2.2. Setting and participants

We included patients undergoing colorectal cancer surgery under general anaesthesia in Denmark 2008–2018. We excluded endoscopic local resections and operations where no tumour resection was performed. During the study period, all colorectal cancer surgery in Denmark was performed in public hospitals reporting to the Danish Colorectal Cancer Group Database. During the study period, the use of minimally invasive surgery increased from about 25%–80% and the median length of postoperative hospital stay decreased from 7 to 4 days (Figures S1 and S2). During the study period, the level of the epidural catheters was categorized as thoracic for 80% of patients undergoing colorectal cancer surgery.

All included patients received general anaesthesia for surgery and were classified according to preoperative insertion of an epidural catheter. Thus, patients receiving preoperative epidural analgesia registered in the Danish Anaesthesia Database constituted the treatment group, and the control group were patients not receiving epidural analgesia preoperatively. Spinal anaesthesia in combination with general anaesthesia is rarely used in Denmark and the duration and physiological impact is substantially different from epidural analgesia. Therefore, we excluded patients undergoing spinal anaesthesia without epidural analgesia.