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
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
Electrocardiogram
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