Introduction
Atrial fibrillation (AF) is the most common arrhythmia following cardiac surgery. Although many risk factors have been studied, none has emerged as a dominant and widely applicable prophylactic measure.

Aim
The aim of our study was to investigate the relationship between the amount of shed mediastinal blood and the occurrence of postoperative AF.

Material and methods
From February 2021 to March 2024, 1140 patients undergoing cardiac surgery with cardiopulmonary bypass (CPB) were evaluated for the occurrence of postoperative AF (POAF). The following parameters were recorded and analyzed: age, gender, diabetes mellitus, hypertension, smoking, dyslipidemia, chronic obstructive pulmonary disease (COPD), history of stroke, pre-operative ejection fraction, CPB, aortic cross clamp time, and amount of mediastinal drainage. All variables were analyzed using logistic regression to assess their association with new-onset AF.

Results
A total of 388 (33.59%) patients developed paroxysmal AF. According to our study, after multiple regression analysis, new onset atrial fibrillation showed statistically significant relationships with age (p = 0.001), hypertension (p = 0.001), duration of CPB (p = 0.04), and the amount of mediastinal drainage (p = 0.04).

Conclusions
The occurrence of postoperative AF is strongly associated with the amount of shed mediastinal blood.