Introduction: In the usual procedures performed on newborns affect by Hypoplastic Left Heart Syndrome (HLHS), phenotypic variations influence the success of the treatment, especially the characteristics related to the atrial septum and foramen ovale (FO) in case of catheter septostomy. Objectives: To analyze macroscopics features of the atrial septum of hearts with HLHS in order to define and guide therapeutic procedures on this structure. Methods: 18 hearts of deceased patients diagnosed with HLHS were evaluated for FO patency and size, bulging of its blade, atresia or patency of the mitral valve and caliber of the ascending aorta and pulmonary trunk. Histological sections of the atrial septum were made for measurements of maximum and minimum thickness of the lamina. Results: The mean age of the patients was 34.5 days (57% male), with a mean atrial septal thickness of 1.90 mm (0.63 - 4.09 mm). The mean diameter of the pulmonary trunk and aorta were 1.16 cm and 0.22 cm, respectively. FO was patent in 39% of cases. The mitral valve was atresic in 21% of the specimens. There was a significant difference in the thickness of the atrial septum in cases with patent or closed FO, being greater in cases where the FO was closed (p = 0.047). The ratio FO/age presented a statistically significant negative correlation with atrial septum thickness (r = -0.76 and p <0.05). Conclusions: The results indicate that the patency and size of the oval foramen have repercussions on the thickness of the atrial septum, suggesting that this factor may limit the success of therapeutic interventions, especially catheter septostomy.
