Etiology of the electrocardiographic pattern of "incomplete right bundle branch block" in atrial septal defect: an electrophysiologic study.

His bundle and right ventricular apical endocardial electrograms were recorded in 18 patients, ages five to 17 years, of whom ten had atrial septal defect of the secundum type and eight had mild aortic stenosis. The H-RVA interval was assumed to approximate conduction time from the level of the His bundle along the RBB to the site of earliest activation in the right ventricle at the RVA. The H-V interval was normal in all patients (less than or equal to 40 msec). All patients in the ASD group had an rsR' or rSr' pattern in Vl with a mean duration of 99 +/- 9 msec (86 to 116 msec); the mean duration of the H-RVA intervals was 54 +/- 6 msec in this group. In patients with AS, the surface electrocardiograms were normal, with a mean QRS duration of 82 +/- 8 msec (70 to 95 msec); the H-RVA intervals averaged 52 +/- 6 msec. The difference in the H-RVA intervals in the two groups was not statistically significant (P greater than 0.50) despite the fact that patients with ASD had significant prolongation of QRS duration (P greater than 0.001). Moreover, when functional RBB block was induced by premature atrial stimulation in the ASD group, the prolongation of QRS duration was accompanied by an increase in H-RVA interval, indicating relatively intact function of the RBB. It is concluded that the rsR' or rSr' pattern in ASD is likely to be a manifestation of right ventricular overload rather than a true conduction delay in the RBB.