Abstract Body (Do not enter title and authors here):

Introduction
Right ventricular (RV) and right atrial (RA) dilatation are important hallmarks of disease severity in patients with pulmonary arterial hypertension (PAH). To obtain more insights on the clinical significance of RA dilatation, we phenotypically characterized patients with right heart dilatation.

Aim
Description of different RV/RA phenotypes in PAH

Methods
203 incident, treatment naïve PAH patients with cardiac magnetic resonance (CMR) imaging were included. Phenotypic clustering was based on RA and RV dilatation. Conform literature, RA dilatation was defined by a RA index maximal area >15 cm²/m². RV dilatation was defined by an RVEDV/LVEDV ratio >1.3 and >2.4 for extreme dilatation.

Results
Four phenotypic patient clusters were identified: no dilatation (n=48), RV dilatation (n=78), RV+RA dilatation (n=53) and extreme RV dilatation + RA dilatation (n=23), Figure 1A. To unravel the additive clinical value of RA dilatation, we mainly compared patients with RV dilatation and patients with RV+RA dilatation. Intriguingly, patients with RV+RA dilatation demonstrated lower 3- and 5-year survival rates (58% and 43% vs. 75% and 55%) (Figure 1B) and were less likely to reverse RV dilatation after treatment initiation than patients with RV dilatation only. Despite the consequences of RA dilatation, we could not identify an explanation for the difference in RA dilatation between both groups. No difference in afterload was observed, shown by comparable pulmonary vascular resistance (9.9 [6.8-13] WU vs. 10 [7.5-12.3] WU, p>0.05). RV function was similarly impaired (RV ejection fraction: 36±9% vs. 33±9%, p>0.05). End-diastolic elastance (0.73 [0.39-1.11] mmHg/mL vs. 0.58 [0.40-0.94] mmHg/mL, p>0.05) and severity of tricuspid regurgitation did not differ. Finally, diuretic use (40% vs. 40%, p>0.05), age (54±16 vs. 57±19 yrs, p>0.05), female gender (70% vs. 60%, p>0.05) and iPAH diagnoses was similar (69% vs. 59%, p>0.05).

Conclusion
PAH-patients with both RA and RV dilatation have a worse clinical profile than patients with only RV dilatation. However, no clear trigger for RA dilatation was found and future mechanistic studies should unravel factors explaining these differences in right heart adaptation.