A 28-year-old man with hypertrophic obstructive cardiomyopathy receives sublingual glyceryl trinitrate for chest pain and develops a marked increase in his outflow tract gradient. The physiological explanation is:

• A Nitrates dilate the abnormal intramural coronary arteries, worsening ischemia
• B Nitrate-induced tachycardia increases contractility through the Bowditch effect
• C Venodilation reduces preload, decreasing left ventricular end-diastolic volume and narrowing the outflow tract ✓
• D Reduced afterload increases ejection velocity, dragging the anterior mitral leaflet toward the septum

Explanation

The dynamic obstruction in HOCM worsens whenever left ventricular volume falls or contractility rises. Venodilation lowers preload, shrinking end-diastolic volume so the hypertrophied septum and mitral leaflet appose more closely, increasing the gradient. Reduced afterload also contributes, but the classic teaching point is that nitrates are contraindicated because they decrease preload. Option B is wrong because nitrates cause reflex tachycardia without a positive inotropic Bowditch effect on the ventricle.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.