Spinal anesthesia in sheep with local anesthetic solutions at pH 4 and pH 7

Spinal anesthesia was performed in sheep, using five widely-used local anesthetic agents. The durations of sensory and motor block were shortest with procaine and longest with bupivacaine and tetracaine. The durations of block with prilocaine and mepivacaine fell between those of procaine and bupivacaine. These results agree with what we know about the relative durations of action of these agents in man. All solutions were tested at pH 4 and pH 7, but, except for prilocaine, there were no differences in the mean durations of sensory and motor block at the two pH levels. Sensory block with prilocaine solution at pH 7 was about 20% longer than at pH 4. With all the compounds except prilocaine, the frequency of motor block in the hind limbs was lower at pH 7 than at pH 4. We conclude that single injections of unbuffered solutions of these agents at pH 4 are as effective in terms of onset and duration as solutions at pH 7. The results of this study reconfirm the usefulness and reliability of the sheep as a model for evaluating and comparing spinal anesthetics.