Ocular microtremor during general anesthesia: results of a multicenter trial using automated signal analysis

Ocular microtremor (OMT) is a fine physiologic tremor of the eye related to neuronal activity in the reticular formation of the brainstem. The frequency of OMT is suppressed by propofol and sevoflurane and predicts the response to command at emergence from anesthesia. Previous studies have relied on post hoc computer analysis of OMT wave forms or on real-time measurements confirmed visually on an oscilloscope. Our overall aim was to evaluate an automated system of OMT signal analysis in a diverse patient population undergoing general anesthesia. In a multicenter trial involving four centers in three countries, we examined the accuracy of OMT to identify the unconscious state and to predict movement in response to airway instrumentation and surgical stimulation. We also tested the effects of neuromuscular blockade and patient position on OMT. We measured OMT continuously by using the closed-eye piezoelectric technique in 214 patients undergoing extracranial surgery with general anesthesia using a variety of anesthetics. OMT decreased at induction in all patients, increased transiently in response to surgical incision or airway instrumentation, and increased at emergence. The frequency of OMT predicted movement in response to laryngeal mask airway insertion and response to command at emergence. Neuromuscular blockade did not affect the frequency of OMT but decreased its amplitude. OMT frequency was unaffected by changes in patient position. We conclude that OMT, measured by an automated signal analysis module, accurately determines the anesthetic state in surgical patients, even during profound neuromuscular blockade and after changes in patient position.