Atrioventricular Conduction in Patients with Incomplete Endocardial Cushion Defect

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Abstract
The cause of left axis deviation noted on the electrocardiogram (ECG) in patients with ostium primum atrial septal defect has been attributed to two differing mechanisms: left ventricular overload secondary to mitral insufficiency and a congenitally anomalous atrioventricular conduction system. In order to define the conduction characteristics of the atrioventricular conduction system in patients free from the hemodynamic abnormalities of ostium primum atrial septal defect, seven patients with an ostium primum atrial septal defect were studied in the late postoperative period. Each ECG showed persisting left axis deviation with no new conduction abnormalities. Hemodynamic evaluation, including left ventriculography, showed no intra-atrial shunting and only minimal mitral insufficiency in three patients. His bundle recording techniques were utilized to study the atrioventricular conduction system. The P-A interval was normal in four patients (25-45 msec) and prolonged in three (50, 60 and 60 msec), while the A-H interval was normal in all seven patients (60-130 msec). This is compatible with abnormal intra-atrial conduction and normal conduction through the atrioventricular node. The H-V interval was normal in four patients (35-55 msec) and short in three (20, 25, and 25 msec). Atrial pacing produced the normally expected A-H prolongation, P-A and H-V remaining constant. These results and the persisting left axis deviation can be explained on the basis of the known anatomy of the atrioventricular conduction system in patients with ostium primum atrial septal defect. The left axis deviation is most probably related to early activation of the postero-basal left ventricular wall (patients with short H-V) and/or late activation of the antero-lateral left ventricular wall (patients with normal H-V). Hemodynamics are not important in producing these findings.