VI. Genesis of the U Wave

Abstract

The best explanation for the U wave of the electrocardiogram is that it corresponds to potential differences produced during the descending limb of a negative afterpotential, just as the T wave corresponds to similar potential differences produced during the descending limb of the action potential proper. This explanation accounts for the timing of the U wave and for the general dependence of its configuration on that of the T wave. All factors which influence the negative afterpotentials have the same influence on the U wave. The negative afterpotentials can be explained by partial depolarization of the membrane by the K ions which had left the cell during the repolarizaton process. Low external K should counteract reabsorption of these ions, and increase the amplitude both of negative afterpotentials and of U waves more than any other single factor. The effects of other factors can be explained largely by their effect on the K flux. Stretching greatly increased the magnitude of the negative afterpotentials, and occasional discrepancies between the direction of T and U can be explained by greater magnitude of negative afterpotentials in the ventricular apex and papillary muscles, which are subjected to greater mechanical load. An ecg is illustrated in which U became taller and appeared earlier after appearance of A-V nodal rhythm, without change of rate or configuration of QRS and T.