Cardiopulmonary bypass (CPB) can lead to cardiac damage due to oxidative stress (OS) and inflammation in heart failure (HF). We tested the hypothesis that preoperative HF patients with reduced ejection fraction (HFrEF) subjected to CBP have higher levels of OS and NLRP3 (NOD-, LRR- and pyrin domain-containing protein 3) in heart and plasma and in those that develop postoperative AF (pAF) as a clinical outcome. HF was categorized for preoperative left ventricular EF: preserved (HFpEF > 50%, n = 27) and reduced EF (HFrEF ≤ 40%, n = 25). Samples of atrial tissue, pericardial fluid, and plasma were collected at surgery to assess NLRP3 expression; 3-nitrotyrosine (3-NT), thiobarbituric acid reaction (TBARS), and nuclear factor erythroid 2-related factor 2 (Nrf2) in atrial tissue; NLRP3, IL-1β, and IL-18 expression in pericardial fluid; and antioxidant capacity, 8-isoprostanes, and malondialdehyde (MDA) in plasma. Reactive oxygen species, 3-NT, and NLRP3 in atrial tissue were determined by immunohistochemistry in a subset of pAF patients. Plasma and atrial tissue 3-NT and MDA were higher in HFrEF compared with HFpEF. Lipid peroxidation products were higher in both plasma and atrial tissue in pAF (n = 29), compared to sinus rhythm (SR) (n = 23). In HFrEF patients, the values of tissue ROS, 3-NT, and NLRP3 were higher than in HFpEF patients. In addition, the expression levels of NLRP3, IL-1β, and IL-18 were higher in atrial tissue and pericardial fluid in HFrEF. Patients with preoperative HFrEF showed higher OS in plasma and the expression of NLRP3, ROS, and 3-NT in atrial tissue biopsies and pericardial fluid. This finding suggests a potential pharmacologic therapy for pAF and clinical complications due to CPB.