Hypokalemia and ECG

Although the presence of U waves may be normal in many people, these waves also may be an early sign of hypokalemia (Fig. 17-37). Usually easily recognized (best seen in lead V3), a U wave may encroach on the preceding T wave and go unnoticed. The T wave may look notched or prolonged when it is hiding the U wave, giving the appearance of a prolonged QT interval. With increased potassium depletion, the U wave may become more prominent as the T wave becomes less so. The T wave becomes flattened and may even invert. The ST segment tends to become depressed, somewhat resembling the effects of digitalis on the ECG. Only at very low serum levels is there reasonable correlation between ECG changes and serum potassium concentrations.

The changes seen in hypokalemia also are observed in other conditions. The U wave may be accentuated in association with digitalis, LVH, and bradycardia. Untreated hypokalemia enhances instability in the myocardial cell. Ventricular premature beats are the most common manifestation of this imbalance, but supraventricular dysrhythmias, conduction problems, and eventually VT and VF may occur. Hypokalemia also increases the sensitivity of the heart to digitalis and its accompanying dysrhythmias, even at normal serum levels. The severity of the dysrhythmias associated with hypokalemia requires early recognition of this problem.