ECG Diagnosis: Hypokalemia

The earliest electrocardiogram (ECG) change associated with hypokalemia is a decrease in the T-wave amplitude.1 As potassium levels decline further, ST-segment depression and T-wave inversions are seen, while the PR interval can be prolonged along with an increase in the amplitude of the P wave.1 The U wave is described as a positive deflection after the T wave, often best seen in the mid-precordial leads (eg, V2 and V3). When the U wave exceeds the T-wave amplitude, the serum potassium level is < 3 mEq/L.2 In severe hypokalemia, T- and U-wave fusion with giant U waves masking the smaller preceding T waves becomes apparent on the ECG.1,2 A pseudo-prolonged QT interval may be seen, which is actually the QU interval with an absent T wave.1 Severe hypokalemia can also cause a variety of tachyarrhythmias, including ventricular tachycardia/fibrillation and rarely atrioventricular block.3 Treatment of hypokalemia involves parenteral and oral potassium supplementation, as well as identification and treatment of the underlying cause.1 Figure 1 12-lead ECG from a 21-year-old man with syncope, generalized weakness, and severe hypokalemia (serum potassium 1.6 mEq/L). Figure 2 12-lead ECG from same patient following oral and intravenous potassium replacement (serum potassium 2.5 mEq/L).