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Box 86.1

Disorders of Plasma Potassium Concentration

Plasma potassiumconcentration (mmol/L)ClassificationClinical and ECG features
>6.5Severe hyperkalaemia

Often asymptomatic. Muscle weakness may occur.

ECG: tall peaked T wave with shortened QT interval Progressive lengthening of the PR interval and QRS duration Sine wave QRS pattern Ventricular standstill or fibrillation.

5.5–6.5Moderate hyperkalaemiaMay be asymptomatic.
5.0–5.5Mild hyperkalaemiaMay be asymptomatic.
3.5–5.0Normal range
3.0–3.5Mild hypokalaemiaMay be asymptomatic.
2.5–3.0Moderate hypokalaemiaECG: arrhythmias may occur, especially in patients with underlying cardiac disorders or taking antiarrhythmic drugs.
<2.5Severe hypokalaemia

Muscle necrosis may occur. Below 2 mmol/L there may be ascending paralysis, resembling Guillain-Barré syndrome. Ileus.

ECG: ST depression, reduced T wave amplitude, increased U wave amplitude.

Arrhythmias commonly occur: extrasystoles, atrial fibrillation, atrioventricular block, ventricular tachycardiac and fibrillation.