Towards the Definition, Measurement and Assessment of the Anaesthetic State

Robb, Henry Morgan (1988) Towards the Definition, Measurement and Assessment of the Anaesthetic State. MSc(R) thesis, University of Glasgow.

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The definition of the anaesthetic state is discussed. We see the anaesthetic state as a central nervous system state, which results from the interaction between the effects of anaesthetic dose and the effects of surgical stimulation on that system. However, the effects of surgical stimulation, although they may be described as an ordered sequence of events, cannot be quantified. And the wide variation in anaesthetic requirement means that anaesthetic dose can, at best be, defined with respect to a single point (or points) on a qualitative scale. It is therefore difficult to assess the various techniques proposed as monitors of the anaesthetic state. These techniques are reviewed and it is apparent, that without a clear definition of the anaesthetic state, these techniques cannot be fully assessed. There is therefore a need for an individually valid anaesthetic state ("gold standard") to which other methods can be compared. Clinical signs form the basis for the administration of thousands of successful anaesthetics each day and, despite some inadequacies, are currently the only practical, universally accepted indicators of anaesthetic adequacy. However, their interpretation is both subjective and variable. A reproducible, individualised description of the anaesthetic state, based on clinical signs would allow this and other techniques to be more fully assessed. The objective of the work presented in this thesis was to investigate whether a clinically acceptable anaesthetic state could be produced by a control system designed to maintain the patients systolic arterial pressure (SAP) at a predetermined level by altering the inspired concentration of enflurane or isoflurane. Such a control system, though limited in that it uses only one clinical sign, should help to standardise the management of the anaesthetic state. A proportional - integral control system was set up with an RML 380Z-D computer interfaced to a Critikon Dinamap 1846 and a vaporizer controller. A program, written in BASIC, was designed to maintain the patients SAP at 90% of that predicted from age and sex standardised tables. The controller took data from the Dinamap, which was set to cycle at one minute intervals, and changed the inspired volatile concentration via the vaporizer controller. In a preliminary study some patients were hypotensive, relative to the target SAP, following induction but prior to surgery. These patients therefore received minimal concentrations of volatile anaesthetic initially. Others received a relative overdose of anaesthetic during the procedure. Two additional rules were incorporated in the control algorithm to limit these effects. First, a minimum inspired concentration of 0.6% was delivered for the first 10 minutes and second, if the inspired dose of volatile anaesthetic exceeded a preset limit, the controller instructed the anaesthetist to give a bolus of morphine and reset the integral. This limit was defined as a total inspired concentration of more than 15% over 5 consecutive vaporizer settings with each individual setting exceeding 2.5%. This rule could only be activated once in 15 minutes. The study was carried out on 57 ASA I and II female patients admitted for routine gynaecological surgery. All patients were artificially ventilated and received a standard, nitrous oxide based anaesthetic supplemented with morphine at induction, and enflurane (22 patients) or isoflurane (35 patients). The dosage of volatile anaesthetic and the requirement of additional morphine were determined by the controller. During surgery cardiovascular data and the inspired volatile requirement were recorded every minute and clinical signs "scored". Recovery times were measured and all patients interviewed post-operatively to discover whether they had been aware during anaesthesia. An adequate anaesthetic state was defined by strict criteria based on cardiovascular variables, the goodness of control, recovery time and absence of awareness post-operatively. No patient recalled any intraoperative event. Overall, 53.8% of cases satisfied all criteria. A failure to achieve two or more of these criteria occurred in 7.5% of patients in the groups not requiring additional morphine compared to 83.3% of the patients who did require additional morphine. The use of SAP as the sole variable to control the delivery of anaesthetic agents, can be criticised on several grounds. However, it is one of the few quantifiable indices available and increases in SAP are commonly treated with anaesthetic agents. Further, we carefully excluded those cases in whom SAP is likely to unreliable e.g. hypertensive patients. Although our method of selecting the TSAP is crude, and those patients who required additional morphine may reflect those in whom the TSAP is low, the controller achieved the immediate aim of controlling SAP and also produced a pattern of clinical signs recognisable as general anaesthesia. Pupil diameter was measured in 34 patients. A significant correlation was found between pupil diameter and SAP in 5 cases. There was no difference in the clinical anaesthetic state in those patients in whom a significant correlation was found and those in whom there was no significant correlation. Spectral edge frequency (SEF) was measured in eight of the patients who received isoflurane using a Neurotrac monitor. Due to limitations in data recording we were not able to correlate SEF with SAP. However, SEF was plotted against SAP for seven patients during relatively stable periods of SAP control. A visual relationship was apparent between SAP and SEF in 4 cases. The significance of this finding is unknown. In a further case the EEG became isoelectric at the time a morphine bolus was requested by the computer. The processed EEG may be a better method of assessing a relative anaesthetic overdose than SAP alone during nitrous oxide-morphine-isoflurane anaesthesia. In summary, a computer control system has been developed and tested. The controller achieved the immediate aim of controlling SAP and also produced a pattern of clinical signs recognisable as general anaesthesia.

Item Type: Thesis (MSc(R))
Qualification Level: Masters
Keywords: Medicine
Date of Award: 1988
Depositing User: Enlighten Team
Unique ID: glathesis:1988-77776
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 14 Jan 2020 11:53
Last Modified: 14 Jan 2020 11:53

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