PLASMA BUPIVACAINE CONCENTRATIONS ASSOCIATED WITH CONTINUOUS EXTRADURAL INFUSIONS IN BABIES

The maximum recommended dose for extradural infusions of bupivacaine i n children older than 1 month is 0.5 mg kg(-1) h(-1) but there are few specific reports of the associated blood concentrations during infusi ons in babies. Toxic symptoms can occur in children at plasma concentr ations of bupivacaine as low as 2 mu g ml(-1). We attempted to measure venous plasma concentrations of total and free bupivacaine in babies aged 3-12 months during extradural infusions given at a rate commonly used in our hospital. We studied eight babies (mean age 33 weeks; mean weight 7.8 kg). After a mean initial dose of 1.2 mg kg(-1) (range 1.1 -1.3 mg kg(-1)), bupivacaine was infused at a mean rate of 0.38 (0.36- 0.39) mg kg(-1) h(-1) for a mean of 31 (4-44) h. Blood was obtained at 4, 8, 16, 24, 32 and 40 h after starting the infusion and plasma sepa rated by centrifugation. Total plasma bupivacaine concentration was me asured using high pressure liquid chromatography (HPLC). Plasma concen trations of total bupivacaine were mostly less than 2 mu g ml(-1). One baby had a concentration of 2.02 mu g ml(-1) at 32 h and showed clear evidence of accumulation of bupivacaine. Babies can accumulate bupiva caine and achieve plasma concentrations above the threshold for toxic side effects, despite infusion rates below the currently accepted maxi mum. The sample size in our study was small but we believe an extradur al infusion rate of 0.375 mg kg(-1) h(-1) is probably an absolute maxi mum for babies younger than 12 months.