Loop formation is the most frequent cause of cecal intubation failure during colonoscopy. To reduce the loop formation, external abdominal pressure is widely used and proved to be helpful. Properly applied pressure can also decrease patients discomfort and shorten the cecal intubation time.

The loop formation during water exchange is less severe as compared with during air insufflation and can be reduced quite readily. Traditionally an assistant is not asked to administer abdominal compression until the endoscopist has struggled for some time and failed to reduce the loops by withdrawal. The colonoscopist can administer the abdominal compression whenever the scope is not advancing smoothly, probably in the early stage of loop formation. We test the hypothesis that colonoscopist administered abdominal compression to remove loops in their early stage of formation hastens cecal intubation.

A total of 120patients will be randomized in a 1:1 ratio (n=60 per group). When the tip of the scope doesn't advance or paradoxical movements occur, loop reduction by withdrawal of the scope will be implemented. If looping persists, abdominal compression will be applied. In the endoscopist-administered abdominal compression (endoscopist) group, the colonoscopist will apply the compression with his right hand and counter the pressure by pushing the back of the patient with his left forearm with the colonoscope in his left hand. The compression will be administered at left lower quadrant when the scope is in the sigmoid colon and at left lower quadrant and upper abdomen, respectively, when the scope tip reaches the transverse or ascending colon. If the formation of loop cannot be overcome, an assistant will apply the abdominal compression instead. In the assistant-administered abdominal compression (assistant) group, an endoscopic assistant will apply abdominal compression when a loop is formed. The assistant will apply the compression at the left lower quadrant initially, but quickly shift to other parts as needed depending on the tip location of colonoscope. If manual compressions fail, then the patients' position will be changed.

Inclusion Criteria:

• In the Buddhist Dalin Tzu Chi Hospital, Buddhist Tzu Chi Medical Foundation, Chiayi, Taiwan.
• Patients who undergo WE colonoscopy performed by the two endoscopists (YHH and CWT) at the endoscopic suite will be included.

Exclusion Criteria:

• Included patient declined to give consent,
• age <20 years old,
• age >80 years old,
• previous partial colectomy, not completely consumed bowel prep regimen, massive ascites, or known colonic obstruction, morbid obesity (BMI ≥ 35).