A Computer Simulation of Anesthesia Machine Gas Flows
Edwin B. Liem, M.D., David Lizdas, Sem Lampotang, Ph.D.
Dept of Anesthesiology, University of Florida
Summary:
We have implemented an interactive, web-based computer
simulation of the flow of oxygen, nitrous oxide, carbon dioxide
and volatile anesthetics in an anesthesia machine. This Virtual
Anesthesia Machine (VAM) simulation can be viewed at: http://www.simanest.org/.
Introduction: There are various approaches for building a
computer simulation of the anesthesia machine. Understanding how
an anesthesia machine works internally is difficult because the
gas molecules are invisible and cannot be traced through the
system. We constructed a transparent model of the anesthesia
system based on a schematic design of the anesthesia system
circuits. We expected this “mental model” to enhance
understanding of the gas flows and implemented this model in a
computer simulation.
Methods: The objective was to design a simplified and accurate
representation of the anesthesia machine, including the
ventilator, and to animate gas flow through it during all phases
of mechanical, manual and spontaneous ventilation. We used
Director 7.02 (Macromedia, San Francisco, CA) to construct the
simulation. To view the simulation, the Shockwave 8 web player
specific to the user's PC platform (Windows or Macintosh) must
first be downloaded (free download) from the Macromedia website.
Results: Unlike the previous version of the Virtual Anesthesia
Machine, the current computer simulation has a software
abstraction layer that contains a mathematical model that
handles the events that can change the flow of gas. This layer
in turn drives the representation of the gas molecules inside
the mental model of the anesthesia system. This version allows
the user to:
- Manipulate many controls similar to a real anesthesia machine
such as the flow meter knobs, connectors, selector knob, etc.
Use ventilator controls to change inspiratory to expiratory
ratios, ventilation rates and tidal volume settings.
- Semi-quantatively change the gas flow to different rates, from
zero flow to flush rates.
- Observe the effects of changing gas flows by observing
breathing bag, scavenging bag, lung and bellows volumes, opening
and closing actions of checkvalves, ventilator relief valve,
positive and negative pressure relief valves and pressure
regulators.
- Observe the particular behaviors of gas molecules in the
different sections of the anesthesia system. Examples are the
absorption of CO2 in the sodalyme canister or the wash-in and
washout of CO2, nitrous oxide and volatile anesthetic in
different parts of the system.
Discussion: Others have animated gas flows using mental models
of anesthesia machine subsystems, without using mathematical
models of gas flows. Goldman developed a mathematical model for
gas flows, but only for the circle system. Beneken et al
developed mathematical models that simulated transport of gases
through both Bain and circle systems, that were validated for
the Bain circuit . To our knowledge no previous attempts have
been made to model the gas flows inside the entire anesthesia
machine. We hope to be able to apply both our mental model and
the mathematical gas flow model to other types of anesthesia
machines and also continue refining these models in order to
show anesthesia machine malfunctions in addition to normal gas
flows. By selecting an implementation as a (free) Web-based
animation, the Virtual Anesthesia Machine simulation remains
affordable and viewable from any location or platform (IBM/Mac)
with Internet access.
