Revised July 2012
ANESTHESIA GAS MACHINE>COMPONENTS & SYSTEMS>INTRODUCTION
- Numbers to remember
- General features of all anesthesia gas machines
- Features required by standard
- Path of gases within the machine
- Five tasks of oxygen
- Supply, Processing, Delivery, Disposal model
Components and systems- 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 older gas machines -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.
However, this generation of gas machines is becoming less common as they approach the time when they will no longer be supported by the manufacturer's technical support and service.
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 and electronic medical record integration. So the differences are more fully described here for new models from GE Healthcare (Aisys, Aespire, Avance, ADU) and Dräger (Apollo, Fabius GS).
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 links 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. Read the 1977 NIOSH recommendation on waste gases.
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
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 ADU, Aisys, Apollo, and Fabius GS. 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)
- pressure control ventilation with volume guarantee (PC-VG)
- synchronized intermittent mandatory ventilation (SIMV) with pressure- or volume-controlled breaths (called. respectively, SIMV-PS, and SIMV-PSV on the Aisys)
- pressure support ventilation (PSV)
- 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, and flow-time waveforms)
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
- 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).
- 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 auxiliary oxygen flowmeter is strongly recommended
- An oxygen flush is present, capable of 35-75 L/min flow which does not proceed through any vaporizers.
- 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
Path of gases within the machine
Oxygen has five "tasks" in the AGM; it powers the
- ventilator driving gas
- flush valve
- oxygen pressure failure alarm
- oxygen pressure sensor shut-off valve ("fail-safe")
It also plays a role in the hypoxic guard system, which maintains the correct proportion between flows of oxygen and nitrous oxide.
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 & Plaus 2010). This is but one way to conceive of the machine- a better way might be the Supply, Processing, Delivery & Disposal model.
The machine components can also be logically conceptualized by the amount of pneumatic pressure they are exposed to:
- The High-pressure circuit consists of those parts which receive gas at cylinder pressure
- hanger yoke (including filter and unidirectional valve)
- yoke block
- cylinder pressure gauge
- cylinder pressure regulators
- The Intermediate pressure circuit receives gases at low, relatively constant pressures (37-55 psi, which is pipeline pressure, or the pressure downstream of a cylinder regulator)
- pipeline inlets and pressure gauges
- ventilator power inlet
- Oxygen pressure-failure device (fail-safe) and alarm
- flowmeter valves
- oxygen and nitrous oxide second-stage regulators
- oxygen flush valve
- The Low-pressure circuit includes components distal to the flowmeter needle
- flowmeter tubes
- check valves (if present)
- common gas outlet
There are two major manufacturers of anesthesia gas machines in the United States.
Dräger Medical Inc. (Telford, Pennsylvania) currently offers the Apollo, Fabius (GS Premium, MRI), Tiro (and Tiro M).
- Fabius GS
- Fabius MRI
- Fabius Tiro
- "Developed to be used in facilities where space is a premium"
- Fabius Tiro M ("mobile")
- "The Fabius Tiro M is a compact anesthesia system that offers the
full spectrum of anesthesia ventilation to military anesthesiologists/
CRNAs in the field of operations. This unit can be used in a
variety of mobile and stationary military applications where general anesthesia is required.
The modular design allows all standard
system components to be stored in a
single container ..."
- Narkomed 6000 / 6400
- Narkomed GS
- Narkomed Mobile, 2C, 3, 4
GE Healthcare (Madison, Wisconsin) currently offers the first four gas machines listed.