A 68-year-old man with chronic atrial fibrillation and a hypertrophied, non-compliant left ventricle notices a marked fall in exercise tolerance compared with when he was in sinus rhythm. Which hemodynamic consequence of lost atrial contraction best explains this?
- A Loss of the atrial kick removes the late-diastolic contribution of roughly 20 to 30 percent of end-diastolic volume ✓
- B Loss of atrial contraction eliminates the entire passive early-diastolic filling phase
- C Atrial fibrillation abolishes the pressure gradient between pulmonary veins and left atrium
- D Absent P waves prevent calcium entry into ventricular myocytes during phase 2
Correct answer: A. Loss of the atrial kick removes the late-diastolic contribution of roughly 20 to 30 percent of end-diastolic volume
Explanation
Most ventricular filling occurs passively during early diastole, but atrial contraction adds the final 20 to 30 percent of end-diastolic volume in late diastole. In a stiff, hypertrophied ventricle, passive filling is impaired and this atrial contribution becomes proportionately larger, so its loss in atrial fibrillation causes a significant drop in preload and output. Passive filling continues without atrial systole, ruling out option B, and ventricular excitation proceeds normally via the AV junction.
Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.
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