Plasma concentrations and pharmacokinetics of bupivacaine with and without adrenaline following caudal anaesthesia in infants.

Abstract

The aim of this study was to determine whether the use of adrenaline 1/400000 added to 0.25% bupivacaine significantly delays the systemic absorption of the drug from the caudal epidural space in young infants. Fifteen infants less than 5 months of age undergoing minor lower abdominal procedures under a standardised general anaesthetic were randomised to receive a caudal block with either 0.25% plain bupivacaine 2.5 mg/kg (n=7) or bupivacaine 0.25% with 1/400000 adrenaline (n=8). Blood samples were drawn at 30, 60, 90, 180, 240 and 360 min according to the infant's weight and analysed for total and free bupivacaine concentrations using a gas chromatography-mass spectrometry (GC-MS) technique. The total C(MAX) and T(MAX) were comparable in both groups. The total bupivacaine concentration at t=360 min was significantly higher in the "adrenaline" group compared to the "plain" group, i.e. a median (range) 742 ng/ml (372-1423 ng/ml) vs. 400.5 ng/ml (114-446 ng/ml), P=0.0080. The median "apparent" terminal half-life (t1/2) was significantly longer in the "adrenaline" group (363 min; range 238-537 min) compared to the "plain" group (n=6) (165 min; range 104-264 min), P=0.0087. The free bupivacaine concentrations (n=3 in both groups) ranged between 13 ng/ml and 52 ng/ml, corresponding to a percentage of free bupivacaine between 1.3% and 6.7%. The addition of 1/400.000 adrenaline prolongs the systemic absorption of caudally administered bupivacaine in infants less than 5 months of age.