Abstract Body (Do not enter title and authors here): Background
In hypoplastic left heart syndrome (HLHS), the single right ventricle (RV) accounts for the entire cardiac stroke volume. This physiology may produce elevated transpulmonary flow velocities and dilation of the right heart structures. Little data exists regarding normal right heart dimensions and transpulmonary velocities in relation to somatic growth in these fetuses. We sought to assess serial changes in the size of right heart structures and pulmonary valve (PV) velocities in fetuses with HLHS and compare findings to fetuses with normal cardiac anatomy.

Methods
A retrospective cohort study of fetuses with HLHS or normal cardiac anatomy confirmed by fetal echocardiogram between 1/1/13 – 12/31/22 was performed, excluding multiple gestation pregnancies. Fetal echocardiograms were assessed by an independent reviewer who obtained RV, PV and tricuspid valve annulus dimensions, and PV flow velocities. Nomograms were created for both HLHS and normal cohorts. The association between gestational age and PV velocity, PV annulus, RV diameter and tricuspid valve annulus was assessed and compared between groups using linear mixed models.

Results
There were 290 total fetal echocardiograms for 177 patients with HLHS compared against 408 normal fetuses. [MG1] PV velocities were higher for patients with HLHS compared to normal cardiac anatomy throughout gestation (p < 0.001). In addition, the PV velocity increased more rapidly with gestational age for the HLHS (+0.02 m/s/week) group compared to those with normal cardiac anatomy (+0.01 m/s/week) (p=0.019) (Figure 1). Similarly, the RV dimension grew more rapidly with gestational age for those with HLHS (+3.2% cm/week) compared to those with normal cardiac anatomy (+2.5% cm/week) (p<0.001) (Figure 2). Lastly, the tricuspid valve annulus grew more rapidly with gestational age for patients with HLHS (+2.6% cm/week) compared to those with normal cardiac anatomy (+2.3% cm/week) (p=0.010) (Figure 3). While the PV annulus size increased with gestational age, there was not a significant difference in growth between the two groups (p=0.119).

Conclusions
In fetuses with HLHS, the PV velocity, RV size and tricuspid valve size increase at a rate greater than normal somatic growth. These nomograms help identify expected RV size, tricuspid valve size and PV velocities for fetuses with HLHS and any who may deviate from the norm.