, full beating heart
• B. Normothermic, empty beating heart
• C. Normothermic, full beating heart
• D. Hypothermic, empty beating heart

Answer: (B)

The main idea is that myocardial oxygen consumption (MVO2) rises with both the heart’s metabolic rate and its work. Temperature affects metabolic rate: at normothermia the heart’s enzymes work at their normal pace, so the myocardium uses more oxygen than when it’s cooled (hypothermia slows metabolism and reduces oxygen demand). Work goes up with preload/afterload: a full ventricle can generate more pressure and perform more stroke work than an empty one, so contracting against a filled ventricle costs more oxygen.

Putting these together, the highest MVO2 occurs when the heart is at normal temperature and actively contracting with normal preload—normothermic, full beating. If the ventricle is empty, there’s less wall stress and lower stroke work, reducing oxygen demand; if the heart is hypothermic, metabolic rate and thus oxygen use are reduced regardless of workload. So to maximize MVO2, you want a normothermic heart undergoing full, normal contraction.