Ocular microtremor: a tool for measuring depth of anaesthesia?

Ocular microtremor (OMT) is a fine high frequency tremor of the eyes caused by extra-ocular muscle activity stimulated by impulses emanating in the br ain stem. Several studies have shown that the frequency of this tremor is r educed in patients whose consciousness is reduced by anaesthesia or head in jury. Therefore, OMT may possibly be used to determine depth of anaesthesia . Twenty-two unpre-medicated subjects undergoing surgery with general anaes thesia were studied. OMT activity was measured at admission using the open eye piezoelectric strain gauge technique. Anaesthesia was induced with prop ofol using a target controlled infusion delivery system (Diprifusor (TM)). OMT activity was then recorded at predicted plasma propofol concentrations of 1,2, 3 and 5 mug ml(-1). The patient's level of consciousness (response to command or stimulation) was assessed after each OMT measurement. OMT act ivity was reduced progressively at predicted plasma concentrations of propo fol of I and 2 mug ml(-1) and then plateaued between 3 and 5 mug ml(-1). Th ere was a significant difference between the last awake OMT recording and t he first recording at loss of consciousness (P<0.001). OMT recording holds promise as a practical indicator of the depth of anaesthesia.