Changes in recording electrode impedance during anaesthesia have no effect on evoked electromyographic responses

Changes in electrical impedance at the recording electrodes have been blamed for the failure of evoked electromyographic responses to return to baseline during the offset of neuromuscular block, but a relationship between electrode impedance and electromyographic responses has never been shown. In 50 fit adults, the resistive and capacitive (reactive) components of the impedance of the recording electrodes were measured during electromyographic monitoring of neuromuscular transmission under enflurane anaesthesia. In 25 patients six 1 mm-deep punctures were made in the skin under both active recording electrodes, and in the other 25 no puncture was made. Electrode impedance was measured using an impedance bridge at a current density less than 60 microA cm-2. Skin puncture made no difference either to the electrode impedance, or to the decrement from the baseline of the first electromyographic response to the train-of-four stimuli after offset of neuromuscular blockade. The electrode impedance decreased in 48 of the patients, and increased slightly in the other two patients. There was no relationship between the decrease in the T1 response from baseline and the change in electrode impedance during electromyographic monitoring.