Postoperative pain and analgesic treatment still remains a challenge in daily perioperative medicine. Skin incisions, intraoperative tissue retraction and -dissection, intravasal cannulations and drainages, sternotomy and pericardiotomy are the most important reasons for postoperative pain. Poorly controlled pain can contribute directly or indirectly to postoperative complications, such as myocardial ischemia, pulmonary dysfunction like hypoventilation, pneumonia and atelectasis, a delayed return of gastrointestinal function and decreased mobility. In addition, prolonged acute pain also results in chronic pain.

Opioids are internationally recognized as the golden standard in the treatment of acute postoperative pain. On one side, the high potency of opioids in pain relief is clearly undisputed, but on the other hand, the administration of opioids is associated with nausea, vomiting, sedation and with the development of bowel dysfunction, which encompasses symptoms including bloating, abdominal spasm, cramps and constipation. Opioid-induced constipation is a frequently reported adverse effect and sometimes requires discontinuation of therapy, which results in analgesic under-treatment, severely impairing quality of life. However, there are many different regimes for the treatment of postoperative pain using opioids. Patient-controlled analgesia (PCA) using morphine is widely used, but requires trained staff and expensive equipment. Once patients are able to tolerate oral medications, the oral route is preferred postoperatively because it is more convenient, noninvasive and less expensive.

Inclusion Criteria:

• Age from 18 to 90 years
• ASA physical status 1-3
• Elective major cardiac surgery requiring sternotomy
• Oral and written consent
• Postoperative extubation within four hours after arrival at the ICU
• Cognitive ability in the use of the PCA pump and the VAS

Exclusion Criteria:

• Chronic use of opioids in the last three months
• Chronic use of tranquilizer or pain medications
• Hypersensitivity against opioids
• Use of monoamine oxidase inhibitors in the last two weeks before surgery
• Alcohol or drug abuse
• Renal dysfunction (GFR < 30 or necessity of dialysis)
• Liver Dysfunction defined as Child-Pugh-Score 7-15
• Ejection fraction (EF< 40%)
• Malabsorption syndrome
• Neurologic or cognitive dysfunction
• Pregnancy
• Participation in another clinical trial
• Severe respiratory depression with hypoxia and/or hypercapnia
• Severe chronic obstructive pulmonary disease
• Severe bronchial asthma
• Non-opioid induced paralytic ileus
• Risk of seizures