The superior QRS axis in ostium primum ASD: a proposed mechanism.

The influence of abnormal hemodynamics, ventricular hypertrophy, and right bundle branch block on the AQRS was studied pre- and post-operatively in 29 patients with OPSD. The AQRS markedly diminishes with the surgical correction of abnormal hemodynamics and the subsequent resolution of RVH or BVH. With the persistence of ventricular hypertrophy postoperatively or the surgical induction of RBBB, the AQRS either remains unchanged or, in the latter instance, becomes more superior and rightward. The dependence of the superior AQRS on these factors suggests that a left anterior hemiblock is not responsible for this AQRS. In OPSD early activation of the posterobasal region of the left ventricle through an abnormally short posterior fascicle results in a minimal superior AQRS which is then exaggerated in the presence of abnormal hemodynamics, ventricular hypertrophy, or RBBB. Thus, the superior AQRS in OPSD with associated RBBB does not represent a true bifascicular block and has a different natural history and clinical significance.