RESUMO
PURPOSE: Response to commands is the gold standard to assess the level of consciousness during anesthesia induction but it only provides an intermittent, binary measure with low temporal resolution. To overcome these limitations, we combined the object hold method with handgrip dynamometry to continuously record the force applied to hold a dynamometer as a surrogate measure of the level of consciousness during induction of anesthesia. METHODS: Fourteen patients scheduled for elective lumbar surgery and 14 age-matched non-anesthetized controls were enrolled. The subjects held the dynamometer with their dominant hand for as long as possible (patients) or until told to stop (controls). After a one-minute baseline, propofol was infused (1.0 mg·kg-1·min-1) to the patient group until the subject dropped the dynamometer, which defined the object hold time. Three additional patients were also asked intermittently to squeeze the dynamometer during the propofol infusion to determine any retained ability to exert a strong grip despite any grip changes during induction. RESULTS: The mean (standard deviation) object hold time was 115 (22) seconds after the start of the propofol infusion. There was a progressive significant linear decrease (R2 = 0.98; P < 0.001) in dynamometry-determined handgrip force starting approximately 74 seconds before object drop. Age was inversely related to the object hold time (R2 = 0.47, P < 0.01). The three additional propofol induction patients had strong intermittent grip strength despite progressive decreases in the hold force. Of the 17 patients who completed the object hold task (14 with the standard protocol and three with intermittent squeeze requests), 16 (94%; 95% confidence interval, 76 to 99%) did not respond to verbal commands after dropping the dynamometer. CONCLUSION: Handgrip dynamometry can be used to continuously track volitional control during induction of anesthesia while also reliably showing a gradual loss of consciousness. This method could be useful for studies investigating mechanisms of anesthesia.