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British Journal of Anaesthesia, 2001, Vol. 86, No. 4 519-522
© 2001 The Board of Management and Trustees of the British Journal of Anaesthesia

Ocular microtremor: a tool for measuring depth of anaesthesia?

S. Bojanic1, T. Simpson1 and C. Bolger2

1Department of Neurosurgery and 2Department of Anaesthetics, Frenchay Hospital, Bristol, UK*Corresponding author: Department of Neurosurgery, The Radcliffe Infirmary, Woodstock Road, Oxford OX2 1HE, UK

Ocular microtremor (OMT) is a fine high frequency tremor of the eyes caused by extra-ocular muscle activity stimulated by impulses emanating in the brain stem. Several studies have shown that the frequency of this tremor is reduced in patients whose consciousness is reduced by anaesthesia or head injury. Therefore, OMT may possibly be used to determine depth of anaesthesia. Twenty-two unpre-medicated subjects undergoing surgery with general anaesthesia were studied. OMT activity was measured at admission using the open eye piezoelectric strain gauge technique. Anaesthesia was induced with propofol using a target controlled infusion delivery system (DiprifusorTM). OMT activity was then recorded at predicted plasma propofol concentrations of 1, 2, 3 and 5 µg ml–1. The patient’s level of consciousness (response to command or stimulation) was assessed after each OMT measurement. OMT activity was reduced progressively at predicted plasma concentrations of propofol of 1 and 2 µg ml–1 and then plateaued between 3 and 5 µg ml–1. There was a significant difference between the last awake OMT recording and the first recording at loss of consciousness (P<0.001). OMT recording holds promise as a practical indicator of the depth of anaesthesia.

Br J Anaesth 2001; 86: 519–22


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