Concentration dependent effects of local anesthetics on reversibility of conduction block in intact cat sciatic nerve

ABSTRACT

Delivery of large doses of local anesthetics for spinal anesthesia by repeated injections or continuous infusion could expose the cauda equina to concentrations of drug that may be neurotoxic per se. This possible neurotoxic effect was studied by assessing recovery from conduction blockade of peripheral nerves after exposure to some of the local anesthetic solutions commonly used for spinal anesthesia. The reversibility of conduction blockade was studied in intact cat sciatic nerve by recording the compound action potentials [CAP] before and during exposure to local anesthetics and during drug washout. The nerves were exposed for 15 minutes to 5% or 2% lidocaine, 0.5% or 0.06% tetracaine, 0.5% bupivacaine or Ringer's solution [without local anesthetic] as control. After 15 minutes in the drug solution, the nerves were washed for 2-3 hours. Exposure of nerves to 5% lidocaine or 0.5% tetracaine resulted in irreversible total conduction blockade, whereas 2% lidocaine or 0.5% bupivacaine caused 25-50% residual block after 2-3 hours wash. Nerves exposed to Ringer's solution or 0.06% tetracaine had 0-10% residual block after 2-3 hours wash. However, the presence of verapamil [calcium-channel blocker] made the nerves more resistant to lidocaine and tetracaine, as these local anesthetics increase the intracellular calcium to a toxic level