CONCENTRATION MEASURES OF VOLATILE ANESTHETICS IN THE AQUEOUS-PHASE USING CALCIUM-SENSITIVE ELECTRODES

Volatile anesthetic concentrations have been difficult to measure, but are an important experimental parameter for in vitro studies of anest hetic actions. Calcium sensitive electrodes were investigated as a mea ns of continuously monitoring anesthetic concentrations in artificial cerebrospinal fluids (ACSF). Anesthetic-induced Ca2+ electrode signals were compared at room (22 degrees C) and physiological (35 degrees C) temperatures. Electrophysiological measures of anesthetic effects on synaptic potentials provided a bioassay. Halothane and isoflurane prod uced negative changes in calcium electrode potentials which were linea rly related to concentrations over a clinically useful range (0.5-1.5 MAC). Anesthetic-induced voltages persisted in nominally zero Ca2+ ACS F and even in deionized water. A good correlation (r > 0.9) was found for calcium electrode measures of anesthetic concentration and synapti c response depression produced by halothane, at both 22 and 35 degrees C. These results support three conclusions: (1) calcium sensitive ele ctrodes provide a useful measure of volatile anesthetic concentrations in aqueous solution. (2) Care must be taken when using these electrod es for Ca2+ concentration measurements,if a volatile anesthetic is als o to be used, since the anesthetic could introduce an appreciable erro r (> 50%). (3) A temperature change of 13 degrees C had surprisingly l ittle effect on Ca2+ electrode responses or on synaptic depression pro duced by anesthetics. (C) 1998 Elsevier Science B.V. All rights reserv ed.