The electrocardiograms shown here and on the next several pages illustrate the effects of a decrease in extracellular potassium concentration. This ECG was recorded on a 61 year old woman whose serum potassium level was 3.6 mM. There are small U waves present in leads V2 and V3 (arrow).These U waves, while suggestive of low serum potassium (hypopotassemia), are still within normal limits(as is the ECG).