• 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

Changes appear when K+ falls below about 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

T wave inversion and prominent U waves in hypokalaemia

Apparent long QT interval with hypokalaemia (actually T-U fusion)

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

Severe hypokalaemia


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

This patient had a serum K+ of 1.7

Example 2


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

The serum K+ was 1.9 mmol/L. 

Example 3

Hypokalaemia Torsades de Pointes

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

Related Topics

Further Reading


  • Ganong, WF. Review of Medical Physiology (22nd edition). Lange / McGraw-Hill 2005.
  • Hampton, JR. The ECG in Practice (5th edition), Churchill Livingstone 2008.
  • Phibbs BP. Advanced ECG: Boards and Beyond (second edition). Elsevier 2006.
  • Slovis C, Jenkins R. ABC of clinical electrocardiography: Conditions not primarily affecting the heart. BMJ. 2002 Jun 1;324(7349):1320-3. Review. PMID: 12039829. Full text.
  • Wagner, GS. Marriott’s Practical Electrocardiography (11th edition), Lippincott Williams & Wilkins 2007.
Print Friendly