Syncope, a Systolic Murmur, and a Dose of Nitroglycerin
A 24-year-old male collegiate rower presents after two episodes of syncope during sprint intervals. He describes chest tightness and lightheadedness with exertion that resolve within a minute of stopping. On exam, a harsh crescendo–decrescendo systolic murmur is heard best at the left lower sternal border, radiating to the apex but not the carotids; it intensifies with Valsalva and softens with squatting. Echocardiography shows asymmetric septal hypertrophy, systolic anterior motion of the mitral leaflet, and a resting left ventricular outflow tract gradient of 62 mmHg; ejection fraction is 72%. In the emergency department, while awaiting admission, he reports chest tightness and is given sublingual nitroglycerin. Minutes later he is diaphoretic and presyncopal, and his blood pressure has fallen from 128/78 to 86/50 mmHg.
1. Explain mechanistically why nitroglycerin produced this deterioration.
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In hypertrophic cardiomyopathy the obstruction is dynamic, not fixed. The gradient depends on three variables: contractility, preload, and afterload.
Nitroglycerin is a venodilator (with some arteriolar effect). Venodilation reduces preload, which shrinks the LV cavity. A smaller cavity brings the hypertrophied septum and the anterior mitral leaflet into closer apposition, so systolic anterior motion worsens and the outflow gradient rises. Reduced afterload adds a second hit: the ventricle empties more completely and at higher velocity, further increasing the Venturi effect that pulls the leaflet into the outflow tract.
The result is a fall in stroke volume and cardiac output despite a vigorously contracting ventricle - hypotension with a small, underfilled left ventricle. The same logic explains why diuretics, ACE inhibitors, and hydralazine-type afterload reducers can all be harmful here, and why volume resuscitation and a vasoconstrictor/pure β-blocker are the appropriate acute response.
2. Explain why squatting softens the murmur and Valsalva intensifies it, and what that tells you about the maneuver's physiologic target.
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Both maneuvers change ventricular volume, which is the variable that governs the dynamic gradient.
- Squatting compresses leg veins and augments venous return → preload rises → the LV cavity enlarges → the septum and mitral leaflet are held apart → outflow obstruction decreases → murmur softens. Squatting also raises afterload, which further enlarges the cavity.
- Valsalva (strain phase) raises intrathoracic pressure and reduces venous return → preload falls → smaller cavity → more obstruction → murmur intensifies.
This is the mirror image of what happens in most other murmurs. Aortic stenosis and mitral regurgitation typically soften with Valsalva because their severity is flow-dependent and flow falls. The distinguishing reasoning step is that in HCM, decreasing preload paradoxically increases obstruction - the murmur behaves opposite to almost everything else, which is exactly why the maneuver is diagnostic rather than incidental.
3. Two years later he develops atrial fibrillation with a ventricular rate of 150/min and becomes markedly dyspneic. Explain why this rhythm is disproportionately poorly tolerated in this patient.
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Three insults stack on top of each other:
- Loss of the atrial kick. A hypertrophied, noncompliant ventricle is heavily preload-dependent; atrial contraction contributes a large fraction of end-diastolic volume. Losing it reduces cavity size and increases the dynamic gradient.
- Short filling time at 150/min. Tachycardia disproportionately shortens diastole, so a stiff ventricle that already fills poorly fills even less - pulmonary congestion develops rapidly.
- Rate-related worsening of obstruction. Faster rates and higher contractility both increase the gradient, so the arrhythmia feeds back on the underlying lesion.
Because of this, maintaining sinus rhythm or at least controlling rate is a priority, and pure afterload-reducing vasodilators remain hazardous. Note the anticoagulation implication as well: AF in HCM carries a high stroke risk independent of conventional risk scores, so anticoagulation is generally indicated.