Fentanyl with ropivacaine infusion for postoperative pain relief in infants and children. Kinetics of epidural fentanyl.

OBJECTIVES: The aim of the study was to evaluate pharmacokinetics of fentanyl administered as continuous epidural infusion with 0.2% ropivacaine for postoperative pain relief in infants and toddlers, and older children undergoing major abdominal and urological procedures.

METHODS: Thirteen infants and toddlers (median age 14 [range 3-36] months, 11 [5-17] kg, Group I) and 11 children (68 [45-131] months, 21 [16-52] kg, Group II) participated in the study. Epidural catheter was placed under general anesthesia in the L1-L2, L2-L3, or L3-L4 epidural space and threaded up to 4 cm into the epidural space. Bolus dose of 0.2% ropivacaine, 0.5 ml·kg(-1) and fentanyl, 2 μg·kg(-1) was given, then followed by continuous infusion of 0.2% ropivacaine, 0.15 ml·kg(-1) ·h(-1) with fentanyl 1.12 μg·kg(-1) ·h(-1) . In the postoperative period, fentanyl dose was reduced to 0.375 μg·kg(-1) ·h(-1) .

RESULTS: With this dosing regimen, fentanyl concentration in plasma was within the range of analgesic concentrations, and did not exceed 1.0 ng·ml(-1) . After discontinuation of epidural infusion, pharmacokinetics of fentanyl was complicated by a slight increase in plasma concentration during the elimination phase. Both elimination half-life of fentanyl (t1/2, MRT ) and mean residence time (MRT) were much longer than those observed after single IV bolus dose, and longer in Group I than in Group II (t1/2 MRT 15.9 [3.6-31.5] h vs 8.0 [7.1-13.3] h, P < 0.05, MRTstop-last 22.9 [5.1-45.5] h vs 11.5 [10.2-19.1] h, P < 0.05). Therefore, monitoring of vital signs seems warranted for several hours after the termination of the epidural infusion because risk of respiratory depression may persist, especially in the younger age group.