A mountaineer at 5500 m altitude develops periodic (Cheyne-Stokes) breathing during sleep. The PRIMARY physiological trigger is:

• A Hypocapnia from hyperventilation reducing CO2 below the apnoeic threshold ✓
• B Hypoxic depression of the respiratory centre causing central apnoea
• C Increased loop gain due to high carotid body sensitivity and prolonged circulation time
• D Pulmonary oedema causing intermittent airway obstruction

Explanation

At altitude, hypoxia drives hyperventilation, which washesout CO2. PaCO2 falls below the apnoeic threshold of the central respiratory centres, triggering a central apnoea. During the apnoea PaCO2 rises and PaO2 falls, restarting ventilation — the cycle repeats as periodic breathing. This is primarily a CO2-mediated phenomenon (loss of hypercapnic drive), not hypoxic depression itself. Loop gain is increased secondarily but the initiating trigger is hypocapnia.

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

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