Purkinje repolarization as a possible cause of the U wave in the electrocardiogram

Our recent clinical studies suggest that the U wave represents Purkinje repolarization. To test this hypothesis, transmembrane potentials of Purkinje fibers (P) and the ventricular muscle (V) were simultaneously recorded from canine P-V preparations perfused in a tissue bath, and effects of various factors causing prominent U waves were studied. These include low stimulating frequency (bracycardia), low potassium (K) concentration, hypothermia, and quinidine. Bracycardia increased the difference between action potential duration of P and V, and decreased the rate of repolarization in P. Similarly, the difference of P and V action potential duration was increased markedly by low K and hypothermia, and slightly by quinidine, while the slope of phase 3 was significantly decreased by all these factors. In several intact animals either hypokalemia or hypothermia was produced using hemodialysis or extracorporeal circulation, and recorded electrocardiograms were compared with the P and V action potentials obtained under similar perfusing conditions in subsequent tissue bath study. Such comparison revealed a good temporal correlation between phase 3 repolarization in P and the electrocardiographic U wave. These results, although indirect, appear to support the theory that P repolarization caused the genesis of the U wave.