[Thinning of ventricular septum in tetralogy of Fallot]

Cross sectional echocardiography was used to evaluate the thickness of the ventricular septum in tetralogy of Fallot (TOF). Forty-six patients with TOF and 20 patients with pseudo-truncus arteriosus underwent echocardiography during a five-year period beginning in 1984. Thicknesses of the right ventricular anterior wall (RVAWT), trabecular septum (IVST) and left ventricular posterior wall (LVPWT) were measured in end diastole on parasternal short axis view at the level of the tips of papillary muscles. The ratios of IVST to RVAWT and IVST to LVPWT were assessed. The ratio of IVST to RVAWT was 1.09 +/- 0.15 in the group aged less than 7 years (less than 7 y.o.) and 0.94 +/- 0.15 in the group aged of 7 years or more (greater than = 7 y.o.). The ratios of IVST to LVPWT were 1.10 +/- 0.14 (less than 7 y.o.) and 0.90 +/- 0.15 (greater than = 7 y.o.), respectively. Both ratios were significantly different (p less than 0.01) in the two age groups, and relative thinning of the septum was demonstrated in the older patients. It is speculated that thinning of the interventricular septum is caused by the lower systolic wall stress of the ventricular septum compared with that of the free walls, which is produced under equal systolic pressure of the two ventricles. It is suggested that this thinning is one of the factors that reduces left ventricular function after repair of TOF.