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

Explanation

## 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 - **Respiratory alkalosis** is the primary acid-base disturbance at high altitude due to hypoxia-driven hyperventilation and CO₂ loss. - **PaCO₂ < 35 mmHg** with **pH > 7.45** defines respiratory alkalosis; at altitude, PaCO₂ may drop to 25–30 mmHg acutely. - **Peripheral chemoreceptors** (not central) sense low PO₂ and trigger hyperventilation; central chemoreceptors respond to CO₂ and H⁺. - **Acute mountain sickness (AMS)** in Indian trekkers presents with headache, nausea, and dyspnea; respiratory alkalosis is the underlying acid-base change. - **Metabolic compensation** (HCO₃⁻ excretion by kidneys) develops over 24–48 hours; acute phase is purely respiratory alkalosis. ## 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)_