A 55-year-old man with chronic duodenal ulcer presents with weeks of vomiting, weight loss, and visible gastric peristalsis. Arterial blood gas shows pH 7.52, bicarbonate 38 mmol/L, sodium 138 mmol/L, chloride 88 mmol/L, and potassium 3.0 mmol/L. Late in the illness his urinary pH becomes unexpectedly acidic despite the alkalosis. What explains the paradoxical aciduria?

• A Renal tubular acidosis secondary to chronic hypokalaemia
• B Volume depletion with aldosterone-driven hydrogen ion reabsorption in exchange for sodium conservation ✓
• C Loss of bicarbonate in vomitus exceeding renal generation
• D Reduced ammoniagenesis from impaired hepatic perfusion

Explanation

Gastric outlet obstruction causes loss of hydrochloric acid, producing hypochloraemic hypokalaemic metabolic alkalosis. Early urine is alkaline from bicarbonate diuresis with sodium and potassium wasting. Once volume depletion becomes severe, aldosterone-mediated sodium retention forces hydrogen excretion in exchange, so the urine turns acid despite systemic alkalosis. This paradoxical aciduria signals serious volume depletion demanding aggressive saline and potassium replacement before definitive surgery.

Reference: Bailey and Love's Short Practice of Surgery, 27th ed.

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Written and medically reviewed by the StethoPrep medical team.