Opioid free anesthesia (OFA) means a technique in which no intraoperative opioid is administered through any route. Perioperative pain management in an obese patient is challenging. The incidence of respiratory depression is higher in obese patients and is exaggerated with opioids, so the investigators are searching for a drug that has analgesic effect without any effect on respiratory function. In this study, the investigators will add melatonin to OFA in obese patients undergoing laparoscopic cholecystectomy.

Obesity leads to a restrictive lung disease, causing reduction in functional residual capacity and total lung compliance.

When an obese patient is supine and anesthetised, the depressant effects of many anesthetic agents and analgesics, particularly opioids, further decrease the lung compliance, leading to increased hypoxemia.

Opioid based general anesthesia in these patients increases the incidence of postoperative respiratory depression, atelectasis, and pneumonia. Also, pain relief with opioids is associated with sedation, hence impeding rapid recovery and early mobilization.

OFA is the use of multimodal or balanced analgesia. The principle of this is to gain additive analgesic effects from different drugs while minimizing side effects, particularly those of opioids. Studies have shown that OFA fast tracks surgery, reduces hospital stay, promotes early mobilization, and enteral nutrition.

Prior studies which investigated opioid free techniques are based on the combination of drugs acting on sympathetic nervous system, perioperative administration of local anesthetics, nonsteroidal anti-inflammatory drugs, and of adjuvant drugs, such as ketamine, magnesium etc.

Laparoscopic surgery is more challenging in obese patients since they have excessive pneumoperitoneal insufflation pressures, longer anesthetic, surgical, and recovery times. Moreover, these procedures are usually done in Trendelenburg position which further leads to increased airway resistance.

Melatonin is mainly secreted from the pineal gland by the suprachiasmatic nucleus. This neurohormone possesses a circadian secretion pattern and regulates the biological clock; it also offers antiemetic, analgesic, and anxiolytic effects. Due to its effect on both acute and chronic pain, melatonin fulfills a beneficial role in reducing postoperative opioid consumption while minimizing nausea and vomiting. In addition, melatonin can be used to moderate the effect of light on the autonomic system.

Several studies have reported that melatonin, as an analgesic, anti-inflammatory, anxiolytic, and anti-agitation premedication, is associated with sedation and anxiolysis without adverse effects on recall and driving performance.

Inclusion Criteria:

1. Age: 18-45 years.
2. Body Mass Index: Over 30 kg/m2.
3. Physical Status: ASA classification I and II.

Exclusion Criteria:

1. Patient refusal.
2. Age: Less than 18 years, more than 45 years.
3. Patients with known history of allergy towards one of the study drugs.
4. Patients with severe cardiac, respiratory, hepatic or renal disease.
5. Body Mass Index: Under 30 kg/m2.