This prospective, randomized controlled trial compares traditional air insufflation with water immersion and water exchange in patients undergoing colonoscopy using on demand sedation. We test the hypothesis that compared with air insufflation the proportion of patients who require on demand sedation during colonoscopy will be significantly lowered by water immersion and water exchange, and water exchange will produce the greatest reduction

The patients will be allocated into 3 groups by computerized randomization. In group A (water exchange), water will be infused and removed at the same time throughout the entire colon during the insertion phase with the air pump turned off. In group B (water immersion), water will be infused in the insertion phase and removed in the withdrawal phase. The water will be used mainly to open the lumen, without attempting to clear the colon contents. In group A and B, warm-to-touch water will be infused mainly infused through the accessory channel of the colonoscope using a foot-switch controlled water pump (JW2, Fujinon, Saitama, Japan). Air insufflation not used until the cecum has been reached. . In group C, air insufflation will be used throughout the procedure. Aliquots of 30 to 50 ml of water will be used for washing residual stool, as needed. In all three groups, loop reduction maneuvers, abdominal compression and change of patient position will be utilized at the discretion of the colonoscopist and recorded. Intubation of the cecum will be defined as successful only if the base of the cecum is touched with the tip of the colonoscope. Detailed examinations will be undertaken during the withdrawal phase.

During colonoscopy, a study nurse will ask the patient to report the level of pain (0 = none, 10 = most severe) at 2 to 3-minute intervals or at any time the patient voiced discomfort. For pain scores ≧2, maneuvers to minimize pain will be implemented. Immediately thereafter, the nurse will offer sedation, which the patients can accept or decline. If accepted, after an initial bolus of 1 mg/kg or 0.5 mg/kg for patients over 65 years propofol (Diprivan, Astra-Zeneca, Stockholm, Sweden) will be titrated in 20- to 30-mg increments to achieve an adequate level of sedation. The need of additional drug will be estimated by patient pain response (moans, grimaces and movements). Usually no more propofol will be added during the withdrawal phase. The initial amount of propofol, and those before and after arrival to the cecum will be recorded.

The following parameters will be evaluated and recorded on the patient data sheet: quality of bowel preparation, cecal intubation time, withdrawal time total procedure time, use of abdominal pressure, need for changing position, presence of polyps and reasons for incomplete colonoscopy.

The procedure will be recorded and stored as digital files.

Inclusion Criteria:

• Patients undergoing colonoscopy performed by the endoscopist at our endoscopic suite

Exclusion Criteria:

• request for sedation, indicated for bidirectional endoscopy, obstructive lesions of the colon, allergy to meperidine or propofol, American Society of Anesthesiology (ASA) risk Class 3 or higher, massive ascites, past history of partial colectomy, or refusal to provide written informed consent.