The Course of Physiologic Incomplete Right Bundle Branch Block During Infancy. ECG Findings Based on Follow-up of Normal Newborns

In order to establish the incidence, characteristics as well as the course of R waves which can be considered as imcomplete right bundle branch block (IRBBB) during the involution of right ventricular hypertrophy, 8 serial ECG's were evaluated starting from 43 normal newborns, 49 after one month and 56 after two months. The right precordial r-waves were classified according to their amplitudes and their occurence before or after the S wave. The duration of qR-interval and its ratio to QRS were evaluated right and left chest leads. The incidence of R'-wave, which was very rare in neonates increased after the first month and occured before the S-wave. Thereafter, the amplitude of R'decreased and changed to Rr'-pattern. This turned into an r' after the S wave and finally r' decreased and evolved into SS'-pattern. An rSR' or qRS-pattern in the right precordial leads were never observed. Since the absolute and relative duration of qR-interval in the right precordial leads was prolonged as compared to V5-6, was postulated that R'-waves were caused by the delay in conduction through the right-sided myofibrils which were reduced in diameter because of involution of right ventricular hypertrophy while being stretched due to the increase of its volume of cavity during growth. The physiologic IRBBB during infancy can be differentiated from the pathologic one by the latter's stationary character of the amplitude of r'-wave.