British Journal of Anaesthesia, Vol 81, Issue 2 161-170, Copyright © 1998 by The Board of Management and Trustees of the British Journal of Anaesthesia
D. M. Beams, F. J. Sasse, J. G. Webster and R. G. Radwin
A general (multiple-gas) three-compartment mass-balance model of the
circle-absorber breathing circuit with intermittent positive-pressure
ventilation has been developed. We propose it as a tool to determine
flowmeter and vapourizer settings for inhalation anaesthesia by low- flow
methods (less than 1 litre min-1 total fresh gas flow). This model
reproduces the results of various previously-published mass-balance models,
but it appears to underestimate slightly the delivery of fresh gases to the
inspired limb of the breathing circuit when tested with clinical data from
surgical cases. Low-flow dosing schedules for 35% inspired oxygen and 1%
inspired halothane were computed with the model and tested in vitro with a
circle-absorber breathing circuit and an active gas-exchange lung at nine
values of simulated patient gas exchanges. The mean inspired oxygen
concentration over all trials was 32.8% (SD = 1.9%), while the mean
inspired halothane concentration was 1.2% (SD = 0.3%). The flow meter and
vapourizer settings calculated from the model appear to have sufficient
accuracy to be useful in the clinical setting in conjunction with active
oxygen and anaesthetic agent monitoring.
CLINICAL INVESTIGATIONS
Model for the administration of low-flow anaesthesia
Department of Electrical Engineering, University of Texas at Tyler, 3900 University Blvd., Tyler, TX 75799 USA; Department of Anesthesiology, University of Wisconsin Center for Health Sciences, Madison, USA; Department of Electrical and Computer Engineering, University of Wisconsin-Madison, Madison, USA; Department of Industrial Engineering, University of Wisconsin-Madison, Madison, USA
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