Extradural pain relief in labour: Bupivacaine sparing by extradural fentanyl is dose dependent

The minimum local analgesic concentration (MLAC) of bupivacaine in labour is defined as the effective concentration in 50% of subjects (EC50). We have used the technique of double-blinded sequential allocation to quantify the bupivacaine sparing effect of the addition of four different doses of extradural fentanyl in 223 labouring women. There were five groups: (1) plain bupivacaine (control); (2) bupivacaine with fentanyl 1 mu g ml(-1); (3) bupivacaine with fentanyl 2 mu g ml(-1); (4) bupivacaine with fentanyl 3 mu g ml(-1); and (5) bupivacaine with fentanyl 4 mu g ml(-1). The MLAC of bupivacaine were 0.069% w/v, 0.057% w/v, 0.048% w/v, 0.031% w/v and 0.015% w/v, respectively. We observed a reduction in MLAC of 18%, 31% (P=0.03%), 55% (P<0.0001) and 72% (P<0.0001) with fentanyl 1, 2, 3 and 4 mu g ml(-1), respectively, demonstrating a significant negative linear trend (P<0.0001) with increasing fentanyl dose. The incidence of pruritus was increased significantly with fentanyl 4 mu g ml(-1) (P=0.0015). Because of this, fentanyl 3 mu g ml(-1) may be the optimal dose when the aim is bupivacaine sparing extradural analgesia during labour.