In Cushing's reflex (vasopressor response to raised ICP), the triad of hypertension, bradycardia and irregular breathing results from:

• A Direct compression of the vagus nerve at the jugular foramen causing parasympathetic excess
• B Hypothalamic stimulation by cerebellar herniation releasing excess cortisol and catecholamines
• C Ischaemia of the vasomotor centre triggering intense sympathetic discharge; hypertension then activates baroreceptors causing reflex bradycardia ✓
• D Cerebral venous congestion stimulating aortic chemoreceptors to raise blood pressure

Explanation

When ICP rises to approach systemic MAP, cerebral perfusion pressure (CPP = MAP − ICP) falls critically, causing ischaemia of the brainstem vasomotor centre (VMC) in the medulla. The ischaemic VMC responds with maximal sympathetic outflow, producing intense systemic vasoconstriction and a dramatic rise in MAP (the vasopressor response). The acute hypertension activates baroreceptors, causing reflex bradycardia. Irregular (Biot's) breathing reflects involvement of the medullary respiratory centres by the same ischaemia/herniation. This triad is a late, ominous sign of transtentorial herniation.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.