Abstract Body (Do not enter title and authors here): Background
Infants with congenital heart disease are at high risk of developing necrotizing enterocolitis (NEC), with increased morbidity and mortality compared to infants without heart disease who develop NEC. Shunted single ventricle patients are at even higher risk. The hybrid stage 1 palliation (HS1P) is an alternative to the Norwood procedure for high-risk single ventricle patients or as initial palliation for patients with future two ventricle repair, as bridge to transplant, or as destination therapy. Few small studies have shown similar or slightly higher rates of NEC after HS1P compared to after Norwood.

Aims
To evaluate the incidence, risk factors, and outcomes of NEC in HS1P versus Norwood patients.

Methods
This is a single-center retrospective cohort study of all neonates who underwent HS1P or Norwood procedure from 2011-2022. Demographic data and episodes of NEC (defined using modified Bell’s criteria) were collected. NEC incidences and risk factors were compared between HS1P and Norwood patients using Chi-square test. For HS1P patients, more extensive data were collected including planned and final physiologic outcomes (single ventricle palliation, biventricular repair, transplant, and destination) and primary patient risk factors that led to HS1P. Risk factors and clinical outcomes of HS1P patients with and without NEC were compared using standard univariate tests.

Results
The cumulative incidence of NEC in the HS1P group was 53% versus 37% in the Norwood group (p=0.01) and HS1P patients had more NEC episodes than the Norwood group (median 2 vs 1, p=0.03). Suspected NEC (stage 1A/1B) accounted for 70-75% of NEC cases in both groups. Only two infants, both in the HS1P group, had surgical (stage 3B) NEC. We found no differences in baseline demographics between patients with and without NEC in both HS1P and Norwood groups. In the HS1P group, NEC was associated with longer hospital length of stay (LOS), (median 49 days vs 23 days, p<0.0001) and greater need for tube feeds at 1 year (32% of the NEC group exclusively orally fed versus 43% of the non-NEC group, p=0.03). Final physiologic outcome was not associated with development of NEC in the HS1P group (p=0.38).

Conclusions
While the exact risk factors or predictors are not clear, NEC is more common after HS1P compared to Norwoods and is associated with longer hospital LOS and late feeding difficulty. Further studies to better understand the physiologic causes of NEC in the HS1P population are needed.