A patient with hypertrophic obstructive cardiomyopathy (HOCM) undergoes Valsalva maneuver during examination. The dynamic outflow obstruction worsens. Which physiological mechanism best explains the worsening obstruction during forced expiration against a closed glottis?
- A Increased venous return increases preload, expanding the ventricle and relieving sub-aortic obstruction
- B Elevated intrathoracic pressure increases afterload, forcing the LVOT open more widely
- C Decreased venous return reduces LV end-diastolic volume, causing the hypertrophied septum to approach the anterior mitral leaflet more closely ✓
- D Increased sympathetic stimulation during straining causes peripheral vasoconstriction that increases afterload
Correct answer: C. Decreased venous return reduces LV end-diastolic volume, causing the hypertrophied septum to approach the anterior mitral leaflet more closely
Explanation
Valsalva's straining phase reduces venous return due to elevated intrathoracic pressure, which decreases LV filling (preload). A smaller LV cavity brings the hypertrophied interventricular septum and the elongated anterior mitral leaflet into apposition more easily during systole — worsening the dynamic sub-aortic obstruction. This distinguishes HOCM from fixed obstructions and is why maneuvers that increase preload (squatting, leg raise) relieve the murmur.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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