Experimental evaluation of local anaesthetic solutions using rat infraorbital nerve block

The analgesic effectiveness of various local anaesthetic solutions was measured in the distribution of the infraorbital nerve of the rat. Using Sprague-Dawley rats sedated with phenobarbitone 25 mg/kg intraperitoneally and a simple stereotactic technique, 0.2 ml of solution was deposited at the infraorbital notch. The onset and duration of analgesia were ascertained by timing the unilateral absence of aversive response to pinching the upper lip, tested at intervals of five minutes. The experiments were performed with coded solutions, the entire code being preserved intact until completion of the study. Solutions were tested on sets of eight animals weighing 500--600 g. The average duration (minutes +/- S.D.) of analgesia produced by the respective injectates was as follows: Lidocaine 0.5% -42+/-12 Lidocaine 1.0% -47+/-10 Mepivacaine 0.5% -58+/-13 Mepivacaine 1.0% -78+/-23 Procaine 0.5% -0+/-0 Procaine 1.0% -34+/-10 Procaine 1.5% -46+/-13 2-Chloroprocaine 1.5% -38+/-9 Bupivacaine 0.5% -100+/-40 Etidocaine 1.0% -59+/-25 Tetracaine 0.2% -0+/-0 Tetracaine 0.3% -0+/-0 Comparison with other animal models used to evaluate conduction block indicates that this technique affords an improved, standardized and reliable experimental guide to the clinical analgesic properties of local anaesthetic agents.