Abstract Body (Do not enter title and authors here): Introduction:
Hypoplastic left heart syndrome (HLHS) with mitral stenosis/aortic atresia (MS/AA) is associated with worse outcomes compared to other anatomic variants. It is unclear whether this is related to left ventricle-coronary artery (LV-CA) fistula.

Research Question:
We sought to compare outcomes for patients with HLHS (MS/AA) with and without LV-CA fistula.

Methods:
Retrospective cohort study of patients with HLHS (MS/AA) who underwent stage 1 palliation at Boston Children’s Hospital 2008-2023. Presence of LV-CA fistula was defined by angiography.

Results:
Among 91 patients, 58 (64%) had LV-CA fistula and 33 (36%) did not. LV-CA fistula was diagnosed prior to initial palliation in 31 (53%) patients. Birth weight (BW) and gestational age (GA) were similar between groups. Mean postnatal echocardiogram ascending aorta diameter z-score was -4.29 ± 0.74 and was similar between groups. In total, 67 (75%) of patients underwent surgical stage 1 palliation and 22 (25%) underwent hybrid palliation; hybrid palliation was more common in the fistula group (36% vs 6%, p=0.007). Transplant-free survival at 1 year was 63% (95% confidence interval [CI]: 49%, 74%) and 79% (95% CI: 61%, 89%) in those with and without fistula, respectively. There was a trend towards shorter time to death or transplant in patients with LV-CA fistula than for those without (Figure 1, p=0.083). In univariate analysis, lower GA (↓1 week; HR 1.34, 95% CI 1.18, 1.51), lower BW (↓0.5 kg; HR 1.69, 95% CI 1.28, 2.23), and initial hybrid palliation (HR 3.55, 95% CI 1.82, 6.94) were associated with an increased hazard of death/transplant (all p<0.001). After adjusting for either GA or BW, there was still a trend towards shorter time to death/transplant in children with LV-CA fistula which was not statistically significant (p=0.09, p=0.13, respectively).

Conclusions:
Mortality remains high for children with HLHS (MS/AA) and there is a trend toward worse transplant-free survival for those with LV-CA fistula.