An isobolographic analysis of diamorphine and levobupivacaine for epidural analgesia in early labour

BackgroundFew data describe the pharmacological interactions between local anaesthetics and opioids. The aim of this study was to measure the median effective concentration (MEC) of diamorphine and levobupivacaine when given separately and as mixtures for epidural analgesia, and determine whether the combination is additive or synergistic.MethodsOne hundred and twenty patients were enrolled in this prospective randomized, two-phase, double-blind study. In the first phase, 60 women were randomized to receive a fixed 20 ml volume of either levobupivacaine or diamorphine epidurally. Dosing was determined using up-down sequential allocation with testing intervals, respectively, of 0.01%w/v and 12.5 µg ml−1. After estimations of the MEC of levobupivacaine and diamorphine, a further 60 patients were randomized in the second phase to one of the three mixtures: (a) diamorphine 70 µg ml−1 (fixed) and levobupivacaine (testing interval 0.004%w/v, starting at 0.044%w/v); (b) levobupivacaine 0.044%w/v (fixed) and diamorphine (testing interval 7 µg ml−1, starting at 70 µg ml−1); and (c) bivariate diamorphine and levobupivacaine (testing intervals of 7 µg ml−1 and 0.004%w/v starting at 70 µg ml−1 and 0.044% w/v respectively).ResultsThe MEC estimates from the first phase were 143.8 µg ml−1 (95% CI 122.2–165.3) for diamorphine and 0.083%w/v (95% CI 0.071–0.095) for levobupivacaine. In the second phase, the MEC and interaction index (γ) of the three combinations were: diamorphine 65.5 µg ml−1 (56.8–74.2), γ = 0.99; levobupivacaine 0.041%w/v (0.037–0.049), γ = 0.98; and for the fixed combination diamorphine 69.5 µg ml−1 (60.5–78.5) and levobupivacaine 0.044%w/v (0.039–0.049), γ = 1.02.ConclusionThe combination of diamorphine and levobupivacaine is additive and not synergistic when used for epidural analgesia in the first stage of labour.