Evacuation of pericardial blood by posterior pericardiotomy or use of a posterior pericardial chest tube lowers postoperative atrial fibrillation (POAF) rates after cardiac surgery by 45-68%. Although it cannot be generalized due to trial undersizing, posterior pericardial chest tube treatment may be a superior alternative to pericardiotomy, given its low risk of procedural complications. This interventional multicenter trial will assess whether prolonged treatment with a posterior pericardial chest tube lowers POAF rates after cardiac surgery. Investigators will randomize 624 patients undergoing routine cardiac surgery at Nordic sites 1:1 to receive a posterior pericardial chest tube as adjunct to standard care for up to 3 postoperative days or standard care alone. The primary outcome is the proportion of patients with POAF up to 7 days post-surgery; the study will be powered to detect a relative risk reduction of 30% in the intervention arm. Secondary outcomes are AF burden; days with chest tubes and their output; proportion of patients with POAF up to 14 days post-surgery; direct current conversions during hospital admission; length of ICU/hospital stay; postoperative complications, mortality, ischemic stroke, and major bleeding at 30/90 days and 1/3/5 years; and quality of life/postoperative recovery at 90 days and 1 year. This trial may provide quality clinical evidence supporting the adoption of a simple method to prevent POAF, thus reducing healthcare costs.