Electrocardiographic correlation of bundle branch block and the U wave

Summary One of the several hypothetical explanations of the genesis of the U wave is that this wave represents the terminal repolarization process in the Purkinje system. In order to test this hypothesis, electrocardiographic correlation of bundle branch blocks and the U wave was studied in 52 cases of right and 35 cases of left bundle branch block, and the following results were obtained. (1) The Q-aU and the T-aU intervals are longer and the U wave is generally of greater amplitude in the presence of left bundle branch block than in right bundle branch block, and (2) positive U waves are more prevalent over the right precordium and negative U waves are often seen in V6 or V5 with left bundle branch block, while the opposite holds true with right bundle branch block. Two other major theories on the genesis of the U wave, or the theory of afterpotential, and that of delayed repolarization of the papillary muscles, were contrasted with the present hypothesis, in the light of these findings. Thus, it is concluded that Purkinje repolarization as the cause of the U wave appears to explain more satisfactorily the observed behavior of this wave in the presence of intraventricular conduction disturbances, when compared with other theories.