Effect of pH of bupivacaine on duration of repeated sciatic nerve blocks in the albino rat. Local Anesthetics for Neuralgia Study Group.

Tachyphylaxis has been ascribed to tissue acidification after repeated injections of acidic local anesthetic solutions. We studied the effect of pH on the duration of action of bupivacaine to determine the validity of this proposed mechanism of tachyphylaxis by injecting bupivacaine solutions adjusted to pH 4.2 or 6.8 into a surgically implanted system created to permit in vivo irrigation of rat sciatic nerves with local anesthetic. Tachyphylaxis developed at both pH values. The results fail to support the acidification hypothesis as there was no statistically significant effect of a 400-fold difference in hydrogen ion concentration on the development of tachyphylaxis or the duration of motor dysfunction.