The goal of this [clinical trial] is to [explore the balance time point between the waiting time for intestinal preparation after gastric emptying and quality of bowel preparation.] in [patients with painless colonoscopy]. The main question[s] it aims to answer are:

[whether the gastric is empty or not 2 hours after finishing drinking 1.5 L of MgSO4 solution within 2 hours] [whether the quality of patients' bowel preparation is superior in a time period of 2-4 hours after finishing drinking 1.5 L of MgSO4 solution] Participants will [ be asked finishing the last dose of 1.5 L MgSO4 solution within 2 hours and received ultrasound gastric assessment immediately 2 or 4 hours after finishing drinking MgSO4 solution in a 15 minutes time interval until the gastric is empty. As soon as the gastric is empty, patients receive painless colonoscopy within 2 hours or 2-4 hours.

Researchers will compare [patients receiving painless colonoscopy within 2 hours or 2-4 hours after finishing drinking 1.5 L of MgSO4 solution] to see if [the quality of patients' bowel preparation is superior in a time period of within 2 hours].](streamdown:incomplete-link)

All subjects ate a light diet the day before the examination and fasted after 19:00 on the night before examination. A segmented intestinal cleansing regimen of 3L MgSO4 was adopted: 1.5L was taken one night before the examination; On the day of the examination, the second dose of 1.5L was taken and finished within 2 hours. The medication method was guided by professional nursing staff. All subjects were instructed to take appropriate activities during the administration of intestinal cleanser to promote excretion. Patients were divided into two groups (group A and group B) using random number table method. Patients in group A received gastric ultrasound examination every 15 minutes 2 hours immediately after taking the last dose of MgSO4 solution. During this period, if the gastric ultrasound showed that the patient had an "empty stomach", and painless colonoscopy was performed within 2 hours while the gastric is empty. If the gastric ultrasound still showed "full stomach" at 4h, the patient was abandoned, but gastric ultrasound assessment continued until the stomach was empty. The patients in group B received gastric ultrasound examination every 15min 4h immediately after taking the last dose of MgSO4 solution on the examination day, and colonoscopy was to completed within 4-6h after taking the MgSO4 solution if the gastric ultrasound showed that the patient had an "empty stomach". If the gastric ultrasound still shows "full stomach" at 6h, it is eliminated, but continuing to perform gastric ultrasound every 15min until the " stomach was empty ". All patients received general anesthesia by the same senior anesthesiologist using propofol 2 mg/kg and remifentanil 0.2 ug/kg. The BIS value of anesthesia depth was controlled in the range of 40-60, and propofol and remifentanil were added as needed. The same chief gastroenterologist performed the colonoscopy and evaluated the quality of intestinal cleaning.

Inclusion Criteria:

1.Patients who receiving elective colonoscopy under general anesthesia

Exclusion Criteria:

1. Patients with severe heart and lung diseases who cannot tolerate anesthesia or colonoscopy
2. chronic constipation
3. history of gastric or intestinal surgery
4. diabetes, Parkinson's disease, stroke,