A Comparison of Two Constant-Dose Continuous Infusions of Remifentanil for Severe Postoperative Pain Enrique Calderón, MD, PhD, Antonio Pernia, MD, Pedro De Antonio, MD, Enrique Calderón-Pla, MD, and Luis-Miguel Torres, MD, PhD Anesthesiology Department, Puerta del Mar University Hospital, Cádiz, Spain We evaluated the analgesic efficacy and safety of two con- tinuous constant-dose infusions of IV remifentanil, with- out infusion rate increments or the addition of boluses, in patients with severe postoperative pain during the first 4 h after general anesthesia with IV propofol-remifentanil. Thirty patients were randomly assigned to two groups of 15 subjects each according to the remifentanil dose admin- istered: 0.1 g · kg -1 · min -1 IV (Group A) or 0.05 g· kg -1 · min -1 IV (Group B). Rescue analgesia was pro- vided with meperidine (0.5 mg/kg IV) when pain inten- sity on the simple verbal scale (SVS) 2. The criteria for adequate analgesia (SVS 0 –1, respiratory frequency 8/ min. and Spo 2 90%) after 4 h were met by 78% and 75% of the patients in Groups A and B, respectively (P = ns). “Meperidine rescue” analgesia was significantly more in Group B (26%) than in Group A (6%) (P  0.05). There were no cases of respiratory depression, and nausea and emesis occurred in one patient in each group (6.5%). We conclude that IV remifentanil is an effective and safe opi- oid for the treatment of postoperative pain at a constant dose of 0.1 g · kg -1 · min -1 with a need for rescue anal- gesia 4 times less than a constant dose of 0.05 g · kg -1 · min -1 . (Anesth Analg 2001;92:715–9) R emifentanil is the latest piperidine-derived opi- oid that acts as a -receptor agonist. The phar- macokinetic properties of remifentanil are unique among the opioids, with the drug having a very rapid plasma clearance and onset time and a very short context-sensitive half-life (2–10 min.). Conse- quently, it can be administered in prolonged infusion without the risk of accumulation (1,2). The absence of residual analgesia after remifentanil-based anesthesia requires the provision of adequate postoperative an- algesia before suspending infusion of the drug, or the maintenance of remifentanil infusion at analgesic doses during the postoperative period (3). In a study by Bowdle et al. (4) adequate postoperative analgesia with IV remifentanil infusions between 0.05– 0.15 g · kg -1 · min -1 was reported. In this study, varia- tions in the infusion rate and supplementary boluses were allowed; a frequent incidence of respiratory ad- verse events (29%) was noted. The most effective analgesic approach and dose reg- imen for remifentanil in the postoperative period when the drug is used as a single analgesic during surgery remains to be determined. We hypothesized that a constant-dose continuous in- fusion of remifentanil 0.1 g · kg -1 · min -1 IV main- tained for 4 h is effective and safe for the treatment of moderate-severe postoperative pain. Thus, the aim of this clinical trial was to compare two different continu- ous constant-small-dose infusions of remifentanil, with- out infusion rate increments or the addition of boluses after surgery, in patients with severe postoperative pain after propofol-remifentanil general anesthesia. Methods After local ethics committee approval and written in- formed consent, 30 patients ASA physical status I-II, 18 yr of age or older, scheduled to undergo elective abdominal or thoracic surgery, were enrolled in this randomized, double-blinded study. The efficacy of two different infusion rates of IV remifentanil for the treatment of severe postoperative pain after propofol- remifentanil general anesthesia. The patients were randomly assigned to two groups of 15 subjects each according to the remifentanil dose administered postoperatively in constant infusion: 0.1 g · kg -1 · min -1 IV (Group A) or 0.05 g · kg -1 · min -1 IV (Group B). Exclusion criteria were cardiovascular or central nervous pathology; allergies to opioids, chronic opioid or psychoactive drug use, or a history of drug addiction or alcohol abuse. Accepted for publication November 29, 2000. Address correspondence to E. Calderon, MD, PhD, Hospital U. Puerta del Mar, Avd Ana de Viya 21. 10. 09 —Cádiz, Spain. ©2001 by the International Anesthesia Research Society 0003-2999/01 Anesth Analg 2001;92:715–9 715