PCA Ketamine-Morphine VS PCA Morphine as Post-Operative Analgesia in Colorectal Surgery

The goal of this clinical trial is to compare the effectiveness of PCA ketamine-morphine versus conventional PCA morphine in postoperative patients undergoing elective laparotomy colorectal surgery under general anaesthesia. The specific objectives are: 1. To compare the post-operative analgesic requirement with PCA ketamine-morphine in comparison with PCA morphine. 2. To compare the postoperative pain scores between PCA ketamine-morphine and PCA morphine. 3. To assess patients\' overall satisfaction with PCA ketamine-morphine in comparison with PCA morphine. 4. To study the incidence of side effects of PCA ketamine-morphine in comparison with PCA morphine. Participants will be screened and recruited at pre-anaesthetic clinic (PAC). Those who consented will be taught to use PCA machine and the potential side effects of the study drugs. They will be randomly allocated into either Group A or Group B by computer generated randomization a day before planned surgery. Researchers will compare Group A and Group B to see post-operative pain scores, patients\' overall satisfaction and any incidence of side effects.