The ECG changes in hypokalemia is mainly due to a delayed ventricular repolarisation. The changes normally do not correlate well with the plasma concentration.
Early changes include flattening or inversion of the T wave, a prominent U wave, ST-segment depression k/a thumbprint-like ST depression, and a prolonged QU interval but the QT interval will be normal.
Severe K+ depletion may result in a prolonged PR interval, decreased voltage and widening of the QRS complex, and an increased risk of ventricular arrhythmias, especially in patients with myocardial ischemia or left ventricular hypertrophy. The QT interval may be normal or lengthened.