Hypokalaemia and hyperkalaemia

Potassium is the dominant intracellular cation: about 98 percent of the body's store sits inside cells, and only a sliver lives in the small extracellular compartment the serum measures. This single fact explains almost everything about potassium disease. A child can lose hundreds of millimoles from the body and still show a near-normal serum level, because insulin, catecholamines and acid-base status keep shifting potassium back and forth across the cell membrane to defend that level. It also explains why a small shift out of cells — a little acidosis, a dose of digoxin, a fast-growing tumour breaking down — can move the serum potassium into the danger zone long before total-body potassium is high. [4] [1]

This page treats the two disorders as a mirror image. Hyperkalaemia is the faster killer — it stops the heart — so it gets the resuscitation framework: calcium, salbutamol, insulin-dextrose, removal. Hypokalaemia is the quieter danger — it weakens muscle and prolongs repolarisation — so it gets the replacement framework: oral versus intravenous, the rate limit, and magnesium. Both share the same organising logic: classify by the three axes, let the ECG set the tempo, and treat the cause while you treat the number. [1] [3]

Overview & Definition

Hypokalaemia is a serum potassium below 3.5 mmol/L. It is graded as mild (3.0 to 3.5), moderate (2.5 to 3.0) and severe (below 2.5); severe disease, or any level with symptoms or ECG changes, is an emergency. Hyperkalaemia is a serum potassium above 5.5 mmol/L. In the newborn the upper bound is slightly higher, but any level above 6.5 mmol/L, or any level with ECG changes at all, is a true emergency. Both thresholds are serum values, and both demand a repeat sample to exclude artefact before you act on an asymptomatic, mildly abnormal result. [1] [4]

The serum potassium is a concentration, not a content. It tells you how much potassium sits in the extracellular water at that instant, not how much the whole body holds. A child in diabetic ketoacidosis may walk in with a serum potassium of 5.0 mmol/L and a total-body deficit of 3 to 6 mmol/kg, because the acidosis has shifted intracellular potassium out into the blood while the urine has been washing it away for days. The number and the deficit point in opposite directions, and this is the trap that catches the unwary prescriber. [4] [3]

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11. [11]Assadi F; Mazaheri M; Rad EM Electrocardiography is Unreliable to Detect Potential Lethal Hyperkalemia in Patients with Non-dialysis Chronic Kidney Disease. Pediatr Cardiol, 2022.PMID 35389084