- 1. previous gastrectomy or colectomy 2. inadequate bowel preparation, 3. obstructive lesions of the colon 4. severe colitis, gastrointestinal bleeding 5. allergy to fentanyl or midazolam 6. American Society of Anesthesiology (ASA) classification of physical status grade 3 or higher, 7. mental retardation 8. regular use of narcotics 9. pregnancy 10. refusal to provide a written informed consent.