The study aimed to validate the European System for Cardiac Operative Risk Evaluation score (EuroSCORE II) in patients with atrial fibrillation (AF). All data were retrieved from the National Registry of Cardiac Surgery Procedures (KROK). EuroSCORE II calibration and discrimination performance was evaluated. The final cohort consisted of 44,172 patients (median age 67, 30.8% female, 13.4% with AF). The in-hospital mortality rate was 4.14% (N = 1830), and 5.21% (N = 2303) for 30-day mortality. EuroSCORE II significantly underestimated mortality in mild- and moderate-risk populations [Observed (O):Expected (E)-1.1, 1.16). In the AF subgroup, it performed well [O:E-0.99), whereas in the very high-risk population overestimated mortality (O:E-0.9). EuroSCORE II showed better discrimination in AF (-) [area under curve (AUC) 0.805, 95% CI 0.793-0.817)] than in AF (+) population (AUC 0.791, 95%CI 0.767-0.816), P < 0.001. The worst discriminative performance for the AF (+) group was for coronary artery bypass grafting (CABG) (AUC 0.746, 95% CI 0.676-0.817) as compared with AF (-) population (AUC 0.798, 95% CI 0.774-0.822), P < 0.001. EuroSCORE II is more accurate for patients with AF. However, it underestimated mortality rates for low-to-moderate-risk patients and had a lower ability to distinguish between high- and low-risk patients with AF, particularly in those undergoing coronary artery bypass grafting.
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