Hypokalaemia

Background

• 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

Definitions

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

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

• Hyperkalaemia
• Hypercalcaemia
• Hypocalcaemia
• Hypomagnesaemia

Further Reading

• ECG BASICS – Waves, Intervals, Segments and Clinical Interpretation
• ECG CLINICAL CASES – Your favourite ECG’s placed in clinical context with a challenging Q&A approach
• ECG and Cardiology Eponymous Syndromes – Cheats guide to eponymous emancipation
• ECG Reference Sites on the WEB – the best of the rest

Author Credits

• Words – Ed Burns
• Pictures – Ed Burns
• Web Editing – Ed Burns

References

• 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.