The association between BMI and postoperative pulmonary complications in adults undergoing non‐cardiac, non‐obstetric surgery: a retrospective cohort study

Summary Introduction Conflicting results have been reported regarding the influence of BMI on postoperative adverse events. The aim of this study was to investigate the association between BMI and postoperative pulmonary complications in adults undergoing non‐cardiac, non‐obstetric surgical procedures. Methods This large‐scale retrospective study included 125,082 adults who underwent surgery at a university‐affiliated tertiary care hospital between 2019 and 2023. The primary endpoint was the incidence of postoperative pulmonary complications. Multivariable logistic regression analyses, subgroup analyses, sensitivity analyses and restricted cubic splines were used to assess the association between BMI and postoperative pulmonary complications. Results A total of 6671 patients (5.3%) developed one or more postoperative pulmonary complications. After adjusting for confounders, compared with those patients with a normal weight (BMI 18.5–24.9 kg.m ‐2 ), patients who were underweight (BMI < 18.5 kg.m ‐2 ) had an increased risk of postoperative pulmonary complications (OR 1.24, 95%CI 1.12–1.39, p < 0.001). Patients who were overweight (BMI 25.0–29.9 kg.m ‐2 ) or living with class 1 obesity (BMI 30.0–34.9 kg.m ‐2 ) had a lower risk of postoperative pulmonary complications (OR 0.88, 95%CI 0.83–0.94, p < 0.001 and OR 0.82, 95%CI 0.70–0.96; p = 0.01, respectively). Patients living with obesity class 2/3 (BMI ≥ 35 kg.m ‐2 ) had a similar risk of postoperative pulmonary complications as patients with a normal weight (OR 1.23, 95%CI 0.91–1.66, p = 0.17). There was a J‐shaped association between BMI and incidence of postoperative pulmonary complications with the lowest risk at a BMI of 27.4 kg.m ‐2 . Discussion Patients who were overweight or living with class 1 obesity undergoing non‐cardiac, non‐obstetric surgery had paradoxically lower risks of postoperative pulmonary complications compared with those of a normal weight. These findings may contradict traditional assumptions about surgical risk and obesity, highlighting the need to re‐evaluate the relationship between BMI and postoperative pulmonary complications.