Computer Assistance in the Control of Depth of Anaesthesia +

Abstract

A control system was used to bring the tension of anaesthetic in the brain to any value specified (in MAC units) by the anaesthetist and then maintain it constant until a new value was specified. The control was applied to a volatile agent but allowance was automatically made for the anaesthetic effect of any nitrous oxide concomitantly administered by the anaesthetist. The inspired concentration required to achieve the desired brain tension was calculated from a model of the patient and set automatically on the vaporizer. The quantification of the model was matched to the patient on the basis mainly of body mass and periodic non-invasive measurements of alveolar ventilation and cardiac output. In order to adapt the model to the patient an arterial blood sample was taken every 30 min to obtain the arterial tension of halothane for use as feedback. The system has been tested on eight Alsatian dogs. After omitting results affected by avoidable errors, the SD of the measured-to-computed arterial tension ratio was less than 10%.

Gaseous Homeostasis and the Circle System

“…7). A similar ability to demonstrate a second gas effect influencing nitrous oxide uptake is present in version "S" of Mapleson's original model (Chilcoat, 1983).…”

The theoretical ideal fresh‐gas flow sequence at the start of low‐flow anaesthesia

“…However, for an inhalation-only anaesthetic, the target should, basically, be a constant partial pressure at the site (or sites) of action which, in terms of the model, must mean a constant brain partial pressure. To achieve this quickly requires end-expired, and even arterial 'overpressure' [27] -not just the inspired overpressure of Figs 3 and 4. With sufficient trial and error, the model could be used to devise a suitable sequence of flows and partial pressures but the assumption of constancy of cardiac output and its distribution would no longer be valid.…”

A Physiological Model for the Distribution of Injected Agents, With Special Reference to Pethidine †

“…These authors further report that, at the moment of induction, there was less hypotension with the slowest injection rate. This could be explained by following Chilcoat, Lunn and Mapleson [29] in assuming that the "blood-pressure control centre" is better perfused than the "induction-of-anaesthesia control centre": the simulation showed that, in a compartment with four times the specific perfusion of the sample brain compartment (actually the kidney), Cs at the moment of induction with the slowest injection rate was 70 % of that with the normal rate.…”