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
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| Fig 1. ECG Changes in Mild Hypokalaemia. [Source] |
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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
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| Fig 2. T wave inversion and prominent U waves in hypokalaemia |
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| Fig 3. Apparent long QT interval with hypokalaemia (actually T-U fusion) |
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| Fig 4. Hypokalaemia: Note the prominent U waves in the precordial leads and apparent long QT in the limb leads |
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| 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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