Signs of membranous subaortic stenosis appearing after correction of persistent common atrioventricular canal

Two cases are presented of persistent common atrioventricular (A-V) canal with interventricular communication in which associated membranous subaortic stenosis became apparent only after correction of the basic malformation and replacement of the mitral valve. The features of the membranous subaortic stenosis present only after operation were a left ventricular outflow pressure gradient and angiocardiographic demonstration of the membrane. In one case, additional echocardiograms demonstrated systolic flutter of the aortic cusps after the first operation (no echocardiograms were taken preoperatively). The flutter disappeared after a second operation in which the subaortic membranous tissue was resected. The appearance of hemodynamic signs of subaortic stenosis after the corrective operation for the persistent common A-V canal is explained by the fact that preoperatively the interventricular communication allowed decompression of the left ventricle so as to preclude a left ventricular outflow gradient. It Is presumed that the use of additional angiocardiographic projections might have demonstrated the anatomic state preoperatively.