Volume 11, No. 2 • Summer 1996

Anesthesia Machines…2

Peter J. Schreiber

To the Editor

This is a response to Dr. Gerald G. Pousho’s “Letter to the Editor” (APSF Newsletter, Winter 1995-96) concerning obsolescence of anesthesia machines. Anesthesia equipment, contrary to automobiles or airplanes, does not become obsolete due to the wearing out of components, but as a result of changes in medical procedures, innovations regarding efforts to increase safety, and changes in the education, training, and experience of personnel responsible for the administration of anesthesia. The decision to retire aging anesthesia equipment cannot be based on the age of the equipment alone, but must be the responsibility of the owner of the equipment after careful evaluation. The following is an attempt to provide some guidelines which may help owners of anesthesia equipment who are in the process of reaching a decision as to whether old anesthesia equipment should be replaced.

If we compare the introduction of safety features into automobiles with the introduction of safety features in anesthesia machines, we can state the following: The introduction of single or double air bags, side door air bags, anti-lock braking systems, automatic traction control, and four-wheel drive on demand has certainly increased the safety of automobiles considerably, but it is questionable if the introduction of any of those features renders automobiles without them “obsolete” and requires their retirement. Similarly, anesthesia equipment has seen numerous improvements (and related development is still ongoing) concerning safeguarding the patient against inadvertent apnea and hypoxic events. There are three different areas in which aging equipment may be compared to the equipment presently on the market: (A) Safety, (B) Performance, and (C) The environment in which the equipment is used.


The ASTM Standard published in 1989, which governs the design of anesthesia equipment, recommends several safety features which were not incorporated in its earlier versions. For the person responsible for deciding on the replacement of older anesthesia equipment, it may be worthwhile to obtain a copy of this standard to be used during the decision-making process.(1) The following recommendations will not cover all features of the above-referenced standard in its entirety, but will be limited to features this author considers to be the most important ones.

Main Switch Interface:

Many anesthesia machines produced after 1980 incorporate a main switch which provides pneumatic and/or electrical signals which may be utilized to control the activation of monitors and alarms. Monitors and alarms which are not interfaced to the main switch have a certain risk of not being activated intentionally or as an oversight. Replacement of anesthesia machines not providing this feature (main switch interface that automatically turns on monitors and alarms) should be seriously considered. Integrated anesthesia workstations or systems composed of equipment and monitors provided by different manufacturers should be capable of electronic communication between the different components of the system. This communication should not be limited to a main switch interface, but should also include other features as described below.

Hypoxic Fresh Gas Flow Safeguard:

Most anesthesia machines produced after 1980 do not permit the administration of a fresh gas mixture with a nominal oxygen content of less than 25% because of the incorporation of devices like the ORMC and ORC by North American Drager or LINK 25 by Ohmeda. Oxygen pressure failure protection devices (so-called “fail-safe” systems), on the other hand, have an extremely limited safety potential (activation only by failure of oxygen pressure) and are not a substitute for the described devices which specifically safeguard against hypoxic fresh gas mixtures. Anesthesia machines not incorporating such a fresh gas ratio protection device or not capable of accepting an in-field installation of one may be serious candidates for replacement.

Hanging Bellows Ventilator:

Until the early part of the 1980s, the majority of anesthesia machines used in the United States incorporated ventilators with bellows which descended during the expiratory phase.2 While these designs have certain advantages over bellows that ascend during expiration, alarm devices utilizing pressure monitoring to detect circuit disconnects can easily be fooled by the design; furthermore, monitoring for expiratory flow to help detect a disconnect is not possible. In addition to this, the bellows will continue its up and down motion during disconnection from the patient, which may fool the operator into believing that the patient is adequately ventilated. Anesthesia machines incorporating a ventilator with hanging bellows are serious candidates for replacement in the event that the manufacturers do not have a conversion kit available to modify the ventilator and eliminate the hanging bellows in favor of the more recent opposite design.

Fresh Gas Hose Locking Device:

For reasons no one can explain today, a 15mm slip fit connection has been standardized for the connector of the fresh gas hose to the anesthesia machine. This standard not only actually can encourage the accidental disconnection of the fresh gas hose from the machine, but also invites the utilization of a cheap plastic connector, normally used with endotracheal tubes. Manufacturers of anesthesia machines have gone through relatively expensive efforts to make the unsafe fresh gas hose coupling (based on a 15mm connector) safe by incorporating an additional locking device. An anesthesia machine which does not incorporate a locking device for the 15mm fresh gas hose connector is not a candidate for replacement based on this feature alone, but a locking device should be installed to eliminate accidental disconnect of the fresh gas hose if there are not enough other features to induce machine replacement.

Default Values for Alarms:

The survey which formed the basis for the Arthur D. Little Study on disconnects (2) states that some operators will use every effort imaginable to silence or to avoid alarms which are considered embarrassing. Also, after a critical incident which was caused by the failure of a monitor’s alarm to warn of an adverse situation, the typical excuse stated by the operator is that he used and trusted the alarm settings dialed in by a prior operator. Alarm “default” settings are alarm limits and sound level settings which the monitor assumes whenever it is turned on. A design such as this eliminates the risk of an operator utilizing a monitor with potentially inappropriate alarm settings dialed in during a previous use, but still permits the adjustment of alarm parameters as desired.

Furthermore, this feature shortens the time necessary to prepare an anesthesia system for use. While the absence of monitors with default values for alarms incorporated into an anesthesia system alone does not justify the replacement of the system or its monitors, certain precautions must then be taken to educate the anesthesia personnel regarding this specific shortcoming. This is especially important if anesthesia is administered by personnel who are not full-time in that anesthesia department, or personnel coming from other medical facilities which provide more modern anesthesia equipment.

