Feedback control of sedation and general anaesthesia
The man aim of my studies was to investigate the safety and efficacy of two modes of feedback control of sedation and anaesthesia. A secondary aim was to add to the body of knowledge on the Bispectral Index (BIS). I also wrote a computer program (BISCLAN) that was used in all the studies as a BIS data management tool, and in some studies for manual or automatic control of a propofol infusion. Two studies did not involve feedback control, but were performed to further our understanding of the BIS. For one, I recorded BIS values and the times at which clinical events occurred during 200 general anaesthetics. and studied memory of perioperative events. Broad variation in BIS values at similar levels of anaesthetic depth was found, although there was good separation between the majority of BIS values found during periods of consciousness and unconsciousness. BIS values on awakening were not predictive of memory for subsequent events. For the second study I investigated the effects of the stimuli used lo generate auditory evoked potentials on consciousness levels and the BIS, during sedation and anaesthesia. No effect was found. Three studies of BIS-guided computer control of anaesthesia and sedation were performed. Control performance was assessed in terms of clinical adequacy of anaesthesia and with recognised mathematical criteria. BISCLAN was able to control anaesthesia successfully. Cardiovascular parameters were stable in all patients. With two exceptions, operating conditions were also adequate. Control parameters during sedation and anaesthesia were acceptable and compare favourably with those found in other studies. Two studies of a second mode of feedback control of sedation (patient-maintained sedation) were performed. In both the goal was to determine if system safety was sufficient to prevent volunteers from purposefully inducing loss of consciousness. Sedation scores, propofol concentrations and physiological data were recorded. Secondary data included BIS values, and tests of memory for words. In one study a revised version of a previously developed blood concentration targeted infusion system was used, and in the other an effect-site targeted system. One subject in the second study became over-sedated, but no subjects lost consciousness. There was correlation among BIS values and propofol concentrations, and among BIS and propofol concentrations and the likelihood of memory for words. Several subjects remained conscious during periods when the BIS was < 60.
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