Coated-wire electrodes with local anesthetic (LA) cation-selective membranes were prepared, and their properties in vitro were investigated. Copper wires (0.8--mm diameter) were coated with gel membranes of 110 mg of poly (vinyl chloride), 5 mg of ion pairs of tetraphenylborate anion with LA cation, 100 mg of dioctylphtalate, and 1.5 mL of tetrahydrofuran. This was the composition determined to be most suitable. Their electromotive force relative to an Ag/AgCl electrode was measured in LA solutions. The lidocaine, dibucaine, and mepivacaine electrodes all showed good Nernstian response at 25°C in aqueous solutions in the concentration ranges of 1 × 10−4 to 1 × 10−2 mol/L, 4 × 10−5 to 1 × 10−2 mol/L, and 5 × 10−2 to 1 × 10−2mol/L, respectively. The response time was within 10 s. The electrode potential decreased as the pH in the solution increased, with a corresponding decrease of the protonated form of LA. The hydrophobic nature of the LA was closely related to the electromotive force and to the selectivity of the electrode toward various LA cations. Dibucaine, the most hydrophobic, had the highest electrode potential. The more hydrophobic the LA of the electrode, the less it is interfered with by other LA molecules. The more hydrophobic the interferent cation, the more it acts on the electrode potential. The electrode system could also measure LA in human plasma at 37°C, although the responsiveness was depressed in the low concentration range owing to binding of LA to the serum protein. Its small size, convenience of handling, good response, and simplicity of measurement indicate the feasibility of the electrode system for in vivo application.
Address correspondence to Dr. Yokono, Department of Anesthesiology and Emergency Medicine, Kagawa Medical School, 1750--1, Ikenobe, Miki-Cho, Kita-Gun, Kagawa, 761--07, Japan.
© 1992 International Anesthesia Research Society