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Causes and ECG Changes in Hypokalaemia

Hypokalaemia is caused by potassium-wasting diuretics, potassium-wasting diarrhoea and hypokalaemic periodic paralysis.

The hallmark of the effect of hypokalaemia on the ECG is the development of large U waves (positive deflection after the T wave). The normal U wave is produced by repolarisation of the His–Purkinje system.


ECG Changes in Hypokalaemia

Fig 1. ECG Changes in Mild Hypokalaemia. [Source]

ECG changes in hypokalaemia 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
With worsening hypokalaemia, these ECG changes occur:
  • 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

Fig 2. T wave inversion and prominent U waves in hypokalaemia
Fig 3. Apparent long QT interval with hypokalaemia (actually T-U fusion)
Fig 4. Hypokalaemia: Note the prominent U waves in the precordial leads and apparent long QT in the limb leads

Fig 5. Hypokalaemia leading to Torsades de Pointes: Note the atrial ectopic causing 'R on T' (or is it 'R on U'?) that initiates the paroxysm of TdP
Fig 2-5 Source


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