A 70-year-old man with chronic atrial fibrillation complains of reduced exercise tolerance. Compared with his previous sinus rhythm state, the loss of the 'atrial kick' reduces left ventricular end-diastolic volume by approximately what proportion?

• A About 5 percent
• B About 75 percent
• C About 50 percent
• D About 25 percent ✓

Explanation

Atrial systole contributes roughly 20 to 30 percent of end-diastolic volume at resting heart rates, so losing coordinated atrial contraction in fibrillation removes about one quarter of the filling volume and lowers stroke volume by the Frank-Starling relation. The larger figures apply only when diastolic compliance is reduced, such as in hypertrophy or stiff ventricles, where atrial kick may contribute up to 40 percent, but the standard textbook figure for the normal resting heart is about 25 percent.

Reference: Ganong Review of Medical Physiology, 26th ed.

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