A patient with severe aortic stenosis develops syncope on exertion. Which physiological mechanism best explains this phenomenon?
- A Fixed cardiac output cannot increase with exercise-induced vasodilation, causing cerebral hypoperfusion ✓
- B Increased afterload causes acute left ventricular failure and pulmonary oedema
- C Reflex bradycardia triggered by aortic baroreceptors reduces cardiac output
- D Elevated LV end-diastolic pressure reduces coronary perfusion pressure at rest
Correct answer: A. Fixed cardiac output cannot increase with exercise-induced vasodilation, causing cerebral hypoperfusion
Explanation
In severe aortic stenosis, cardiac output is fixed by the obstructed valve orifice and cannot appropriately increase during exercise. Peripheral vasodilation during exertion drops systemic vascular resistance, but output cannot compensate, resulting in a fall in mean arterial pressure and cerebral hypoperfusion. Options B and D may occur in advanced disease but are not the primary exertional syncope mechanism; option C is not the causative reflex in this setting.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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