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

Background: The aim of this study was to determine whether the use of adrenaline 1/400 000 added to 0.25% bupivacaine significantly delays the systemic absorption of the drug from the caudal epidural space in young infants. Methods: 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 1/400 000 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. Results: 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 (t(1/2)) 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%. Conclusion: The addition of 1/400.000 adrenaline prolongs the systemic absorption of caudally administered bupivacaine in infants less than 5 months of age.