Postoperative Bowel Function Recovery

Investigators will examaine the effects of Analgesia Nociception Index (ANI)-guided analgesia on postoperative bowel function recovery in laparoscopic colorectal surgery. This is a prospective randomized-controlled study. Investigators will randomly divide the patients into two groups. In ANI group, remifentanil infusion rate during anesthesia will be adjusted according to ANI monitoring. In Control group, remifentanil infusion rate during anesthesia will be adjusted according to the conventional method of blood pressure and heart rate monitroing. And Investigators will evalualte the bowel function recovery in both groups after surgery, and compare between the two groups.