Circulation 84,122 • Volume 23, No. 2 • Summer 2008   Issue PDF

Why Don’t All Vaporizers Have an “Empty” Alarm?

Terry W. Bejot, MD

Q Dear Q&A,

Since anesthesia awareness is now in the public forum, I was wondering why the non-desflurane vaporizers don’t have an “empty” alarm. Seems like an easy fix to prevent the vaporizer from running out during a case that goes unnoticed. Perhaps it has already been done and we are waiting for the upgrades to occur, but thought I would pass it along.

The above would prevent empty vaporizers, but it would not prevent failure to turn the vaporizer on during a case. With electronic charting being a future reality, is the APSF going to require all electronic charting to flag end-tidal anesthetic gas concentrations that approach awareness MAC values? While the programs are being written it would be of great value if the APSF came out with a list of requirements for electronic charting to prevent mishaps rather than leaving the algorithms to the programmers who usually know nothing about anesthesia.

Terry W. Bejot, MD
Lincoln, NE

A Dear Dr. Bejot,

The consensus is that low agent alarms would be useful on all vaporizers. However, the suggestion of minimizing the risk of patient awareness through the use of a low agent alarm would not be as efficacious as properly measured exhaled agent concentration, which yields an estimate of Minimum Alveolar Concentration (MAC) of the agent. It has been suggested that an exhaled agent concentration greater than 0.8 MAC will minimize the risk of awareness.1

The low agent alarm would be a very useful adjunct to monitoring the exhaled agent concentration to indicate the need to refill the vaporizer before it’s operation is compromised by having too little agent in the vaporizer. Concern was expressed about retrofitting current vaporizers with reliable low level alarms, because most of the currently available vaporizers are purely mechanical and the alarm system would add complexities of a sensor, potential leak of agent, electronic circuitry, and battery power and maintenance to name a few issues. Add to that the complexities of a sensor/alarm system that must be immune from extraneous radio frequencies (cautery, cell phones, and other devices), and must alarm when the battery is low and when the agent is low or when the device is not working properly.

Some non-desflurane electronic vaporizers already provide a low-level alarm, while some manufacturers have indicated they do plan for monitoring the amount of liquid in all vaporizers on future anesthesia machines. Most likely, the market place will drive all manufacturers in this direction. Meanwhile, the use of exhaled end-tidal agent concentration should be employed and properly monitored to minimize the likelihood of patient awareness.

With regard to electronic charting, the Committee on Technology is not a standards-setting committee and will not be promulgating standards or requirements for medical records of any kind including computerized anesthesia records. Thank you for bringing this important topic to us for discussion.

The Committee on Technology


  1. Hardman J G, Aitkenhead AR. Awareness during anesthesia. Continuing Education in Anaesthesia, Critical Care & Pain. 2005 5(6):183-186.

Numerous questions to the Committee on Technology are individually and quickly answered each quarter by knowledgeable committee members. Many of those responses would be of value to the general readership, but are not suitable for the Dear SIRS column. Therefore, we have created this simple column to address the needs of our readership.

The information provided is for safety-related educational purposes only, and does not constitute medical or legal advice. Individual or group responses are only commentary, provided for purposes of education or discussion, and are neither statements of advice nor the opinions of APSF. It is not the intention of APSF to provide specific medical or legal advice or to endorse any specific views or recommendations in response to the inquiries posted. In no event shall APSF be responsible or liable, directly or indirectly, for any damage or loss caused or alleged to be caused by or in connection with the reliance on any such information