During a tilt-table test, a patient develops profound hypotension and bradycardia when tilted to 70 degrees upright. Which reflex mechanism is responsible for this vasovagal response?
- A Activation of cardiac C-fibers (Bezold-Jarisch reflex) by vigorous contraction of a volume-depleted ventricle ✓
- B Carotid sinus hypersensitivity from mechanical distortion of the neck
- C Peripheral chemoreceptor activation by venous pooling-induced hypoxemia
- D Baroreceptor resetting to a lower operating range
Explanation
In vasovagal syncope on upright tilt, venous pooling reduces ventricular filling; the nearly empty ventricle contracts vigorously, activating mechanosensitive C-fiber afferents (Bezold-Jarisch reflex). These fibers travel via the vagus to the NTS, producing paradoxical withdrawal of sympathetic tone and vagal bradycardia, causing the syncopal episode. Carotid sinus hypersensitivity (option B) would be triggered by neck pressure, not upright tilting. Peripheral chemoreceptors are not activated by simple venous pooling. Baroreceptor resetting is a chronic adaptation, not an acute syncope mechanism.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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