Calibration of Vaporizers

George Burgess, MD

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.

Dear Q&A:

One of my hospitals plans on purchasing new vaporizers for anesthetic delivery. Their machines already have anesthetic gas monitors mounted and used on every case. The vendor states that the "standard" vaporizer check/recalibration occurs every 3 years. He offers an annual check/calibration of each vaporizer for additional cost.

Does the APSF have any recommendation as to annual vs. every-3-year calibration of vaporizers?

And, if no recommendation, standard, or guideline exists for interval of calibration, do you have any opinion about whether the presence of anesthetic gas monitors in the circuit might influence the interval between calibrations?

Thank you.

George Burgess MD
VP, Innovation & Technology
Franciscan Missionaries of Our Lady Health System
Baton Rouge, LA

Dear Dr. Burgess:

From a clinical use standpoint, I believe that gas analyzers sampling at the endotracheal tube are far more useful to me than what the dial or output of the vaporizer actually is. We know that the actual vaporizer output is not what the patient inhales. Fresh gas flow, inspired flow, carrier gas composition, mixing capacity of the circuit, circuit design, uptake, degradation, gas sampling extraction, etc., are all contributing factors. Thus, if the engineers and manufacturers believe the internal calibration mechanisms are stable for up to 3 years, then I would say that more frequent calibrations are not clinically helpful. I would further add that gross errors in output, secondary to internal failure, might occur suddenly and should be detectable by gas analysis. Vaporizer output must also be checked under highly regulated conditions with controlled carrier gas composition.

Dear Dr. Burgess:

The above response is absolutely right. The vaporizer setting is only meaningful when an open circuit is used. In all other configurations, the vaporizer setting is different from what the machine delivers at the Y-piece. So the only adequate number is what the gas analyzer measures as "inspiratory Agent"/"expiratory Agent" value. During low flow anesthesia the vaporizer setting may differ greatly from the inspired agent concentration. That’s why a gas analyzer is so helpful. The only reason for a vaporizer check (as opposed to a calibration), may be to verify its functionality in general.

Dear Dr. Burgess:

I would defer to the vaporizer manufacturers, as they have the most definitive information regarding field verification and factory calibration process. Informally, I can tell you that very little is, or can be done, in the field, other than identifying an errant vaporizer and pulling it off the machine.

Most vaporizers on the market today, regardless of their manufacturer, cannot be calibrated in the field. They can only be checked. Usually the first inkling that calibration may be off comes from a disagreement with the multi-gas monitor. However, since the possibility exists that the gas analyzer could be misreading, then both devices need to be checked. Normally, vaporizer calibration should be checked with a reference device such as the ones made by Riken.

Different vaporizer manufacturers make greatly varying claims about how often vaporizers need to be checked or sent back for calibration. I remember that one manufacturer claims a 10-year service-free life for its vaporizer. There is also the school of thought that may suggest that "if it ain’t broke, don’t fix it," meaning that if the multi-agent monitor agrees with the vaporizer’s setting, then all is well.

Dear Dr. Burgess:

I suggest that one point deserves more emphasis: I had this type of discussion with my vaporizer service people who insisted on a 6-monthly check which seemed excessive apart from halothane and thymol accumulation. They made the point that most of the trouble came from foreign substances in the vaporization chamber, necessitating disassembly of the unit. In one case there was blood in a unit, and not infrequently watery solutions that could be corrosive over a long period. The wrong agent can confuse the issue. This type of problem may not be detected by a check of concentration output.

Dear Dr. Burgess:

While there is no formal standard that addresses either a service, or calibration interval for inhalation anesthetic agent vaporizers, the vaporizer manufacturers normally have both device- or model-specific recommended intervals for each. These intervals depend on the age of the device and the degree to which technology has advanced both the accuracy and reliability of the vaporizer. Since vaporizers may be conventional pneumatic, pneumatic with electronic monitoring, or entirely electronic, the intervals will vary.

The original design for vaporizers included no electronic means to verify vaporizer output, vaporizer fill status, or any number of other potentially important pieces of information the user may wish to know. The advent of electronics in vaporizers has permitted a more careful observation of the current overall status of the vaporizer. Electronics also provide the opportunity to notify the user when something untoward has occurred with the vaporizer that may otherwise remain unnoticed by the user. The inclusion of electronics affects both the service and calibration intervals. As a result of these variations many institutions select either one single vaporizer technology or a group of similar technologies that require calibration and servicing at similar intervals.