Can the ECG diagnose electrolyte abnormalities?

ECG abnormalities due to electrolyte imbalance, particularly potassium and calcium, have been evident for years. However, because multiple electrolyte may be involved and because of the patient’s underlying disease or even drug effects, the ECG should not be used in lieu of direct laboratory electrolyte determination. When an abnormality has been identified, the ECG may be used as a guide to the effectiveness of therapy.

The classic ECG changes of hyperkalemia are tall, narrow, peaked T waves, intraventricular conduction defect, decreased amplitude or absence of P waves, bradyarrhythmias, and AV blocks (see figure on p. 22). Hypokalemia causes ST-segment depression, decreased T amplitude, prominent U waves, cardiac arrhythmias, and rarely QRS prolongation. Serum calcium primarily alters the QT interval, with calcium excess causing shortening and deficiency causing prolongation. Currently, serum magnesium levels are assuming importance, but unfortunately are unlikely to be detected by the ECG.

Mild hyperkalemia. Note the tall, peaked T waves, most prominent in V2-V5. (From Seelig CB: Simplified EKG Analysis, Philadelphis, Hanley & Belfus, 1992, pp 107-110, with permission.)

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