In mild hypokalaemia there are usually no ECG changes.

Moderate to severe hypokalaemia may cause increased P wave amplitude, PR prolongation, U waves, T wave flattening and ST segment depression. Due to myocardial hyperexcitability, ectopics may occur. With worsening hypokalaemia, arrhythmias including atrial fibrillation, atrial flutter, ventricular tachycardia or ventricular fibrillation may result.

This ECG shows hypokalaemia with U waves.