Three patients, one night, hypokalemic metabolic alkalosis
Three patients are admitted on the same night with hypokalemic metabolic alkalosis. All have normal serum creatinine, and no diuretic is detected on the admission urine screen.
Patient 1: 71 years, four days of vomiting after gastroenteritis. BP 104/62 supine and 86/54 standing, pulse 106 on standing. Na 137, K 2.8, Cl 86, HCO₃⁻ 39, PCO₂ 48, creatinine 1.1 mg/dL. Urine Cl 7 mmol/L, urine K 12 mmol/L.
Patient 2: 34 years, known hypertension, no vomiting or diarrhoea. BP 168/98. Na 141, K 2.6, Cl 93, HCO₃⁻ 36, PCO₂ 47. Urine Cl 44 mmol/L, urine K 38 mmol/L. Plasma renin activity suppressed; aldosterone 31 ng/dL.
Patient 3: 26 years, three days of vomiting, no orthostasis. K 3.0, HCO₃⁻ 34, PCO₂ 45. Urine Cl 62 mmol/L, urine K 26 mmol/L. The specimen was collected 90 minutes after 2 L of 0.9% saline in the emergency department.
Serum magnesium is 1.9–2.1 mg/dL in all three.
1. Classify each patient's alkalosis as saline-responsive or saline-resistant, with the mechanism for each.
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Patient 1 is saline-responsive. Urine chloride below 20 mmol/L means the kidney is avidly reclaiming chloride, which is the signature of chloride and volume depletion. Vomiting loses HCl; the resulting bicarbonate load would normally be excreted, but the tubule cannot excrete bicarbonate without a counter-ion to accompany sodium, and chloride is the relevant one. Volume depletion also raises aldosterone, driving potassium and hydrogen secretion and sustaining the alkalosis. Treatment is 0.9% saline plus potassium chloride - the KCl matters as much as the saline, because potassium depletion itself drives hydrogen secretion and perpetuates the alkalosis.
Patient 2 is saline-resistant. Urine chloride above 20 mmol/L in a hypertensive patient with suppressed renin and a high aldosterone points to primary aldosteronism. The mineralocorticoid receptor is being stimulated directly, so expanding volume with saline does not correct the defect; it worsens the hypertension. Treatment is a mineralocorticoid receptor antagonist or surgery.
Patient 3 looks saline-resistant at first glance, with a urine chloride of 62, but that specimen was drawn after 2 L of saline. The infused chloride is being excreted, so the value describes the therapy, not the pathophysiology. The classification cannot be made from this sample.
2. How would you resolve Patient 3, and what alternative diagnoses would you then consider?
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Repeat the urine chloride after the saline has been off for a few hours, or use any pre-treatment specimen still available. Pre-analytic timing is the single biggest trap in urine electrolyte interpretation, and a value drawn during resuscitation should be treated as uninterpretable rather than as evidence.
If the repeat is below 20 mmol/L, the patient is simply volume and chloride depleted from vomiting and should follow Patient 1's path: saline plus KCl, with attention to magnesium.
If it remains above 40 mmol/L with normal blood pressure, think about the saline-resistant causes. Surreptitious diuretic use is the common one - the admission screen was negative, but screens are time-limited and a dose taken after admission would be missed; repeat it on a spot sample. Gitelman syndrome is the next consideration: it produces hypokalemic metabolic alkalosis with a high urine chloride and, crucially, low or low-normal serum magnesium with inappropriately high urinary magnesium and hypocalciuria, none of which is present here. Bartter syndrome is similar but typically presents earlier and with more profound potassium wasting. Current vomiting with ongoing losses can occasionally give a higher urine chloride than expected once volume is restored. Serum magnesium and urine calcium are the cheapest discriminating tests.
3. Check whether Patient 1's respiratory compensation is appropriate, and state what would change the interpretation.
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The expected PCO₂ for metabolic alkalosis is approximately 0.7 × HCO₃⁻ + 21, with an acceptance range of roughly ±5 mmHg.
For a bicarbonate of 39: expected PCO₂ = 0.7 × 39 + 21 = 48.3 mmHg. The measured value is 48, so compensation is entirely appropriate - there is no second respiratory disorder to explain.
The anion gap is also worth checking before stopping: Na − (Cl + HCO₃⁻) = 137 − (86 + 39) = 12 mmol/L, which is normal, so there is no hidden metabolic acidosis. Had it been elevated, the picture would be a mixed metabolic alkalosis plus a high-anion-gap acidosis. A normal gap in a hypoalbuminemic patient can still conceal an acidosis, so albumin should be checked and the gap corrected if it is low.
The interpretation changes in two ways. A PCO₂ substantially below the expected value (for example 34) would indicate an additional respiratory alkalosis and would prompt a search for pain, sepsis, hypoxaemia or salicylate exposure. A PCO₂ above about 60 is approaching the ceiling of compensation and suggests either a coexisting respiratory acidosis or that the alkalosis is more chronic and severe than the bicarbonate alone suggests.