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Supervisory Self-Organising Control of Relaxant Anaesthesia

Submitted by D A Linkens wiith M F Abbod,29.11.2001,IBA D

Problem

To provide on-line automation of muscle relaxant drug administration during surgery. The system should cater for possible faults in the equipment, changes in patient dynamics and variability between patients. Apart from catastrophic faults in the Relaxagraph, there are also signal changes caused by movement of electrodes on the skin, and incipient drift caused by sweating etc. Intra-patient variability can be caused by a range of conditions, such as blood loss (giving reduced drug sensitivity ) etc.

Solution

A supervisory layer has been added to the Self-Organising Fuzzy Logic Control (SOFLC) structure of Procyk and Mamdani. This provides for fault surveillance and monitoring of gradual changes in the environment and patient conditions. In particular, controller initialisation from the supervisory level determines the approximate time delay in the overall system, the SOFLC scaling factors, and the selection of an initial rule base. Also,a gain scheduler estimates the patient sensitivity via prior knowledge of the chosen drug pharmacokinetics and pre-operation room bolus drug responses. Three categories of sensitivity are defined, being Low, Medium and High. The supervisory layer monitors the system performance during an operation in terms of basic closed-loop behaviour of steady-state, oscillatory, divergent or convergent conditions. It adjusts the system response accordingly.

Status and results

The system has been developed and validated for management under both catastrophic and incipient measurement (Relaxograph) and actuator faults/changes. It has been demonstrated to cope under simulation conditions with significant changes in intra-patient dynamics via its SOFLC level. Further, the gain schedulig for inter-patient variability has been shown to give satisfactory performance in terms of stability and speed of response.

Adaptivity and portability

It achieves Level 1 adaptivity in the EUNITE definition, in that it caters for changes in the environment, including alterations in either the process (the patient),the measuring equipment (Relaxograph) or the actuator (drug syringe-driver).It achieves Level 2 adaptivity via gain scheduling, since it incorporates patient-specific information on drug sensitivity elicited in the pre-operation room prior to commencement of surgical procedures.

More information

Further details can be found in:

M F Abbod, "Hierachical supervisory fuzzy control for muscle relaxation anaethesia", in "Intelligent Control in Biomedicine", ed. D A Linkens, 1994, pp133-173.

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