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M ajor shoulder surgery is associated with severe postoperative pain, particularly in the first 48 hours. Interscalene analgesia by either continuous or patient-controlled infusion of local anesthetic is currently used. Ropivacaine has been used in this context and has a greater margin of safety than bupivacaine. Information about the pharmacokinetics of ropivacaine during continuous interscalene infusion is not available. This open, randomized study determined the pharmacokinetics, clinical efficacy, and safety of a 48-hour continuous interscalene infusion of ropivacaine for postoperative pain relief in 24 patients undergoing open major shoulder surgery.Patients were allocated to receive a continuous interscalene infusion of 2 mg/mL ropivacaine at a rate of either 6 or 9 mL/h (12 or 18 mg/h). Total and unbound plasma concentrations of ropivacaine and 2,6-pipecoloxylidide (PPX), a major active metabolite, were determined during and up to 6 hours after the interscalene infusion. Postoperative pain at rest was assessed by a visual analog scale, and the need for supplementary analgesics and the occurrence of adverse events were recorded.Plasma concentrations of total and unbound ropivacaine were proportional to the total dose. At the end of the interscalene infusion of 9 mL/h, the mean ± SD plasma concentrations of total and unbound ropivacaine were 1.40 ± 0.54 and 0.03 ± 0.01 mg/L, respectively, and of total and unbound PPX, 0.70 ± 0.38 and 0.30 ± 0.20 mg/L, respectively. Plasma levels of unbound PPX and unbound ropivacaine, added together, were well below threshold levels for systemic CNS toxicity.Pain scores at rest were similar in both groups as were time to first administration and dose of supplementary analgesics. Pain relief was considered as excellent in 63% of the 9-mL/h group and 43% in the 6-mL/h group. Blood pressure, pulse rate, respiratory, and oxygen saturation remained stable throughout the study. No signs or symptoms of local anesthetic toxicity were seen in any patient.Using the protocol of this study, satisfactory postoperative pain relief was obtained after major shoulder surgery. There was a dose-proportional increase in the plasma concentration of total and unbound ropivacaine; however, unbound plasma concentrations of ropivacaine and PPX were well below the threshold for systemic CNS toxicity.

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