On the relative potency of amino-amide local anaesthetics in vivo

With the aim of comparing the analgesic effectiveness of lidocaine, prilocaine, bupivacaine and etidocaine in vivo, a study of the relationships between dose and duration of infraorbital nerve block (IONB) of various intensities (IONB degrees 3-10) was performed in the rat. With increasing doses longer durations of action were obtained. Further analyses were performed using multiple regression analysis. The log (dose)-duration lines for bupivacaine and etidocaine were found to be linear, whereas those for lidocaine in degree 3 and degree 10 IONB and for prilocaine in degree 10 IONB were so only after omission of some data. The only comparisons yielding no deviation from parallelism of log (dose)-duration lines were etidocaine vs. lidocaine at IONB degree 10 and lidocaine vs. prilocaine and bupivacaine vs. etidocaine at IONB degree 3. The difference between these agents with respect to their duration of action at all dose levels amounted to 11 +/- 3 minutes (M +/- s.e.m.) for etidocaine vs. lidocaine (IONB degree 10), 27 +/- 4 min for prilocaine vs. lidocaine and 54 +/- 5 min for bupivacaine vs. etidocaine (IONB degree 3). For all other comparisons the log (dose)-duration lines deviated from parallelism, i.e. differences between agents with respect to their duration of action were found to be dose-dependent. The slopes of the log (dose)-duration lines were found to correlate closely to the log (partition coefficient) and log (protein binding) of the investigated agents.