Abstract Body (Do not enter title and authors here): Background: Many children born with single ventricle physiology are palliated with Fontan operation, which may include a fenestration creation. This has been demonstrated to improve short-term outcomes after surgery. Reduced exercise capacity has been shown to be a predictor of prognosis in Fontan patients.

Aim: We sought to compare cardiopulmonary exercise test (CPET) data between fenestrated and non-fenestrated Fontan patients.

Methods: We conducted a retrospective cohort study of 185 patients referred for CPET from 2011 to 2024. Patients with complete CPET data were included. Type of Fontan, comorbidities, and echocardiogram data were ascertained electronically. Patients were divided into 2 different groups according to the fenestration patency. Fisher’s exact test or chi-square was used to compare categorical variables. T-test and Turkey-Kramer were used to compare continuous variables. Adjustment using General Lineal Models procedure were used to identify any confounder that could affect the results. P value <0.05 was considered significant.
Results: 142 patients were classified in the non-fenestrated (NF) group, and 43 in the fenestrated group (FF). Baseline characteristics of both groups are shown in Table 1. From the evaluated CPET variables, the minute ventilation-carbon dioxide production (VE/VCO2) was higher in the FF (p=<0.001), and oxygen saturation at peak exercise (SatO2) was lower in this group (p= <0.001) (Figure 1). There was no significant difference between peak oxygen consumption (VO2) between groups. When adjusted by comorbidities only VE/VCO2 nadir, slope and peak SatO2 were statistically significant different between groups (Table 2).

Conclusion: Ventilatory variables showed significant differences when fenestration was presented, secondary to V-Q mismatch from increased right to left shunt. Despite the theoretical advantage of increased cardiac output from Fontan fenestration, exercise performance was similar between both groups.