Which factor most directly impacts myocardial oxygen supply?

• A. Heart rate
• B. Coronary perfusion
• C. Systemic vascular resistance
• D. Venous oxygen content

Answer: (B)

Oxygen delivery to the heart muscle comes from the blood flowing through the coronary arteries, so the factor that most directly determines myocardial oxygen supply is coronary perfusion. The amount of oxygen delivered equals the product of coronary blood flow and the oxygen content of that blood. Coronary perfusion is driven by perfusion pressure, mainly diastolic aortic pressure minus right atrial pressure, and flow to the heart is greatest during diastole. When perfusion is reduced—such as by low diastolic pressure, coronary artery blockage, or conditions shortening diastolic time—oxygen supply to the myocardium drops, even if arterial oxygen content is normal.

Heart rate influences oxygen demand more than supply: a faster heart rate increases how much oxygen the heart uses. Systemic vascular resistance affects afterload and thus myocardial work (demand) rather than directly altering the delivery of oxygen. Venous oxygen content reflects how much oxygen remains after tissues have extracted what they need and is not the direct determinant of how much oxygen reaches the heart muscle.