A 45-year-old man with hypertrophic obstructive cardiomyopathy (HOCM) has a systolic murmur that increases with Valsalva maneuver and decreases with squatting. The pathophysiological basis of the Valsalva-induced change is:

• A Reduced venous return decreases LV preload, worsening the dynamic outflow obstruction ✓
• B Increased intrathoracic pressure directly compresses the outflow tract
• C Sympathetic activation during Valsalva increases septal hypertrophy transiently
• D Elevated LV end-diastolic pressure dilates the outflow tract, paradoxically increasing flow

Explanation

In HOCM, the outflow obstruction is dynamic and worsens when the left ventricle is less filled. Phase II of Valsalva maneuver reduces venous return, decreasing LV preload; the smaller LV cavity brings the hypertrophied septum and anterior mitral leaflet closer to the outflow tract, intensifying obstruction and the murmur. Squatting increases venous return (preload) and afterload, distending the LV cavity and relieving obstruction. Direct mechanical compression and sympathetic septal thickening are not the operative mechanisms.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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