Characteristics of included studies.

Note: ASA: American Society of Anesthesiology; LC: laparoscopic cholecystectomy; OC: open cholecystectomy; LCR: laparoscopic colorectal cancer resection; OCR: open colorectal cancer resection; LADG: laparoscopic assisted distal gastrectomy; OADG: open assisted distal gastrectomy; LGBP: laparoscopic gastric bypass; OGBP: open gastric bypass; NA: not available;