Background Obstruction to left ventricular outflow can be seen after surgical repair of congenital heart disease in which a ventricular septal defect (VSD) is closed by means of a baffle to the systemic great artery arising from the right ventricle (Rastelli operation, intraventricular repair, for conotruncal anomaly).We investigated the hypothesis that obligatory volumetric changes that occur after this operation lead to acute alterations in ventricular geometry and VSD size, resulting in subsequent subaortic stenosis in patients who were thought before operation to have a large, nonrestrictive VSD orifice.Methods and Results Preoperative and postoperative echocardiograms and medical records of 24 patients with conotruncal anomaly who underwent conventional Rastelli operation or intraventricular repair in which the VSD was used as part of a new left ventricular outflow were reviewed.Eleven patients had transposition of the great arteries with pulmonic stenosis, 7 had double-outlet right ventricle, and 6 had subaortic atresia or stenosis with a normal-size left ventricle and underwent Norwood's palliation in infancy. All had large, nonrestrictive VSDs at preoperative cardiac catheterization. The mean age at the time of surgery was 32+-24 months. The following measurements were made from two- dimensional echocardiographic images obtained before and 5+-4 days after surgery from the subcostal views at end diastole: (1) VSD diameter; (2) short-axis left ventricular internal diameter (LVID); (3) left ventricular posterior wall thickness (LVPW); and (4) systemic great artery diameter (arising from the right ventricle). VSD diameter diminished significantly after surgery (11.6+-3.6 versus 10.1+-3.7 mm, P<.0001), as did LVID (34.9+-5.0 versus 31.7+-5.1 mm, P<.001). LVPW thickness increased significantly (5.7+-1.0 versus 6.7+-1.1 mm, P<.0001), while great artery diameter was unchanged (16.2+-4.0 versus 16.7+-3.8 mm, P=NS). Percent change in VSD dimension correlated with percent change in LVPW/LVIDD ratio (degree of ventricular "contraction"). Nine patients subsequently developed subaortic obstruction at the VSD orifice level and had a greater degree of early diminution in VSD size (21+-8% versus 10+-8%, P<.002) as well as postoperative change in LVPW/LVID ratio (0.24+-0.04 versus 0.20+-0.02, P<.002) than those who did not develop subsequent subaortic obstruction.Conclusions The left ventricle undergoes geometric change after Rastelli operation or intraventricular repair, surgeries in which the VSD is used as the new left ventricular outflow.These changes are manifested as increased wall thickness, decreased cavity dimensions, and a decrease in VSD size. Patients who subsequently develop left ventricular outflow obstruction have the greatest degree of ventricular contraction and VSD diminution early after surgery. (Circulation. 1994;90(part 2):II-13-II-19.)