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Home | ECG Library | ECG Diagnosis | Hypokalaemia

Hypokalaemia

by Dr Ed Burns, last update March 16, 2019

↪  ECG Library Homepage

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

Hypokalaemia:

  • 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

Hypokalaemia

  • 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

Related Topics

  • Hyperkalemia
  • Hypokalemia
  • Hypercalcemia
  • Hypocalcemia
  • Hypomagnesemia

LITFL Further Reading

  • ECG Library Basics – Waves, Intervals, Segments and Clinical Interpretation
  • ECG A to Z by diagnosis – ECG interpretation in clinical context
  • ECG Exigency and Cardiovascular Curveball – ECG Clinical Cases
  • 100 ECG Quiz – Self-assessment tool for examination practice
  • ECG Reference SITES and BOOKS – the best of the rest

Advanced Reading

  • Brady WJ, Truwit JD. Critical Decisions in Emergency and Acute Care Electrocardiography
  • Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric
  • Wagner GS. Marriott’s Practical Electrocardiography 12e
  • Chan TC. ECG in Emergency Medicine and Acute Care
  • Rawshani A. Clinical ECG Interpretation
  • Mattu A. ECG’s for the Emergency Physician
  • Hampton JR. The ECG In Practice, 6e

ECG LIBRARY 700

ECG LIBRARY

Electrocardiogram

EKG Library

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About Dr Ed Burns

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

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