The role of pH in the development of tachyphylaxis to local anesthetic agents.

Abstract

Evidence for tachyphylaxis to the local anesthetic agents lidocaine and mepivacaine has been found in a study of continuous caudal anesthesia in obstetrical patients. Seduced response to these drugs is shown to be independent of the increasing pain of labor. An experimental study of tachyphylaxis in the dog was undertaken, utilizing 14C-lidocaine and 24C-procaine administered by a continuous spinal technique. Repeated injections of the acid anesthetic salt were associated with continued decreases in pH of the cerebrospinal fluid and proportionate increases in the concentration of local anesthetic- Calculations of the percentages of the free base form (lipid-soluble) indicated significant shifts with minor changes in pH. It is suggested that tachyphylaxis to local anesthetic agents may be associated with changes in pH and shifts in the amounts of ionized and nonionized forms. The limited buffer reserve available in the cerebrospinal fluid (and presumably, the extradural space) makes these areas especially vulnerable.