Vomiting, Diuretics, and a Chloride-Responsive Alkalosis
A 58-year-old man with hypertension treated with hydrochlorothiazide 25 mg daily presents after 3 days of profuse vomiting and poor oral intake. He is drowsy but oriented. BP 102/64 mmHg, pulse 112/min, mucous membranes dry, skin turgor reduced; JVP is low, lungs are clear, and there is no oedema. Laboratory results: sodium 132 mmol/L, potassium 2.9 mmol/L, chloride 88 mmol/L, bicarbonate 38 mmol/L, pH 7.48, PaCO2 48 mmHg, PaO2 82 mmHg, creatinine 1.4 mg/dL (baseline 0.9), urine sodium 18 mmol/L, urine potassium 32 mmol/L, urine chloride 8 mmol/L. He denies diarrhoea and has not taken NSAIDs or laxatives. His wife reports that he has taken his antihypertensive only intermittently. There is no history of renal failure, diabetes, or other diuretic use. A medical student asks whether the acid-base disturbance is fully explained by vomiting alone and whether immediate bicarbonate would be appropriate.
1. Calculate the anion gap and assess whether respiratory compensation is appropriate. What is the acid-base diagnosis?
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Step 1 - identify the primary disorder. pH 7.48 is alkalemia, and bicarbonate 38 mmol/L is elevated, so the primary disorder is metabolic alkalosis.
Step 2 - check compensation. Expected PaCO2 = 0.7 × HCO3 + 20 ± 5 = 0.7 × 38 + 20 = 46.6 mmHg. The measured PaCO2 of 48 mmHg falls within ±5 mmHg of this value, so respiratory compensation is appropriate.
Step 3 - calculate the anion gap. AG = Na − (Cl + HCO3) = 132 − (88 + 38) = 6 mmol/L. This is normal, so the alkalosis is not accompanied by a high-anion-gap metabolic acidosis.
Conclusion: hypochloremic, hypokalemic, chloride-responsive metabolic alkalosis caused by vomiting plus intermittent thiazide use, with appropriate respiratory compensation. The normal PaO2 and the absence of hypercapnia beyond the expected range argue against a superimposed respiratory disorder.
2. Explain the low urine chloride and the high urine potassium. Why would bicarbonate therapy be harmful here?
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Vomiting loses hydrochloric acid and sodium chloride, producing volume depletion and chloride deficiency. The kidney avidly conserves sodium and chloride, so urine chloride falls below 20 mmol/L - a marker of a chloride-responsive alkalosis. Volume depletion also activates the renin-angiotensin-aldosterone axis. Aldosterone increases distal sodium reabsorption and drives potassium and hydrogen secretion. Because bicarbonate is a non-reabsorbable anion in the distal tubule, it increases the negative luminal charge and promotes further potassium wasting; the result is high urine potassium and paradoxical aciduria despite systemic alkalosis.
Bicarbonate would worsen the alkalosis and shift potassium intracellularly, increasing the risk of arrhythmia and tetany. It is not indicated for this chloride-responsive alkalosis. The correct treatment is volume and chloride repletion with 0.9% saline plus potassium chloride; bicarbonate is reserved for specific situations such as severe alkalemia with organ dysfunction or ongoing renal bicarbonate loss, neither of which is present here.
3. Outline your initial fluid and electrolyte prescription for the first 24 hours and the monitoring you would order.
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Give 0.9% sodium chloride 1 L over the first 2 hours, reassessing BP, pulse, urine output and pulmonary exam after each 500 mL; then continue 0.9% saline at approximately 100–150 mL/hour, adjusted to urine output and haemodynamics. Add potassium chloride 20–40 mmol/L to the maintenance fluid, aiming for serum potassium above 3.5 mmol/L, and check magnesium because hypomagnesemia perpetuates hypokalemia. Avoid dextrose-only fluids, which can worsen hyponatremia and do not correct chloride depletion.
Monitor BP, pulse, urine output, and neurological status hourly initially. Check sodium, potassium, chloride, bicarbonate, creatinine and magnesium every 4–6 hours until stable, with ECG monitoring if potassium falls further or arrhythmia develops. Expect bicarbonate to fall as saline corrects volume and chloride; do not try to normalise it rapidly. If oliguria or worsening renal function develops, reassess the fluid rate and consider central venous access and urinary catheterisation.