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Last Updated 6/27/02
Introduction to the Anesthesia Machine
for Beginning Anesthesiology Residents

Sem Lampotang, Ph.D.
Associate Professor of Anesthesiology
Department of Anesthesiology
University of Florida, Gainesville, Florida

June 27, 2002
©2002 University of Florida

Clinical learning objectives

1. What does an anesthesia machine do?

1.1. Gas mixing

1.2. Vaporization of volatile anesthetics

1.3. Ventilation

1.3.1. Mechanical Volume controlled ventilation How to set initial tidal volume and titrate Pressure controlled ventilation When to use How to set initial target pressure and titrate Low flow and closed circuit anesthesia

1.3.2. Manual The adjustable pressure limiting (APL) or “pop-off” valve as a variable pressure threshold, not variable orifice, device Minimum APL valve pressure setting

1.3.3. Spontaneous

1.4. Supporting sub-systems

1.4.1. CO2 absorber

1.4.2. Monitors (FiO2, pressure, volume, gas analyzer, physiological monitors)

1.4.3. Scavenging

2. Understanding the configuration and dynamics of anesthesia machines

2.1. Closed system configuration during mechanical inspiration

2.2. Set vs. delivered (“what you set ain’t necessarily what you get”)

2.2.1. Inspired gas and vapor concentrations

2.2.2. Tidal volume

2.3. Time constants or delays

3. Understanding the rationale and correct execution of the recommended steps in the
1993 FDA pre-use check

4. Understanding the potential failure modes of certain anesthesia machines

4.1. O2 flush during mechanical inspiration

4.2. FGF augmentation of delivered VT

4.3. Pipeline supply gas failure

4.4. Hypoxic “O2” pipeline supply

4.5. Loss of electrical power

4.6. Potential for misreading cylinder pressures

4.7. Bellows leak

5. Identifying failed steps in the FDA pre-use check

Recommended on-line resources and reading:

1. The UF Virtual Anesthesia Machine (VAM) web site: http://vam.anest.ufl.edu

VAM is an interactive Web-based program that can be accessed 24/7 without charge anywhere there is an Internet connection (including at home and in the OR). Users interact with a mouse with more than 20 anesthesia machine controls and settings and visualize in real time the effect of their interventions on gas flows, volumes, pressures and inspired gas compositions. Gas molecules are not only made visible but are also color-coded. VAM includes animations that explicitly incorporate the crucial dimension of time that is often missing in static figures in traditional educational materials like textbooks and slides and may thus provide a more engaging and effective learning experience to novice anesthesia machine users.

The program is pretty intuitive to use but I am obviously biased as one of the developers! A Help section is available at the VAM web site to help users who might need assistance in using the animation.

2. Lampotang S, Good ML: The anesthesia machine, anesthesia ventilator, breathing circuit and scavenging system in Kirby RR, Gravenstein N, Gravenstein JS, Lobato EB, Gravenstein JS (eds): Clinical Anesthesia Practice, ed 2. Philadelphia, WB Saunders, 2002, pp 277-302.

Post-test questions

1. During mechanical ventilation with a gas driven bellows ventilator and low flow anesthesia, what is the largest consumer of oxygen?

2. Gas escapes to the scavenging system during which phase of manual ventilation – inspiration or expiration?

3. When can you trust a cylinder pressure reading without first depressurizing the O2 pipeline supply and making the O2 cylinder pressure gauge read zero?

4. What are the consequences of a bellows leak in an O2-driven ventilator on VT, FiO2, airway pressure, anesthesia and room contamination?

5. What is the pressure setting when an APL (“pop-off”) valve is wide open?

6. If you suspect that there is a problem with an anesthesia machine, what would you do if you were in the middle of a general anesthetic?