A Comparison of Ketamine Infusion Versus Placebo in Opioid Tolerant and Opioid Naive Patients After Spinal Fusion

Subjects aged 16 to 75, ASA I to III, scheduled for an elective lumbar fusion surgery of at least two levels under general anaesthesia, were prospectively studied. We defined opioid-tolerant as the daily use of opioid pain medication (oxycodone, morphine, hydromorphone, fentanyl, methadone, or tramadol) during the two weeks before surgery. Patients who did not fulfil that criterion were deemed to be opioid-naïve. Exclusion criteria were poorly controlled hypertension, severe cardiac or pulmonary disease, elevated intraocular pressure, severe hepatic or renal dysfunction, pregnancy, a history of psychiatric disorder, inability to speak English, inability to understand the numerical pain scale or to operate the patient-controlled analgesia pump, and known allergy to ketamine or hydromorphone.

Patients were allocated to the opioid-naïve or opioid-tolerant arm as defined above, and subsequently randomized into two groups, for a total of 4 groups: the opioid-naïve ketamine, the opioid-naïve placebo, the opioid-tolerant ketamine, and the opioid-tolerant placebo. Patients in each group were randomized to receive either a ketamine infusion (ketamine bolus 0.2 mg/kg over 30 minutes, started on arrival in post-anesthesia care unit (PACU), followed by a fixed-rate infusion of 0.12 mg/kg/h for 24 hours), or placebo (identical volume/rate of normal saline). Randomization within each group was performed by the study coordinator using a computer-generated random number list in a 1:1 ratio. Study medication and placebo were produced according to the randomization list in identical and consecutively numbered 250 mL bags. The pharmacist was not involved in patient care. Information about treatment was concealed but available for unblinding in case of acute complications. During the entire study period investigators performing the postoperative assessments, medical staff (nurse, anesthesiologist, and surgeon), and subjects were blinded to group allocation.

In all patients, general anesthesia was induced with propofol based on patient weight. Rocuronium 0.6 - 1.2 mg/kg was used to facilitate endotracheal intubation. Anesthesia was maintained with propofol (variable rate to maintain bispectral index (BIS) at a level acceptable for surgical anesthesia), desflurane \<1.5% mixed of air and oxygen, fentanyl, sufentanil, hydromorphone and morphine at the discretion of the anesthetic staff. Blood pressure was maintained within 20% of baseline, and hypotension was treated at the discretion of the anesthetic staff with isotonic sodium chloride solution, hetastarch, ephedrine and phenylephrine intravenously.

Most patients received 1000 mg of IV acetaminophen at the end of surgery (see Table 2). NSAIDs were not used in the immediate perioperative period because of the surgeons' concern that they might impede bone healing and proper fusion.

For all patients, postoperative pain treatment during the first 24 hours consisted of standard care of IV patient controlled analgesia (PCA) with hydromorphone (0.2 mg/dose, lockout 6 min, maximum 2 mg/h), started on arrival in PACU. Preoperatively, the patients were educated by the nursing staff in the use of the PCA pump and the numerical pain scale. Rescue medication of IV hydromorphone 0.2 to 0.3 mg as needed was administered by a nurse with the goal to reduce the numerical pain score (NPS; 0 = no pain, 10 = worst imaginable pain) below 4. Opioid pain medication was restricted to hydromorphone in order to allow for valid comparison between the groups. After 24 hours, the PCA was discontinued and all patients were treated according to the surgical department's standard regimen. Diazepam 2 mg IV was administered for severe muscle spasms as needed.

Moderate to severe nausea or vomiting was treated with IV ondansetron 4 mg. If ondansetron was ineffective, IV metoclopramide 10 mg was administered.

All postoperative assessments were performed by the study investigators or trained nurses blinded to group allocation. Cumulative IV hydromorphone consumption was calculated from 0 to 24 hours postoperatively. NPS were recorded at arrival to PACU, then every 30 min during the first 2 hours, and then every 2 hours on the floor during the first 24 hours after surgery while patients were awake.

The primary outcome was cumulative hydromorphone consumption during the first 24h after surgery. Secondary outcome was NPS in the same time period. We also recorded central nervous system adverse events during the same period.