Hypokalaemia Overview

Potassium is vital for regulating the normal electrical activity of the heart. Decreased extracellular potassium causes myocardial hyperexcitability with the potential to develop re-entrant arrhythmias.

  • Hypokalaemia is defined as a potassium level < 3.5 mmol/L
  • Moderate hypokalaemia is a serum level of < 3.0 mmol/L
  • Severe hypokalaemia is defined as a level < 2.5 mmol/L

Effects of hypokalaemia on the ECG

ECG changes when K+ < 2.7 mmol/l

  • Increased amplitude and width of the P wave
  • Prolongation of the PR interval
  • T wave flattening and inversion
  • ST depression
  • Prominent U waves (best seen in the precordial leads)
  • Apparent long QT interval due to fusion of the T and U waves (= long QU interval)

With worsening hypokalaemia…

  • Frequent supraventricular and ventricular ectopics
  • Supraventricular tachyarrhythmias: AF, atrial flutter, atrial tachycardia
  • Potential to develop life-threatening ventricular arrhythmias, e.g. VT, VF and Torsades de Pointes
ECG U wave Hypokalemia

T wave inversion and prominent U waves in hypokalaemia

ECG QU interval Hypokalemia

Long QU interval in hypokalaemia

Handy tips
  • Hypokalaemia is often associated with hypomagnesaemia, which increases the risk of malignant ventricular arrhythmias
  • Check potassium and magnesium in any patient with an arrhythmia
  • Top up the potassium to 4.0-4.5 mmol/l and the magnesium to > 1.0 mmol/l to stabilise the myocardium and protect against arrhythmias – this is standard practice in most CCUs and ICUs

ECG Examples
Example 1
ECG severe hypokalemia serum potassium 1.7


  • ST depression.
  • T wave inversion.
  • Prominent U waves.
  • Long QU interval.

This patient had a serum K+ of 1.7

Example 2
ECG severe hypokalemia serum potassium 1.9


  • ST depression.
  • T wave inversion.
  • Prominent U waves.
  • Long QU interval.

The serum K+ was 1.9 mmol/L.

Example 3
ECG severe hypokalemia serum potassium 2.0 TDP

Hypokalaemia causing Torsades de Pointes

  • Another ECG from the same patient.
  • Note the atrial ectopic causing ‘R on T’ (or is it ‘R on U’?) that initiates the paroxysm of TdP

Advanced Reading



LITFL Further Reading




Emergency Physician in Prehospital and Retrieval Medicine in Sydney, Australia. He has a passion for ECG interpretation and medical education | ECG Library |

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