The membranous portion of the interventricular septum in neonates. An anatomic study in neonatal cadavers.

The membranous portion of the interventricular septum (MPIS) has been described as a small, oval portion of the cardiac septum that lies immediately below the aortic valve. Its location and relationship with the aortic valve are very important in surgical interventions. Several studies have been reported in adults but few in neonates. For this reason, studies of the MPIS were made in the hearts of 26 (12 male, 14 female) fulterm neonates. The left side of the MPIS was photographed with transillumination from the right ventricle. The shapes of the septum were assessed from the photographic images. In addition, in accordance with the ruler in the photographs a millimetric scale was prepared on the tracing paper and the surface area of the MPIS and its distance from the superior border of the aortic valve were measured. In these 26 cases, the following shapes of MPIS were found: semilunar, 7 (26.92%); triangular, 6 (23.07%); quadrangular, 5 (19.23%); oval, 4 (15.38%); circular, 2 (7.69%); irregular, 2 (7.69%). Its surface area varied from 2-21 mm2 (mean 5.84 mm2). The superior border of MPIS was in close relationship with the aortic valve. In 10 cases (38.46%) the MPIS was in direct continuity with the attachments of both the right and posterior aortic cusps, in 8 (30.76%) only with the posterior aortic cusp, in 5 (19.23%) only with the right aortic cusp, while in 3 (11.53%) the superior border of MPIS was below the attached portion of both cusps. The distance between the superior border of the MPIS and the attachment of the right or posterior aortic cusps did not exceed 3.5 mm. There were no statistically significant sex-related differences in shape, surface area and relationship with the aortic valve.