Pleural effusions are associated with adverse outcomes after cardiac surgery: a propensity-matched analysis

All patients undergoing cardiac surgery between 2006 and 2019 at a tertiary care university hospital were included in this observational, cross-sectional analysis. With the written consent of the federal data protection officer and the hospital ethics commission, routine clinical data from all eligible patients were extracted from the hospital’s IT system to an anonymized study database. The study was registered on ClinicalTrials.gov (identifier NCT03409055). Cardiac surgery was defined as a documented open procedure on the valves or vessels in proximity to the heart or coronary vessels identified by German OPS codes (5-35 and 5-36, excluding 5-35 A, i.e., minimally invasive valve replacement). Presence of postoperative pleural effusion was derived from the respective ICD-10 codes (J90, J91: pleural effusion; J94.2: hemothorax). We included the codes for hemothorax to capture the full spectrum of patients diagnosed with fluid around their lungs during this study period. By defining the index surgeries for this study using OPS codes, we excluded surgeries e.g. for mediastinitis. The senior physician in the ICU, together with a dedicated, trained administrative staff, completed all coding according to national coding guidelines. Patients were divided into three groups: Group 0 consists of patients with no coding for pleural effusion, Group 1 consists of patients with effusions that either had no postoperative interventions or whose first postoperative intervention was not an isolated secondary drainage procedure, and Group 2 consists of patients with effusions that received an isolated secondary drainage procedure in the ICU as their first intervention following the initial cardiac surgery (“index operation”). An isolated secondary drainage procedure was defined as the coding of a time stamp and at least one OPS code for a secondary drainage procedure throughout the hospital stay (8-152.1: thoracentesis; 8-144: minimal invasive placement of supplemental chest tube; 5-340.0: surgical placement of supplemental chest tube), with no OPS codes indicating any other procedures sharing the same time stamp. In addition to basic patient characteristics, Charlson Comorbidity Index score, surgery type, priority (i.e., elective, urgent, or emergency), duration of surgery (as a marker of its complexity), and post-operative APACHE II admission scores were assessed in order to characterize the study population and to identify possible confounders. Pre-existing medical conditions were derived from ICD-10 diagnostic codes available from the electronic patient records. Redo surgery for evacuation of pleural effusions was defined as thoracotomy, pericardiotomy, or mediastinotomy.

At the completion of surgery, all patients received at least one commercially available 30 French mediastinal chest tube connected to a vacuum for the initial postoperative blood evacuation after the primary surgery via a subcostal approach. In addition, intraoperative pleural tubes were placed when the pleural space was opened, e.g., harvesting LIMA. In case of significant preoperative pleural effusion, e.g., due to congestive heart failure, these patients also received a pleural drainage via subcostal approach during the index surgery. Those “primary” tubes were established by the cardiac surgeon. In case of significant postoperative pleural effusion, an additional “secondary” thoracentesis and/or chest tube was inserted in the anterior axillary line via mini-thoracotomy by the attending ICU practitioner. All chest tubes were actively drained by nurses as needed. All patients received a combined care pathway in the ICU and Intermediate Care Unite (IMCU). Postoperative medication, including diuretics, was supervised by the attending senior physician of the ICU according to common guidelines.

Primary end points were in-hospital mortality and length of stay; deceased patients were set to missing. Duration of mechanical ventilation in the ICU, incidence of renal replacement therapy (RRT) in patients without a medical history of chronic renal failure, or acute kidney injury (rise of creatinine > 0.3 mg/dl in 48 h during the hospital stay) during the hospital stay were defined as secondary end points.

Descriptive analyses and statistical testing were performed using R (version 3.6.3; R Foundation for Statistical Computing). When normal distribution was ruled out using the Kolmogorov-Smirnov test, results were given as medians and interquartile ranges (IQR), otherwise they were given as mean ± standard deviation (SD). Qualitative observations were characterized by numbers with percentages. Statistical significance among groups was analyzed univariately by the exact nonparametric Kruskal-Wallis test and (pairwise) with the exact Mann-Whitney U test. We removed the effect of baseline confounder variables by pairwise next neighbor matching (1:1:1). Pairing was done by using the propensity score method, as described previously by Kurth et al. [9]. We created a propensity score and performed next neighbor matching for the first and second group, followed by an additional propensity score creation and matching for the second and third group, with group order 0, 1, and 2. The following variables, which were judged to present major confounders, were used: age, sex, BMI, type of surgery, duration and priority of surgery, ASA classification, NYHA score, congestive heart failure, Charlson Comorbidity Index score, APACHE 2 score upon ICU admission, coronary heart disease, chronic obstructive pulmonary disease, peripheral vascular disease, diabetes, and chronic renal failure. A two-tailed p-value < 0.05 was considered statistically significant. The data sets generated and analyzed during this study are not publicly available due to reasons of data privacy; however, they are available from the corresponding author (FB) upon reasonable request.