AIM: No clear consensus exists for extracorporeal cardiopulmonary resuscitation (ECPR) patient selection in refractory cardiac arrest. Evidence suggests that current binary criteria yield either low favorable outcome rates, when being liberal, or exclude potential survivors, when being restrictive. The Pre-ECPR score, which uses logistic regression to give a single predicted probability of favorable outcome (modified Rankin Scale 0-3), demonstrated significantly better discriminatory ability than the Extracorporeal Life Support Organization "Example of selection criteria for ECPR". This protocol outlines the methodology for external validation of the Pre-ECPR score using the ECPR-treated patients from the randomized Prague Out-of-Hospital Cardiac Arrest (OHCA) trial. METHODS: The validation cohort comprises the 92 ECPR-treated patients of the Prague OHCA trial. The Pre-ECPR score incorporates eight predictors: age, no-flow time/initial rhythm combination, total cardiac arrest duration, signs of life, pupil dilation, end-tidal CO2, regional cerebral oxygen saturation, and arterial pH, all measured during resuscitation. Model performance will be evaluated through calibration (calibration-in-the-large, calibration slope, and calibration plots); discrimination (area under the receiver operating characteristics curve and Tjur's discrimination index); and risk stratification analysis at probability thresholds. If needed, model recalibration or refitting will be performed, with internal validation based on k-fold cross-validation. Outcome probabilities will be calculated blinded to study outcomes. CONCLUSION: This external validation will indicate whether the Pre-ECPR score retains its predictive performance in an independent OHCA population. The results will provide essential evidence for its potential to improve ECPR patient selection beyond conventional binary selection criteria.

2nd Department of Internal Medicine Cardiovascular Medicine General University Hospital and 1st Faculty of Medicine Charles University Prague Czech Republic

3rd Faculty of Medicine Charles University Prague Czech Republic

Department of Anesthesiology and Intensive Care Medicine Sahlgrenska Academy University of Gothenburg Gothenburg Sweden

Department of Cardiology Hospital AGEL Trinec Podlesi a s Trinec Czech Republic

Department of Cardiothoracic Anesthesia and Intensive Care Sahlgrenska University Hospital Gothenburg Region Västra Götaland Sweden

Department of Molecular and Clinical Medicine Institute of Medicine Sahlgrenska Academy University of Gothenburg Gothenburg Sweden

Institute for Heart Diseases Wroclaw Medical University Wrocław Poland

Statistiska Konsultgruppen Sweden Gothenburg Sweden

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