What acid-base disturbance is expected in a person trekking at high altitude due to hyperventilation?

Correct Answer: D. Respiratory alkalosis

At high altitude, the partial pressure of oxygen (PO₂) in the atmosphere decreases, triggering hypoxemia. The peripheral chemoreceptors sense this drop in PO₂ and stimulate the respiratory center to increase ventilation rate—a compensatory mechanism called hypoxic ventilation. This hyperventilation causes excessive elimination of CO₂, reducing the partial pressure of arterial CO₂ (PaCO₂) below the normal range (35–45 mmHg). According to acid-base physiology, a primary decrease in PaCO₂ with an increase in pH defines respiratory alkalosis. The kidneys attempt to compensate by increasing bicarbonate (HCO₃⁻) excretion, but this metabolic compensation lags behind the acute respiratory change. In Indian trekkers ascending to high-altitude regions (e.g., Ladakh, Himalayas), acute mountain sickness (AMS) is common, and respiratory alkalosis is the initial acid-base disturbance. The mechanism is purely respiratory (hyperventilation-driven CO₂ loss), not metabolic, making this a primary respiratory process. This is distinct from chronic altitude adaptation, where metabolic compensation becomes more prominent over days.

Why the other options are wrong

A. Metabolic alkalosis — This is wrong because metabolic alkalosis requires a primary elevation in HCO₃⁻ or loss of H⁺ ions (e.g., from vomiting, diuretics, or contraction alkalosis). At high altitude, the acid-base disturbance is respiratory in origin—driven by hyperventilation, not by metabolic loss of acid or retention of base. Metabolic alkalosis may occur secondarily during chronic altitude acclimatization, but it is not the primary disturbance in acute hyperventilation. B. Respiratory acidosis — This is wrong because respiratory acidosis results from hypoventilation and CO₂ retention (elevated PaCO₂). At high altitude, the opposite occurs: hyperventilation causes CO₂ loss and a decrease in PaCO₂. This is an NBE trap—students may confuse altitude-related breathing difficulty with respiratory depression, but the actual physiological response is increased ventilation, not decreased ventilation. C. Metabolic acidosis — This is wrong because metabolic acidosis requires a primary decrease in HCO₃⁻ or accumulation of H⁺ ions (e.g., from lactic acidosis, ketoacidosis, or renal failure). At high altitude, hyperventilation causes CO₂ loss, which raises pH acutely. Although hypoxia may trigger anaerobic metabolism and lactic acidosis in severe cases, the immediate acid-base disturbance from hyperventilation is alkalosis, not acidosis.

High-Yield Facts

Mnemonics

HIGH = Hyperventilation → Increased Gases Expelled → Hypocapnia At HIGH altitude, HYPERventilation causes CO₂ loss → HYPOcapnia → alkalosis. Use when you see 'altitude + hyperventilation' to lock in respiratory alkalosis. CHEMO: Chemoreceptors Hypoxia Elevated Minute ventilation → Oxygen Peripheral chemoreceptors sense hypoxia → drive minute ventilation up → CO₂ out → respiratory alkalosis. Helps distinguish hypoxia-driven hyperventilation from other causes.

NBE Trap

NBE may pair 'altitude' with 'breathing difficulty' to lure students into thinking respiratory acidosis (hypoventilation). However, the physiological response to hypoxia is hyperventilation, not hypoventilation—the trap is confusing symptom (dyspnea) with mechanism (increased ventilation).

Clinical Pearl

Indian trekkers ascending to Ladakh or high Himalayan passes often develop AMS within 6–12 hours. The hallmark triad—headache, nausea, dyspnea—reflects respiratory alkalosis from hypoxia-driven hyperventilation. Acetazolamide (a carbonic anhydrase inhibitor) is the standard prophylaxis in Indian mountaineering, as it induces metabolic acidosis to counteract respiratory alkalosis and reduce AMS incidence.

_Reference: Guyton & Hall Textbook of Medical Physiology, Ch. 42 (Regulation of Respiration); Harrison's Principles of Internal Medicine, Ch. 297 (Acute Mountain Sickness)_