The pharmacodynamics and serum concentration of tramadol during continuous intravenous infusion for postoperative analgesia

Objective To examine pharmacodynamics and serum concentration of tramadol during continuous intravenous infusion for postoperative pain relief Methods 500 ASA Ⅰ Ⅱ patients undergoing operation on extremities, spine or abdomen under anesthesia were studied Premedication included phenobarbital 0 1g and atropine 0 5mg im Anesthesia was induced with midazolam 0 08 0 12mg·kg -1 , fentanyl 5 6μg·kg -1 and vecuronium 0 12 0 14 mg·kg -1 and maintained with continuous intravenous infusion of propofol 3 0 4 5 mg·kg -1 ·h -1 , fentanyl 2 8 3 4μg·kg -1 ·h -1 and vecuronium 0 06 0 08 mg·kg -1 ·h -1 supplemented with inhalation of 0 8% 1 0% isoflurane At the end of operation a loading dose of tramadol 1 5 mg·kg -1 was given intravenously over 2 min, followed by continuous intravenous infusion of tramadol for 72h The patients were divided into two groups: group Ⅰ (n=246) received tramadol intravenous infusion at a rate of 8mg h -1 and group Ⅱ (n=254) received tramadol infusion at a rate of 10mg h -1 Venous blood samples were taken from 10 patients in group Ⅱ at 0, 0 5, 1, 3, 6, 12, 24, 30, 42, 48, 54, 60, 72h during postoperative tramadol infusion for determination of serum concentration of tramadol by high performance liquid chromatography (HPLC) Efficacy of analgesia was assessed by VAS score and side effects were recorded Results The two groups were comparable with regard to age, sex, weight, types of operation and the amount of fentanyl used during operation There was significant difference in the mean VAS scores between group Ⅰ (1 16±1 15) and group Ⅱ (0 83±1 33) (P0 01) The overall satisfaction rate was 84 15% in group Ⅰand 91 33% in group Ⅱ(P0 01) However there was no significant difference in side effects such as nausea, vomiting and drowsiness between the two groups except the occurrence of urine retention which was more frequent in group Ⅱ (10 6%) than in group Ⅰ (6 25%) The mean serum concentration of tramadol during tramadol iv infusion at a rate of 10 mg·h -1 was (0 521±0 216)μg·ml -1 Couclusions Continuous intravenous infusion of tramadol 10 mg·h -1 can provide satisfactory postoperative pain relief with fewer side effects The therapeutic serum concentration of tramadol for postoperative analgesia is about 500ng·ml -1