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Acid and Alkaline Solutions of Local Anesthetics: Duration of Nerve Block and Tissue pH

The effect of solution pH on the duration of rat infraorbital nerve blocks produced by 1% lidocaine or 0.25% bupivacaine at pH 5.0 and 7.4, with and without epinephrine was investigated in a double-blind study. The time course of tissue pH changes subsequent to injections into the infraorbital area or abdominal musculature of rats was measured with a tissue pH microelectrode. Injectate pH had little or no effect upon the duration of block. Tissue pH was minimally changed by the injection of solutions at pH 7.4, but decreased appreciably with injections of solutions at pH 5.0, or if the injectate contained epinephrine.