British Journal of Anaesthesia 1991; 66: 25-30 PLASMA CONCENTRATIONS OF BUPIVACAINE AND TWO OF ITS METABOLITES DURING CONTINUOUS

An interscalene brachial plexus block was performed via a catheter with 20-28 ml of 0.75 % bupivacaine plus adrenaline for surgery of the shoulder region in 12 patients. Constant infusion of 0.25 % bupivacaine 0.25 mg kg~ ' /?" ' was continued for 24 h. During surgery light general anaesthesia, without analgesics, was maintained. Plasma concentrations of total and unbound (free fraction) bupivacaine, desbuty/bupivacaine (DBB), 4-hydroxybupivacaine (4-OHB) and alpha,-acid g/ycoprotein (AAG) were measured at predetermined intervals during the continuous block. The greatest mean plasma concentrations of bupivacaine were measured at 30 min (1.63 (SD 0.55) ng ml-1) and 60 min (1.38 (0.48) figmt'). There was a small but statistically significant increase in the plasma concentration of bupivacaine between 12 and 24 h of infusion. The mean unbound concentration of bupiv-acaine in plasma decreased from 0.044 (0.015) ng ml- ' (3.6 (1.1) % of total bupivacaine concentration) at 3 h to 0.023 (0.011) ng ml~' (2.1 (1.0) %) at 24 h. The AAG concentration in plasma increased by 38 % in 24 h. The metab-olites DBB and 4-OHB were detectable in plasma from 30 min. with a gradual increase during infusion. At 24 h the mean concentrations of DBB and 4-OHB were 0.33 (0.22) ng ml-1 andO. 13 (0.04) fig ml-1, respectively. There were no toxic reactions during the blocks