Hypokalaemia

Hypokalaemia is defined as a serum potassium level of < 3.5 mmol/L. ECG changes generally do not manifest until there is a moderate degree of hypokalaemia (2.5-2.9 mmol/L). The earliest ECG manifestation of hypokalaemia is a decrease in T wave amplitude.

ECG features of hypokalaemia (K < 2.7 mmol/L)

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

The push-pull effect

• Hypokalaemia creates the illusion that the T wave is “pushed down”, with resultant T-wave flattening/inversion, ST depression, and prominent U waves
• In hyperkalaemia, the T wave is “pulled upwards”, creating tall “tented” T waves, and stretching the remainder of the ECG to cause P wave flattening, PR prolongation, and QRS widening

Pathophysiology

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.

Degree of hypokalaemia	Potassium level (mmol/L)
Mild	3.0 – 3.4
Moderate	2.5 – 2.9
Severe	≤ 2.4

Handy tips

• Hypokalaemia is often associated with hypomagnesaemia, which increases the risk of malignant ventricular arrhythmias
• Check both potassium and magnesium levels in any patient with an arrhythmia
• Replace potassium to ≥ 4.0 mmol/L and 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:

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

This patient had a serum K+ of 1.7

Example 2

Hypokalaemia

• ST depression and T wave inversion best noted in inferior leads
• 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 Mastery: 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
• Smith SW. The ECG in Acute MI. 2002 [PDF]

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