Mathematical modelling and clinical testing of low flow breathing systems

  • Patrick Magee

Student thesis: Doctoral ThesisPhD


It is hypothesized that anaesthetic breathing systems are poorly designed, and would benefit from engineering modeling to improve function. The use of breathing systems in different environments is described. Bath fp, a software package, developed in the Department of Mechanical Engineering for mathematically modelling hydraulic systems such as breathing systems, is introduced. Bath fp was used to model the behaviour of anaesthetic circle systems at high, medium, low and minimum fresh gas flows (FGFs). Using 50% nitrous oxide in oxygen ('entonox') as a gas model for all such volatile anaesthetic agents, the behaviour of the system was observed in terms of gas concentrations, volumetric flow-rates and airway pressures. Anaesthetic circle systems studied included standard adult and paediatric systems, and a coaxial system. In the standard systems, different tubing lengths and diameters were modelled, and for the coaxial system, different diameter ratios of outer and inner tubes were modelled. Gas concentration changes at different points in the system were used to clarify system function at different FGFs. Management of exhaled carbon dioxide was examined, including changing absorber volume. A venturi was modelled and added to a valveless coaxial circle system, to see how the venturi changed system function, and under what conditions it might fail.Clinical data were collected to validate modelling results. Nineteen adult volunteers were recruited to a trial, in which entonox was breathed through standard and coaxial systems for short periods, at different FGFs, with high flow air-breathing periods in between. Data on gas concentrations were collected from three points in each system. Results were analysed, and function was compared between different systems, and between systems and modelling results.The simulation model clarified circle system function, and showed that function depended more on FGF than on geometry. The clinical data supported the use of simulation to predict the behaviour of breathing systems.
Date of Award28 Oct 2014
Original languageEnglish
Awarding Institution
  • University of Bath
SupervisorRoger Ngwompo (Supervisor) & Derek Tilley (Supervisor)

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