During the diving reflex (triggered by cold water facial immersion), the coordinated autonomic response includes:

• A Tachycardia with peripheral vasodilation to increase cerebral blood flow
• B Sympathetic activation causing tachycardia and redistribution to skeletal muscle
• C Apnoea with simultaneous cardiac arrest to conserve all oxygen
• D Bradycardia mediated by the vagus with peripheral vasoconstriction conserving O2 for vital organs ✓

Explanation

The mammalian diving reflex is a powerful O2-conserving response. Trigeminal nerve afferents from facial cold receptors (especially around the nose) trigger: (1) apnoea via respiratory centre inhibition, (2) vagally-mediated bradycardia (heart rate can fall >50%), and (3) sympathetic peripheral vasoconstriction shunting blood to the heart and brain while reducing flow to less vital organs. This reflex is exploited therapeutically to terminate SVT using ice water facial immersion.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.