Etiology of Prolongation of the P-R Interval

Conduction time from the region of the sinus node to the region of the atrioventricular (A-V) node was studied during open heart surgery in 13 patients with an endocardial cushion type atrial septal defect, eight patients with a secundum type atrial septal defect, one patient with a sinus venosus type atrial septal defect, and eight patients with an intact atrial septum. Internodal conduction time was prolonged by an average of 37 msec (range 15-69 msec) in patients with an endocardial cushion defect when compared with the other three groups of patients. However, when the atria were paced from the A-V nodal region, the resulting P-R intervals in patients with an endocardial cushion defect were similar to those of patients in the other three groups, a fact that indicates normal A-V nodal-His-Purkinje conduction in patients with an endocardial cushion defect. It was therefore concluded that the prolonged P-R interval which patients with an endocardial cushion defect often manifest is due simply to prolonged intemodal conduction time. Further, we have suggested that the data from this study support the concept that specialized atrial internodal pathways play an important role in conduction between the sinus and A-V nodes.