Renal & Urology

Hypokalaemia

Hypokalaemia is a serum potassium below 3.5 mmol/L, usually from gastrointestinal or renal loss or a transcellular shift; assess the ECG and symptoms urgently, replace potassium safely, correct magnesium and treat the cause.

In a nutshell

Hypokalaemia is serum potassium below 3.5 mmol/L. Think loss or intracellular shift, check magnesium and ECG, and treat severity, symptoms and cause together. Oral potassium is preferred when safe; severe or symptomatic disease, ECG changes or inability to absorb usually needs monitored IV replacement under current BNF/local policy.

Classic presentation

A patient taking a thiazide has weakness and palpitations, a potassium of 2.7 mmol/L, low magnesium and U waves: monitor, replace potassium and magnesium safely, and review the diuretic and cause.

Key points

The ECG may be normal despite clinically important hypokalaemia; assess it alongside symptoms and risk.
Low magnesium can make potassium replacement ineffective.
Use oral potassium when the patient can swallow and absorb it; reserve IV replacement for severe, symptomatic or non-enteral situations.
Never give concentrated IV potassium as a rapid bolus; use a prepared infusion, pump and the current local/BNF protocol.
In DKA, potassium can fall rapidly after insulin starts, so follow the relevant NICE protocol and monitor frequently.
Hypokalaemic cardiac arrest has a separate Resuscitation Council UK emergency algorithm.

First-line investigation

Repeat serum potassium with U&E, renal function, magnesium and acid-base assessment, plus a 12-lead ECG.

Management

Assess cardiac and neuromuscular risk

Urgently escalate severe, symptomatic or ECG-positive hypokalaemia, arrhythmia, weakness with respiratory involvement, renal impairment or rapidly falling potassium for monitored replacement.^1,4

Confirm and classify the cause

Repeat potassium, measure magnesium and renal function, obtain an ECG, and use the history, medication review, acid-base status and urinary potassium when the cause is unclear.^1,3,7

Replace potassium safely

Use oral replacement when safe; use prepared, glucose-free IV potassium under current BNF/local safeguards when severe, symptomatic or unable to absorb. Correct magnesium concurrently and monitor potassium, magnesium, renal function and ECG as indicated.^1,4,2

Treat the cause and prevent recurrence

Review diuretics and other medicines, treat gastrointestinal losses, follow DKA or refeeding protocols where relevant, and seek renal/endocrine input for persistent, recurrent or hypertensive hypokalaemia.^5,10,3,8

Exam traps

Always check magnesium when hypokalaemia is unexplained or resistant.
IV potassium is a high-risk medicine: use a prepared infusion and never give a concentrated rapid bolus.
A low potassium during DKA is a treatment emergency because insulin can drive it lower.
Hypokalaemia plus hypertension suggests a renal or mineralocorticoid cause, not simply poor dietary intake.
Do not use the cardiac-arrest potassium regimen for routine ward replacement.
In renal impairment or oliguria, standard replacement can cause dangerous hyperkalaemia.

Illustrations

ECG changes in hypokalaemiaAn ECG strip showing flattened T waves, ST depression and prominent U waves after the T wave, the characteristic pattern of hypokalaemia.James Heilman, MD, Wikimedia Commons · CC-BY-SA-3.0

Key sources

This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.