ECG changes in hypokalemia

Prominent U waves combined with depressed ST segments and flattened T waves ("roller-coaster effect")
 

- note the depressed ST segments with flattened/equiphasic T waves in leads V4 and V5 and V6 and how the prominent U wave is not seen in those leads

- note the small flattened T wave in leads V2 and V3 and how the ST segment dips down slightly (just after the flattened T wave) before rising to the prominent U wave (that has merged with the following P wave) creating a "roller-coaster" effect

- note how the prominent U wave has merged with the following P wave in lead V1