Hypokalaemia

• Robert Buttner and Ed Burns
• Jun 3, 2021

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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)

• Increased P wave amplitude
• Prolongation of PR interval
• Widespread ST depression and T wave flattening/inversion
• Prominent U waves (best seen in the precordial leads V2-V3)
• Apparent long QT interval due to fusion of 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

Hypokalaemia: T wave inversion and prominent U waves

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QU interval: The apparent pseudo-prolonged QT interval is actually the QU interval with an absent T wave

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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

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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

Classification of severity of hypokalaemia

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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

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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

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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.

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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