Hypopotassemia-induced U wave in electrocardiogram (an experimental study for possible mechanism)

To define the mechanism of hypopotassemia-induced U wave change, we performed open-chest experiments in 20 dogs and obtained following results: (1) During hemodialysis U wave amplitude of epicardial electrogram increased significantly (p less than 0.05) from 0.4 +/- 0.3 (mean +/- SD) to 1.2 +/- 0.5 mV with lowering serum potassium (K) concentrations from 3.4 +/- 0.4 to 2.2 +/- 0.2 mEq/l (p less than 0.001) in 6 dogs. (2) These hypopotassemia-induced U waves disappeared after instillation of high K+ solution into the regional coronary artery, without detectable increase in systemic plasma K+ concentration. (3) In 10 dogs, huge (0.5-1.0 mV), delayed (corrected Q-U apex = 0.66 +/- 0.16) positive U waves were induced at the epicardial surface by infusion of K+-free, Ca++-rich Tyrode solution into the regional coronary artery, whereas no appearance of negative U waves was shown at the endocardial surface. (4) In all of 6 dogs with infusion of K+-free, Ca++-rich solution, monophasic action potential registered with suction electrode showed early afterhyperpolarization, which corresponded to appearance of huge positive U waves. These data indicate that hypopotassemia-induced U wave may reflect early afterhyperpolarization developed in the ventricular muscle.