Colonoscopy is one of the most commonly performed outpatient procedures throughout the world as a screening, diagnostic, and therapeutic tool to evaluate the large intestine from the distal rectum to the cecum.Propofol is the most commonly used hypnotic agent in sedation during endoscopic procedures for its advantages of fast onset and quick recovery. However, propofol may cause respiratory depression and cardiovascular events, which are easier to appear in cases of old age and high speed administration of propofol.Magnesium is a non-specific calcium channel inhibitor and a non-competitive N-methyl-D-aspartate(NMDA)receptor antagonist, which has analgesic and mild sedative properties .

Colonoscopy is one of the most commonly performed outpatient procedures throughout the world as a screening, diagnostic, and therapeutic tool to evaluate the large intestine from the distal rectum to the cecum . Colonoscopy is currently an important examination method for colorectal diseases. It can provide clear images of lesions and provide biopsy for biopsy. It can improve the accuracy and sensitivity of colorectal cancer diagnosis .The most common complications in gastrointestinal endoscopy are not related to the procedure, but they are related to sedation; they include cardio- respiratory adverse events such as hypoxemia, hypoventilation, apnea, dysrhythmias, hypotension and vasovagal episodes .Propofol is the most commonly used hypnotic agent in sedation during endoscopic procedures for its advantages of fast onset and quick recovery. However, propofol may cause respiratory depression and cardiovascular events, which are easier to appear in cases of old age and high speed administration of propofol . Moreover, high doses of propofol may cause dose-dependent hemodynamic instability in older or feeble patients. Therefore, it is essential to minimize the dosage of propofol to prevent cardiopulmonary complications associated with sedation, especially in the elderly, by combining with an adjuvant medication. Although benzodiazepines or opioids combined with propofol can reduce the consumption of propofol, elderly patients usually feature a higher overall body fat content than younger patients which may delay the metabolism of lipid-soluble propofol, opioid, and benzodiazepines . Magnesium is the fourth most plentiful cation in the body and the second most plentiful intracellular cation after potassium. Approximately one half of total body magnesium is present in bone and 20% in skeletal muscle. Magnesium is necessary for the presynaptic release of acetylcholine from nerve endings and may produce effects similar to calcium-entry-blocking drugs . Magnesium is a non-specific calcium channel inhibitor and a non-competitive N-methyl-D-aspartate(NMDA)receptor antagonist, which has analgesic and mild sedative properties . It has been demonstrated that intravenous administration of magnesium sulfate can reduce the overall use of intraoperative propofol and the postoperative analgesia requirement effectively in various types of surgery .These findings implied that magnesium sulfate may be used as a promising adjuvant drug for colonoscopy sedation due to its analgesic and sedative properties .

Inclusion Criteria:

• 1. The subject is scheduled for elective colonoscopy. 2. The subject is ≥ 40 years and ≤ 70 years. 3. Both sexes. 4.No obvious abnormalities in preoperative ECG, blood routine, electrolytes, and other tests. 5. ASA class 1-3.

Exclusion Criteria:

• 1.Subject is known or believed to be pregnant or lactating women. 2. Patients are known hypersensitivity to any of the drugs that would be used in the study. 3. Severe cardiac, renal, lung, or liver diseases. 4. Hypotension (systolic blood pressure< 90 mm Hg), uncontrolled hypertension (systolic blood pressure> 170 mmHg, diastolic blood pressure> 100mmHg). 5. Chronic Opioid Use (daily or almost daily use of opioids for> 3 months). 6. Patients that are immunologically compromised. 7. Sleep apnea syndrome or difficult airway. 8. preexisting hypoxemia (Spo2< 90 %). 9. History of adverse events with prior sedation. Additionally, patients who had taken any sedative drugs within the previous 24h