A medical student compares the cardiac cycle at a heart rate of 75 bpm (cycle length 0.8 seconds) with that at 150 bpm. Which statement correctly describes the change in cycle phases?
- A Diastole shortens much more than systole ✓
- B Systole and diastole shorten proportionally
- C Systole shortens much more than diastole
- D Both phases remain nearly constant regardless of heart rate
Correct answer: A. Diastole shortens much more than systole
Explanation
Ventricular systole has a relatively fixed duration of about 0.3 seconds across physiological heart rates, so as heart rate rises the cycle shortens almost entirely at the expense of diastole. At 150 bpm the cycle is roughly 0.4 seconds, leaving only about 0.1 second for diastole. This disproportionate loss of filling time is why very rapid rates reduce stroke volume despite increased contractility from the force-frequency relationship.
Reference: Ganong's Review of Medical Physiology, 26th ed.
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