Abstract Body (Do not enter title and authors here): Background: Patients with single right ventricles (SRV) are exposed to conditions that predispose them to myocardial fibrosis and ventricular dysfunction. The assessment of extracellular volume (ECV) by cardiac magnetic resonance (CMR) as a surrogate for myocardial fibrosis has been reported to be feasible in the right ventricles of patients with congenital heart disease.

Research Question: The relationship between ECV and invasive reference standard measures of ventricular stiffness has not been performed in SRV patients.

Aim: The purpose of this study was to assess the relationship between CMR derived ECV and reference standard measures of ventricular stiffness derived from pressure-volume loop analysis.

Methods: Subjects aged 2 to 6 years with SRV undergoing pre-Fontan catheterization were prospectively recruited to undergo PVL analysis and CMR. All patients first underwent CMR, then catheterization the same day under the same anesthetic. CMR was performed on a 3 Tesla scanner. A Modified Look Locker sequence T1 mapping pre- and post-Dotarem was performed and offline processing was done in a blinded fashion (MyoMaps, Siemens Healthcare) (Figure 1). ECV was calculated with the formula: ECV=((Δ[1/T1myocardium]/Δ[1/T1blood pool])×[1−hematocrit]). Δ represents the difference between the postcontrast and native T1 values. Pressure volume loop analysis was performed using microconductance catheters. The end diastolic pressure volume relationship was obtained via balloon occlusion of the bidirectional Glenn and fitted to the equation αe^βEDV, where β is the the reference standard measure of ventricular stiffness (Figure 2). Pearson's correlation was used to determine the association between β and ECV.

Results: A total of 22 SRV patients had CMR, 19 had T1 analyses adequate for analysis. Mean age was 4.1 ± 0.7 years. Median β was 0.022 mL-1 (IQR 0.012, 0.062). The median T1 pre-contrast was 1042 (1026, 1114) ms, post-contrast T1 was 480 (363, 626) ms, and ECV was 28 (20, 32)%. There was a moderate correlation between stiffness and ECV by CMR (r = 0.65, p = 0.003) (Figure 3).

Conclusion: The assessment of ECV in SRV patients is feasible. We found a good correlation between ECV and the reference standard measure of ventricular stiffness in ECV patients. Future studies should evaluate ECV’s potential to identify SRV patients with stiffness abnormalities. This study supports the use of CMR derived ECV in assessing diastolic function in SRV.