Ethics committee approval was obtained for this study, which was planned as a prospective, randomized, controlled, single-center trial to be conducted in patients undergoing coronary artery bypass grafting (CABG) surgery between December 2025 and March 2026.

Patients eligible for inclusion were randomized into two groups at a 1:1 ratio using a random number list generated via the random.org website. Randomization was performed by an independent investigator who was not involved in the study procedures. The allocation sequence was placed into sequentially numbered, opaque, sealed envelopes. Upon enrollment of each patient, the next numbered envelope was opened to determine group assignment.

Patients who did not complete the study protocol or did not receive the allocated medication were excluded from the analysis, and a per-protocol analysis approach was applied. The randomization list was kept confidential and was accessible only to authorized statistical personnel. Patients were followed in two groups: the Control Group (Group C) and the Morphine Group (Group M).

All procedures were performed at the Istanbul University-Cerrahpaşa Cardiology Institute. After obtaining written informed consent, all patients were transferred to the operating room, where an 18G intravenous cannula was inserted, followed by premedication with midazolam 0.03 mg/kg (IV).

Standard monitoring included 5-lead electrocardiography (ECG), heart rate measurement, invasive arterial blood pressure monitoring, and peripheral oxygen saturation assessment. Anesthesia induction was standardized for all patients using midazolam 0.15 mg/kg, fentanyl 10 µg/kg, and rocuronium 0.6 mg/kg (IV). Orotracheal intubation was performed using an endotracheal tube of appropriate size according to the patient's age and sex.

Following intubation, mechanical ventilation was initiated and adjusted to maintain end-tidal carbon dioxide (EtCO₂) levels between 35 and 45 mmHg. Anesthesia maintenance was achieved using a total intravenous anesthesia (TIVA) technique with continuous infusions of propofol (1-10 mg/kg/h), fentanyl (2-4 µg/kg/h), and rocuronium (20 mg/h), targeting a bispectral index (BIS) value between 40 and 60. Prior to surgical incision, patients in the Morphine Group (Group M) received 5 mg intravenous morphine, administered with careful consideration of their hemodynamic status.

During surgery, all patients were placed on cardiopulmonary bypass (CPB). After completion of graft anastomoses and stabilization of vital signs, patients were weaned from CPB. At this stage, patients in Group M received an additional 5 mg intravenous morphine. In contrast, patients in the Control Group (Group K) continued to receive perioperative fentanyl infusion.

Postoperatively, patients were transferred to the intensive care unit (ICU) while intubated and mechanically ventilated. Extubation was performed according to clinical criteria. In the ICU, all patients received intravenous paracetamol 1000 mg every 6 hours and tramadol 100 mg intravenously. Pain intensity was assessed using the Numerical Rating Scale (NRS) / Visual Analog Scale (VAS). If the pain score exceeded 4, 3 mg intravenous morphine was administered, and total morphine consumption was recorded.

Preoperative variables collected included age, sex, height, weight, body mass index (BMI), and American Society of Anesthesiologists Physical Status (ASA PS) classification. Intraoperative variables included administered drug doses, duration of surgery, cardiopulmonary bypass time, and total anesthesia duration.

In the postoperative period, the duration of mechanical ventilation in the intensive care unit (ICU), the presence of nausea and vomiting, postoperative ileus, and respiratory depression will be recorded. In addition, pain intensity at the thoracic tube insertion site, sternum, and saphenous vein graft incision will be assessed at the 1st, 2nd, 4th, 8th, and 24th hours following extubation using the Numerical Rating Scale (NRS) / Visual Analog Scale (VAS). All pain assessments will be performed and recorded by a blinded observer. Pain associated with the urinary catheter will also be evaluated using the NRS.

Prolonged postoperative ileus will be defined as the presence of abdominal distension and absence of bowel sounds in a patient without mechanical intestinal obstruction who experiences nausea or vomiting and fails to pass flatus or stool for more than three postoperative days. The severity and frequency of postoperative nausea and vomiting (PONV) will be assessed. Respiratory depression will be defined as a suppressed ventilatory response to hypercapnia or hypoxemia.

Pain intensity at the thoracic tube insertion site, sternum, and saphenous vein graft incision will be assessed at the 1st, 2nd, 4th, 8th, and 24th hours following extubation using the Numerical Rating Scale (NRS) / Visual Analog Scale (VAS).

Inclusion Criteria:

• patients aged 18 to 80 years
• classified as American Society of Anesthesiologists (ASA) III
• who are scheduled for elective coronary artery bypass grafting.

Exclusion Criteria:

• Chronic opioid use.
• Severe liver or kidney disease.
• History of alcohol or substance abuse.
• Left ventricular ejection fraction below 40%.
• Inability to be weaned from mechanical ventilation within 12 hours after completion of surgery.
• Undergoing combined cardiac surgery, including valve procedures.
• Requirement for intra-aortic balloon pump (IABP) support.
• Surgery performed on a beating heart.
• Emergency surgery.
• Reoperation due to bleeding or other complications.
• Refusal to participate in the study.