A physician-validated, board-style question from the Active Transport QBank. Try it, then check the reasoning for every option.
A woman with coronary artery disease is starting to go for a walk. As she begins, her heart rate accelerates from a resting pulse of 60 bpm until it reaches a rate of 120 bpm, at which point she begins to feel a tightening in her chest. She stops walking to rest and the tightening resolves. This has been happening to her consistently for the last 6 months. Which of the following is a true statement?
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A
Increasing the heart rate increases the amount of time spent during each cardiac cycleIncorrect. Increasing heart rate DECREASES the time per cardiac cycle (shorter R-R interval), not increases it; this option reverses the relationship.
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B
Increasing the heart rate decreases the relative amount of time spent during diastoleCorrect. As heart rate rises, diastole shortens disproportionately, reducing the time available for coronary (left ventricular) perfusion and precipitating ischemia in patients with fixed coronary stenoses.
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C
Perfusion of the myocardium takes place primarily during systoleIncorrect. Left ventricular myocardial perfusion takes place primarily during DIASTOLE because intramural compression during systole impedes flow; this option states the opposite.
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D
Perfusion of the myocardium takes place equally throughout the cardiac cycleIncorrect. Perfusion is heavily diastolic, not equal across the cycle; otherwise tachycardia would not preferentially worsen ischemia.
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E
Increasing the heart rate increases stroke volume to a greater extent than it shortens diastoleIncorrect. While increased heart rate can transiently augment contractility (Bowditch effect), it disproportionately shortens diastole and reduces coronary perfusion time, which is what precipitates angina in this patient — the option overstates the stroke volume effect.
↑ Tap an answer to reveal the reasoning
Answer: B. A patient with stable angina develops chest tightness consistently when heart rate climbs to 120 bpm. The mechanism is demand-supply mismatch: at higher heart rates, myocardial oxygen demand rises while myocardial perfusion paradoxically falls. The key physiologic point: coronary perfusion of the left ventricle occurs primarily during DIASTOLE, when the aortic valve is closed and intramyocardial pressures are low enough to allow flow through the coronary circulation. During systole, contracting myocardium compresses the intramural coronary arteries and largely halts left ventricular perfusion.
As heart rate increases, the time per cardiac cycle decreases, and the relative shortening is disproportionately greater for diastole (systole's duration is more fixed). At rest with HR 60, diastole accounts for roughly two-thirds of the cycle; at HR 120, diastole shrinks toward half or less. This compresses the window for coronary perfusion, especially in patients with fixed coronary stenoses whose flow already depends critically on diastolic filling time.
Beta-blockers help angina by slowing heart rate and prolonging diastolic perfusion time, on top of reducing inotropy and oxygen demand.
**Why each option:**
**A.** Increasing heart rate DECREASES the time per cardiac cycle (shorter R-R interval), not increases it; this option reverses the relationship.
**B.** Correct. As heart rate rises, diastole shortens disproportionately, reducing the time available for coronary (left ventricular) perfusion and precipitating ischemia in patients with fixed coronary stenoses.
**C.** Left ventricular myocardial perfusion takes place primarily during DIASTOLE because intramural compression during systole impedes flow; this option states the opposite.
**D.** Perfusion is heavily diastolic, not equal across the cycle; otherwise tachycardia would not preferentially worsen ischemia.