Hypokalaemia

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

T wave inversion and prominent U waves in hypokalaemia

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

Hypokalaemia:

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

This patient had a serum K+ of 1.7

Example 2

Hypokalaemia

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

The serum K+ was 1.9 mmol/L.

Example 3

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

Advanced Reading

Online

• Wiesbauer F, Kühn P. ECG Yellow Belt online course: Become an ECG expert. Medmastery
• Wiesbauer F, Kühn P. ECG Blue Belt online course: Learn to diagnose any rhythm problem. Medmastery
• Rawshani A. Clinical ECG Interpretation ECG Waves
• Smith SW. Dr Smith’s ECG blog.

Textbooks

• Mattu A, Tabas JA, Brady WJ. Electrocardiography in Emergency, Acute, and Critical Care. 2e, 2019
• Brady WJ, Lipinski MJ et al. Electrocardiogram in Clinical Medicine. 1e, 2020
• Straus DG, Schocken DD. Marriott’s Practical Electrocardiography 13e, 2021
• Hampton J. The ECG Made Practical 7e, 2019
• Grauer K. ECG Pocket Brain (Expanded) 6e, 2014
• Brady WJ, Truwit JD. Critical Decisions in Emergency and Acute Care Electrocardiography 1e, 2009
• Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric 6e, 2008
• Mattu A, Brady W. ECG’s for the Emergency Physician Part I 1e, 2003 and Part II
• Chan TC. ECG in Emergency Medicine and Acute Care 1e, 2004

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