Compared with performing each procedure individually, performing two combined procedures such as colonoscopy consecutive Esophagogastroduodenoscopy (EGD) cause more bowel gases, abdominal distension and post-procedure pain because of longer procedure time related to more bowel insufflated gas than one procedure. To the best of the investigators knowledge, there has been no randomized controlled trial of CO2 versus air for insufflations during combined two procedures, colonoscopy with consecutive EGD. And there are a few well randomized trials concerned CO2 insufflation in patients receiving sedation during colonoscopy.