The important electrolytes as far as the electrocardiogram is concerned (Figure) are potassium and calcium and the relative concentration of these substances to each other. Acidosis also has an effect. Other electrolytes, such as sodium and magnesium, which are known to alter the ECG in experimental animals, do not produce changes in man because they do not reach the low levels of concentration in man as are seen in the experimental situations.
Hyperkalemia has been shown to depress the atria, the A-V node, and the ventricles but has less effect on the sinus node. As a consequence, increasing concentrations of potassium produce the following: prolonged P-R interval; low P waves; sino-atrial block with small or absent mechanical contractions of the atria; tenting of T waves (unusually tall and narrow at the base); intraventricular block with widening of the QRS complexes; abnormal S-T segment shifts; and finally ventricular fibrillation and ventricular
Winsor T. Electrolyte Abnormalities and the Electrocardiogram. JAMA. 1968;203(5):347. doi:10.1001/jama.1968.03140050031007
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