Relationship between the ECG, ventricular activation, and the ventricular conduction system in ostium primum ASD.

SUMMARY A combined electrophysiologic-anatomic study was performed in a dog with a congenital atrial septal defect (ASD) primum. Comparisons were made between the dog and electrophysiologic data obtained in four patients with ASD primum. The ECGs and VCGs in both the dog and patients revealed the characteristic features of superior orientation, counterclockwise rotation, and slow-rightward terminal forces in the frontal plane QRS. In both the dog and the patients, epi-cardial ventricular activation revealed early posterior LV wall excitation and later anterior left ventricular (LV) depolarization. Also, there was a marked delay in the activation of the RV in both dog and patients. In the dog, Purkinje (Pj) potentials were recorded from the endocardium of the anterior and posterior LV wall and the anterior RV wall. The endocardial-Pj activation times of these three regions were correlated with the measured lengths of the three major divisions of the conduction system supplying them. Premature activation of the posterior LV Pj and delayed activation of the anterior LV and RV Pj was observed. The marked asynchrony of Pj activation was due to a developmental asymmetry of the ventricular conduction system, characterized by markedly different lengths of the three major divisions. Purkinje conduction velocity was 1.5 m/sec, and there were no areas of block or slow conduction. The agreement between the dog and human data indicated an identical mechanism for patients with ASD primum. The terms left anterior-superior hemiblock and right bundle branch block are inappropriate descriptions of this lesion.