A 19-year-old medical student faints while standing for a long duration during a blood donation camp. Just before losing consciousness she feels nausea and sweating, then collapses. On lying flat she recovers within seconds. The immediate event producing her loss of consciousness was:
- A Sudden sympathetic withdrawal with paradoxical vagal activation causing bradycardia and peripheral vasodilatation ✓
- B Massive sympathetic discharge causing severe hypertension and cerebral vasoconstriction
- C Failure of cerebral autoregulation at a mean arterial pressure of 95 mmHg
- D Acute rise in intracranial pressure compressing cerebral vessels
Correct answer: A. Sudden sympathetic withdrawal with paradoxical vagal activation causing bradycardia and peripheral vasodilatation
Explanation
Vasovagal syncope combines emotional or postural triggers with vigorous cardiac contraction of an underfilled ventricle. Mechanoreceptor afferents then produce a paradoxical burst of vagal activity and loss of sympathetic tone, giving bradycardia plus marked venous and arteriolar dilatation, so cerebral perfusion falls abruptly. Cerebral autoregulation maintains flow down to a mean arterial pressure near 60 mmHg, so it does not fail at 95 mmHg. Recovery on lying flat restores venous return immediately.
Reference: Ganong's Review of Medical Physiology, 26th ed.
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Written and medically reviewed by the StethoPrep medical team.