OBJECTIVE--To clarify the morphological features relating to an intermediate axis or a right axis deviation in atrioventricular septal defect (AVSD). SUBJECTS--135 patients with typical AVSD and with nine patients with atypical AVSD, characterised by a well formed atrial septum, a milder downward displacement of the atrioventricular valves, and a shorter length of the ostium primum defect. MAIN OUTCOME MEASURES--Relation between morphology and electrocardiographic and vectorcardiographic findings; prevalence of Down's syndrome and of other cardiac anomalies. RESULTS--All nine patients with atypical AVSD had an unusual mean frontal QRS axis compared with six of the 135 patients (4%) with typical AVSD (p < 0.01). All eight patients who underwent the vector analyses showed atypical movement of the QRS loop--that is, an initial left inferior movement in the frontal loop (eight patients) and counter-clockwise rotation in the sagittal loop (seven). The corresponding values for 119 patients with typical AVSD were 20 and 22 patients (p < 0.01). Seven patients with atypical AVSD (78%) and 55 (41%) with typical AVSD had Down's syndrome (p < 0.05). None of the twenty one patients with additional cardiac anomalies had atypical AVSD, an unusual QRS axis, or unusual movement in the QRS loop. CONCLUSIONS--The atypical morphology, supposedly related to the degree of posteroinferior displacement of the conduction system, was one of the causes of unusual movement of the QRS loop in AVSD.
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