ANESTHESIA GAS MACHINE>COMPONENTS & SYSTEMS>INTRODUCTION

• Introduction
• Numbers to remember
• General features of all anesthesia gas machines
  • Path of gases within the machine
  • Five tasks of oxygen
  • Supply, Processing, Delivery, Disposal model
• Manufacturers

Components and systems- Introduction

Introduction

The anesthesia gas machine is a device which delivers a precisely-known but variable gas mixture, including anesthetizing and life-sustaining gases.

The components and systems as described in this document are typical for a hypothetical generic anesthesia gas machine. The differences between gas machines common in the workplace today -such as the Ohmeda Modulus, Excel, or Aestiva and the Dräger Narkomed GS, Mobile, MRI, 2B, 2C, 3 or 4- are less than their similarities. Therefore only the differences with the most impact on clinical practice are described for this generation of machines.

Differences between models have more clinical impact with the latest generation of gas machines because of the higher degree of computer-controlled basic systems and monitor integration. So the differences are more fully described here for the Datex Ohmeda S/5 (also called System 5 or AS/3 ADU), and Dräger Julian, Fabius GS, and Narkomed 6000.

Required components of an anesthesia workstation

The current anesthesia gas machine (workstation) standard is ASTM F1850 (a standard promulgated by American Society for Testing and Materials). The European standard is EN740.

F1850 specifies what is needed for an anesthesia workstation. The components are typically built into new gas machines, or they may be added to older machines. Required components include:

• Battery backup for 30 minutes
• Alarms
  • Grouped into high, medium, and low priority.
  • High priority alarms may not be silenced for more than 2 minutes.
  • Certain alarms and monitors must be automatically enabled and functioning prior to use, either through turning the machine on, or by following the pre-use checklist: breathing circuit pressure, oxygen concentration, exhaled volume or carbon dioxide (or both).
  • A high-priority pressure alarm must sound if user-adjustable limits are exceeded, if continuing high pressure is sensed, or for negative pressure.
  • Disconnect alarms may be based on low pressure, exhaled volume, or carbon dioxide.
• Required monitors
  • Exhaled volume
  • Inspired oxygen, with a high priority alarm within 30 seconds of oxygen falling below 18% (or a user-adjustable limit).
  • Oxygen supply failure alarm
  • A hypoxic guard system must protect against less than 21% inspired oxygen if nitrous oxide is in use.
  • Anesthetic vapor concentration must be monitored.
  • Pulse oximetry, blood pressure monitoring, and EKG are required
• Pressure in the breathing circuit is limited to 12.5 kPa (125 cm water).
• The electrical supply cord must be non-detachable or resistant to detachment.
• The machine must have at least one oxygen cylinder attached.
• The hanger yoke must be pin-indexed, have a clamping device that resists leaks, and contain a filter. It must have a check valve to prevent transfilling, and a cylinder pressure gauge. There must be cylinder pressure regulators. The machine must use pipeline gas as long as pipeline pressure is greater than 345 kPa (50 psi).
• Flowmeters:
  • Single control for each gas
  • Each flow control next to a flow indicator
  • Uniquely shaped oxygen flow control knob
  • Valve stops (or some other mechanism) are required such that excessive rotation will not damage the flowmeter.
  • Oxygen flow indicator is to the right side of a flowmeter bank
  • Oxygen enters the common manifiold downstream of other gases
• An oxygen flush is present, capable of 35-75 L/min flow which does not proceed through any vaporizers.
• Vaporizers
  • Concentration-calibrated
  • An interlock must be present
  • Liquid level indicated, designed to prevent overfilling
  • "Should" use keyed-filler devices
  • No discharge of liquid anesthetic occurs from the vaporizer even at maximum fresh gas flow
• Only one common gas outlet at 22 mm outer diameter, 15 mm inner diameter, which is designed to prevent accidental disconnection
• Pipeline gas supply
  • Pipeline pressure gauge
  • Inlets for at least oxygen and nitrous oxide
  • DISS protected
  • In line filter
  • Check valve
• Checklist must be provided (it may be electronic, or performed manually by the user)
• A digital data interface must be provided
• An auxiliary oxygen flowmeter is strongly recommended

Numbers to remember

The hospital pipeline is the primary gas source at 50 psi, which is the normal working pressure of most machines. Cylinders - Oxygen is supplied at around 2000 psi (regulated to approximately 45 psi after it enters the machine).

Oxygen flush is a "straight shot" from supply to delivery point, 35-75 L/min.

OSHA Fact Sheet (1991) on Waste Anesthetic Gases (WAGs) gives the NIOSH recommendation to OSHA - occupational exposure should be limited to (an eight hour time-weighted average of) not more than 2 ppm halogenated agents (0.5 ppm if nitrous oxide in use), and not more than 25 ppm nitrous oxide.

Tubing sizes- scavenger 19 or 30 mm, ETT or common gas outlet (CGO) 15 mm, breathing circuit 22 mm.

General features of all anesthesia gas machines

Path of gases within the machine

Oxygen has five "tasks" in the AGM; it powers the

1. ventilator driving gas
2. flush valve
3. oxygen pressure failure alarm
4. oxygen pressure sensor shut-off valve ("fail-safe")
5. flowmeters.

Diagram of the five tasks of oxygen. Click on the thumbnail, or on the underlined text, to see the larger version (26 KB).

The path of gases through the machine is illustrated in J Andrews (Miller), or Dorsch & Dorsch 4th ed (pg. 81), or M Dosch in Nurse Anesthesia (Nagelhout & Zaglaniczny 2001). This is but one way to conceive of the machine- a better way might be the Supply, Processing, Delivery & Disposal model (Dosch 2001; figure and table in Nagelhout & Zaglaniczny p. 247). Another good resource is Explore! (Book and CD ROM published by Ohmeda).

Manufacturers

There are two major manufacturers of anesthesia gas machines in the United States.

The basic pneumatic-mechanical design of the anesthesia gas machine has become familiar to a generation of providers. The basic design has been called upon to perform more complicated functions since 1990, with the advent of computer-controlled monitors into the operating room, especially pulse oximetry, capnography, and gas analysis. Our gas machines have become top-heavy with the monitors we have added to their basic design.

Now, the advent of the computer gives us a new generation of anesthesia gas machines, which have a great deal of added functionality in a small package, designed from the start to be microprocessor controlled. The prototypes of this new wave are the Datex-Ohmeda AS/3 ADU, the Narkomed 6000, Fabius GS, and Julian. These gas machines are being purchased because they

• enhance patient safety
  • more accurate basic components such as ventilators, vaporizers, flowmeters
  • integrated, computer-controlled alarms
• feature advanced ventilation modes
  • pressure control ventilation (PCV)
  • synchronized intermittent mandatory ventilation (SIMV)
• perform compliance and leak testing of the breathing circuit, and compensate for these to produce unprecedented accuracy in delivered tidal volumes, which may lessen the need for non-rebreathing circuits for children
• are smaller and lighter (in some cases) because they have integrated monitoring
• allow automated record keeping more easily than traditional designs
  • electronic capture of fresh gas flow
  • microprocessor integration
• feature improved monitors,and innovative new monitoring capabilities (spirometry)