How does tachycardia affect diastolic coronary perfusion time and myocardial oxygen supply?

• A. Increases diastolic time and increases myocardial oxygen supply.
• B. Shortens diastolic time and potentially reduces oxygen supply.
• C. Has no effect on diastolic time but increases oxygen demand.
• D. Extends systole, not diastole.

Answer: (B)

Tachycardia reduces the duration of diastole, and this matters because coronary blood flow to the myocardium mainly occurs during diastole when the heart muscle is relaxed. With a faster heart rate, diastole shortens more than systole, so there is less time for the coronary vessels to be perfused. At the same time, myocardial oxygen demand rises with increased rate, contractility, and wall stress. The combination can lower oxygen supply while increasing demand, creating a mismatch that can compromise myocardial oxygen delivery. That’s why shortening diastolic time and potentially reducing oxygen supply is the best description.