Situation: A 57-year-old woman is on post-operative day 3 after surgery for a small bowel obstruction. She has a nasogastric (NG) tube on low intermittent suction that drained 1,800 mL in the past 24 hours, and she receives 5% dextrose in water at 80 mL/h. She is on subcutaneous enoxaparin to prevent venous thrombosis. This morning she reports numbness and tingling of her fingers. Her serum potassium is 3.1 mEq/L (normal 3.5–5.0 mEq/L). Besides the potassium lost in gastric fluid, which mechanism is lowering her serum potassium?

Acid-base changes shift potassium across cell membranes: in acidosis potassium moves out of cells (hyperkalemia), and in alkalosis it moves into cells (hypokalemia). Her metabolic alkalosis therefore adds to the potassium she loses in gastric fluid, and volume depletion also stimulates aldosterone, which increases potassium loss in the urine. Replacing volume with 0.9% sodium chloride and potassium chloride helps correct both problems.

Clinical context

This patient has several simultaneous processes that explain why her serum potassium is 3.1 mEq/L (normal 3.5–5.0 mEq/L). The NG tube on low intermittent suction has drained 1,800 mL of gastric fluid in 24 hours, and gastric secretions are rich in hydrochloric acid. Losing large volumes of HCl generates a metabolic alkalosis, and that acid-base disturbance directly shifts potassium across cell membranes.

Why alkalosis lowers serum potassium

Potassium is the predominant intracellular cation, and its distribution between the intracellular and extracellular compartments is tightly linked to acid-base status ^[1]. Hydrogen ions and potassium ions exchange across the cell membrane to buffer pH changes. In metabolic alkalosis, hydrogen ions move out of cells into the extracellular fluid to help correct the elevated pH. To maintain electroneutrality, potassium moves from the extracellular fluid into the cells, which lowers the measured serum potassium even though total-body potassium may already be depleted from gastric losses ^[1]. This transcellular shift is the mechanism asked for in the question, and it is distinct from the direct potassium loss through NG drainage.

Why the other options are incorrect

Option	Mechanism	Why it does not fit this patient
1. Acidosis drives potassium out of cells and into the urine	In acidosis, hydrogen ions enter cells and potassium exits cells, raising serum potassium	This patient has alkalosis from gastric HCl loss, not acidosis. The direction of potassium shift is opposite.
2. Rapid breathing lowers carbon dioxide and moves potassium into cells	Respiratory alkalosis can shift potassium intracellularly	There is no evidence of hyperventilation or a primary respiratory alkalosis. The primary disorder here is metabolic alkalosis from upper GI losses.
4. Enoxaparin blocks aldosterone and causes potassium loss	Enoxaparin can rarely suppress aldosterone, but this leads to potassium retention, not loss	Low-molecular-weight heparin may reduce aldosterone synthesis in some patients, which would raise serum potassium. It does not cause potassium wasting.

Putting the whole picture together

The patient loses potassium through two routes. First, gastric fluid itself contains potassium, so NG suction directly removes potassium from the body. Second, the loss of HCl produces metabolic alkalosis, and the alkalosis shifts potassium from the extracellular fluid into cells, further lowering the serum concentration ^[1]. In addition, the volume depletion caused by NG drainage activates the renin-angiotensin-aldosterone system, and aldosterone promotes renal potassium excretion. However, the question specifically asks for the mechanism besides direct gastric potassium loss, and the correct answer is the transcellular shift driven by alkalosis.

Key point! Acid-base disorders change the serum potassium independently of total-body potassium stores. Metabolic alkalosis lowers serum potassium by moving potassium into cells, while metabolic acidosis raises it by moving potassium out of cells ^[1]. This is why a patient with alkalosis can have a low serum potassium even before renal losses are fully accounted for.

Watch out! Serum potassium reflects only a small fraction of total-body potassium, so a normal or mildly low serum value may still represent significant intracellular and total-body depletion ^[1]. In this patient, the 3.1 mEq/L value likely underestimates the true potassium deficit because ongoing alkalosis keeps potassium shifted into cells. Correction requires both chloride-containing volume replacement with 0.9% sodium chloride to reverse the alkalosis and direct potassium chloride supplementation.