A 62-year-old man with chronic atrial fibrillation remains stable at rest but develops marked dyspnea and hypotension during a febrile illness with a heart rate of 140 bpm. Which physiological explanation best accounts for his exertional intolerance?

• A Reduced ejection fraction caused by irregular ventricular activation alone
• B Shortening of ventricular action potential duration reducing contractile force
• C Increased afterload from loss of coordinated atrial systole
• D Loss of the atrial contribution to ventricular filling, whose relative importance rises at high heart rates ✓

Explanation

At rest, atrial contraction contributes roughly 20 to 30 percent of end-diastolic volume. At high heart rates, when diastolic filling time shrinks, this atrial kick becomes proportionately more important for maintaining stroke volume. Loss of organized atrial systole in atrial fibrillation therefore causes decompensation mainly during tachycardia. Action potential shortening and afterload changes do not explain the rate-dependent deterioration, and irregularity itself is tolerated at lower rates.

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

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