A man is climbing a mountain for trekking. Based on his physiological response to the high altitude, what is the most likely primary acid-base abnormality in his blood?

Correct Answer: D. Respiratory alkalosis

At high altitude, atmospheric oxygen partial pressure (PO₂) decreases, leading to hypoxemia. The carotid and aortic chemoreceptors sense this drop in arterial oxygen saturation and trigger the respiratory centre to increase minute ventilation—a process called hypoxic ventilatory response. This hyperventilation causes excessive CO₂ elimination from the lungs, reducing arterial PaCO₂ below the normal range (35–45 mmHg). Since CO₂ is an acid, its loss raises blood pH above 7.45, defining respiratory alkalosis. This is the PRIMARY acid-base abnormality in the acute phase of high-altitude exposure. The kidneys may later compensate by increasing bicarbonate excretion, but the initial and dominant disturbance is respiratory alkalosis. This occurs within minutes to hours of altitude exposure, before metabolic compensation becomes significant. Indian trekkers ascending to Himalayan altitudes (>2500 m) commonly experience this response as part of acute mountain sickness (AMS) prodrome.

Why the other options are wrong

A. Respiratory acidosis — Respiratory acidosis (elevated PaCO₂, low pH) occurs when ventilation is inadequate, not excessive. At altitude, the hypoxic drive increases ventilation, causing CO₂ loss—the opposite of what causes respiratory acidosis. This trap confuses students who think 'altitude = breathing problem = acidosis,' ignoring the direction of ventilatory change. B. Metabolic acidosis — Metabolic acidosis requires loss of bicarbonate or accumulation of organic acids (lactate, ketones). While prolonged altitude exposure may eventually trigger anaerobic metabolism and lactic acidosis, the primary disturbance is respiratory alkalosis from hyperventilation. Metabolic acidosis is a secondary or late phenomenon, not the initial acid-base abnormality. C. Metabolic alkalosis — Metabolic alkalosis (elevated HCO₃⁻, high pH) requires loss of acid or gain of base. At altitude, the kidneys do eventually excrete bicarbonate to compensate for respiratory alkalosis, but this is a compensatory response, not the primary abnormality. The question asks for the primary disturbance, which is respiratory in origin.

High-Yield Facts

Mnemonics

HIGH ALTITUDE = HYPERventilation = ALKalosis ↑ Altitude → ↓ O₂ → ↑ Breathing → ↓ CO₂ → ↑ pH = Respiratory ALKalosis. Use when asked about primary acid-base at altitude. CAPO mnemonic for altitude response Chemoreceptors sense hypoxia → Alveolar hyperventilation → PaCO₂ drops → Outcome = Respiratory alkalosis. Helps recall the causal chain.

NBE Trap

NBE pairs 'altitude' with 'breathing difficulty' to lure students into selecting respiratory acidosis, ignoring that altitude triggers hyperventilation (excessive breathing), not hypoventilation. The trap exploits confusion between the stimulus (hypoxemia) and the response (increased ventilation).

Clinical Pearl

Indian trekkers ascending to Ladakh or Nepal above 3500 m often report headache and dizziness within 6–12 hours—these are early signs of respiratory alkalosis and hypoxemia, not dehydration. Recognition of this acid-base shift guides preventive measures (slow ascent, acetazolamide) and helps differentiate AMS from other causes of altitude illness.

_Reference: Guyton & Hall Textbook of Medical Physiology, Ch. 42 (Respiration); Harrison's Principles of Internal Medicine, Ch. 298 (High-Altitude Medicine)_