There are issues of calibration of anesthetic vaporizers that arise from the choice of carrier gas. We have evidence both from the literature and from our own investigations that the choice of carrier affects both the actual and the indicated concentration of agent in a properly calibrated vaporizer. This note explores the principle factors of that variation and their significance.
The issue of carrier gas arises because at least four different gasses are involved in the use and the measurement of the vaporizer: air, nitrogen, oxygen, and a mixture of oxygen and nitrous oxide. Air is the most convenient carrier for some environments, and Primary Medical evaluates calibration in air, oxygen, or both as appropriate for the application. In typical application in a surgical theater, the actual carrier is a mixture characteristically, 30% oxygen and 70% nitrous oxide. Tests made in that theater are usually conducted with oxygen primarily because the typical test instrument (Riken) is not calibrated for mixed gasses. All instrumentation is calibrated against test gasses provided with carriers of oxygen or nitrogen; at high concentrations, only nitrogen is available.
Because of the relatively low precision of the Riken interferometer, Primary Medical uses a Miran infrared spectrometer for verification. The Riken is sensitive to the total index of refraction of the gas; its readings therefore depend critically on the nature of the carrier, and calibration is accurate only when the same carrier is used for testing and for operation. In contrast, the Miran operates in bands in which none of the carriers has significant absorption; therefore, its measurement is essentially independent of the choice of carrier. Simply put, the Miran accurately measures concentration of agent regardless of carrier, where the Riken's reading must be corrected as the carrier is changed.
At low concentrations, the percentage of agent is not highly dependent on the choice of carrier. At concentrations of 3% and above, the carrier alters the concentration of agent. For example, a vaporizer perfectly calibrated in air would provide 2% of agent at a 2% dial setting whether the carrier was air, nitrogen, oxygen, or oxygen/nitrous oxide mixture. However, at 5% dial setting, the true concentration of agent in air would be 5%, but with oxygen carrier it may be 4.7%. The value in mixed gasses or nitrogen would be comparably different from both 4.7% and 5%. (We are currently attempting to estimate that variation.)
The problem is further compounded by the fact that the test gasses available for calibration of the instruments are provided only with a nitrogen carrier in the 5% concentration (where carrier is most significant).
The conclusion of all of the above is that the accuracy of calibration at high concentrations (3 to 5%) is difficult to determine and of questionable value. We recognize that testing done in the hospital is likely to be carried out with oxygen carrier and a Riken interferometer. To avoid the appearance of calibration error, Primary Medical is now calibrating against an oxygen reference. We do not believe that the result will truly be more accurate relative to operation with mixed gasses, or that it will indicate better performance against the nitrogen reference, but this change in procedure may reduce confusion in the hospital.