Stand-Alone Monitors and Alarms:

The primary purpose of an alarm system is to provide surveillance when the operator’s attention is focused elsewhere. It is known that the development of an adverse situation, an anesthesia critical incident, may result in alarm messages generated by several different monitors; for example, a patient disconnect may result in a pressure alarm, CO2 alarm, expiratory volume alarm and, eventually, a pulse oximetry alarm and an alarm on the cardiac monitor. In a system with various stand-alone monitors of the types just described, the proliferation of competing alarm sounds and the management of the different monitors and their signals may occupy the main effort of the equipment operator, leaving little time or energy to identify and correct the problem with the patient. This potential scenario is sometimes offered as justification by anesthesia equipment operators who simply turn off all the alarms all the time. Many operators want to trust their human vigilance instead of the electronic monitoring and alarm messages. Truly up to date anesthesia equipment incorporates what is called a “structured” alarm system. A structured alarm system is based on classifying alarm messages into three separate levels:

  • Warning requires immediate response.
  • Caution requires prompt response.
  • Advisory requires awareness.

The display of the various messages should be centralized through key word phrases which identify the problem triggering an alarm as accurately as possible. The audible annunciation of the arrival of an alarm message should be distinctly different for the three different levels of alarms. There should be only one silencing button for all alarms and, in general, temporary silencing should be approximately 120 seconds maximum. In the event that it is necessary to silence one alarm permanently, a clearly visible message that a specific alarm has been silenced permanently should be displayed. Permanent silencing of alarms should be automatically canceled when the system is turned off. Alarm limits should be displayed in such a way that the operator is informed regarding the existing alarm limits at all times. While an anesthesia system which does not incorporate a structured alarm system similar to that described here is not necessarily a candidate for replacement for that reason alone, the owner of such equipment must be aware that anesthesia systems incorporating less desirable alarm systems may represent slightly higher safety risks. Education of personnel in charge of the administration of anesthesia and posting of advisories on the equipment is indicated.



The 1980s saw significant progress in the development of ventilators utilized during the administration of anesthesia. Features such as PEEP, peak pressure limit, extended respiratory frequency ranges, and clearly marked controls for frequency and inspiratory-expiratory face time ratios were incorporated. The decision whether the performance of older style ventilators satisfies the need of an institution or not depends very much on the procedures performed with the equipment.

Automated Record Keeping:

Many anesthesia machines produced after the mid 1980s permit data acquisition through electronic interfaces directly from the equipment. In the event that an institution plans to introduce automatic record keeping or other means of data management, there may be advantages to replacing old equipment not having the features required for data management instead of attempting to modify old equipment.


In recent history, new volatile anesthetic agents introduced into the market require the installation of new vaporizers into the anesthesia system. While older anesthesia machines may have had vaporizers mounted in parallel or vaporizers mounted in tandem (permitting the simultaneous administration of more than one agent), newer standards require that only one vaporizer can be activated at any given time. The mounting of a vaporizer downstream of the fresh gas outlet is extremely dangerous for many reasons and should not be done at any time. Anesthesia machines which do not permit the installation of a new vaporizer for a new volatile anesthetic may need to be replaced simply for this shortcoming.

Personnel Considerations

Because of the explosive development regarding anesthesia systems during the last ten years, the configuration and the safety aspects of anesthesia machines vary widely. Historically, anesthesia systems were relatively simple devices which permitted an operator to use one or another system without specific education. All anesthesia operators primarily relied on their own vigilance to detect the advent of an adverse situation. The existence of certain safety and performance features in the rather complex anesthesia systems of today may not be immediately apparent to a user not familiar with a specific system, which creates an increased responsibility for an institution or department (the owner of the equipment) to assure that potential operators are familiar with the specifics of a system. An institution which has a relatively small turnover of personnel providing anesthesia services obviously has an easier time to permanently educate the users. Institutions which have a diversification in personnel using the equipment are subject to a significantly higher risk that an operator may expect certain safety features in older equipment similar to equipment he has used at other institutions. This scenario is emphasized in circumstances where a younger generation of anesthesiologists has been trained on modern integrated work stations and is forced on occasion to use old equipment not incorporating today’s safety features.


It must be noted that the safety and performance aspects of older anesthesia equipment differ from equipment manufactured today. Certain missing safety and performance features in older equipment may not in and of themselves necessarily warrant the replacement of this equipment (like the absence of an anti-lock braking system in a car does not necessarily warrant the replacement of the car), but an institution which makes a conscious decision not to replace such equipment must be aware of its responsibility to take measures to reduce the risks inherent with the use of the equipment (which the newer innovations are designed to decrease), specifically including proper education of the personnel regarding missing safety features and possibly acquiring manufacturers’ assistance for the modification of the equipment as an interim measure.

Peter J. Schreiber, President North American Drager Telford, PA


  1. F1161-88 (Reapproved 1994) “Standard Specifications for Minimum Performance and Safety Requirements for Components and Systems of Anesthesia Gas Machines,” American Society for Testing and Materials, Published March 1989; Address: 100 Barr Harbor Drive, West Conshohocken, PA 19428-2929, Phone 610-832-9585; FAX 610-832-9555.
  2. Arthur D. Little “Accidental Breathing Systems Disconnections,” U.S. Department of Health and Human Services, Public Health Service, Food and Drug Administration, Center for Devices and Radiological Health, January 1986.