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Measuring the Humidity of Anesthetic Gases

Gilmour, Ian J. MD

Letter to the Editor: In Response

Cardiopulmonary Services, The University of Minnesota Hospital and Clinic, Minneapolis, MN 55455-0392.

In Response:

We wish to thank Drs. Strau beta et al. for their interest in our article [1]. They have raised several points to which we wish to respond. First, we did not use "the sensor integrated into the humidifier" to measure circuit temperature. The humidity probe that we used has an integrated thermistor so that temperature was indeed determined "in close vicinity to the humidity sensor."

Second, Vaisala Instruments (Helsinki, Finland) stands by its assertion that this probe achieves a 90% response within 5 s (personal communication). Even though the probe was initially heated outside the circuit (to prevent condensation on the head during this process), the probes were in the circuit for a much longer period. In addition, as the experiment was conducted in an identical fashion for all humidifiers with both types of circuits, one would expect that any error related to humidity sensor response time would be consistent and that the outcome would be the same.

Third, Strau beta et al. are correct in their assertion that integrated humidifier temperature probes are relatively inaccurate. We believe that this inaccuracy explains humidity values in excess of 100% when recalculated back to 37 degrees C. This is particularly true when unheated circuits are used because, as Strau beta et al. point out, their use is associated with very high humidifier temperatures, which, given the limitations of these systems, may well cause distal circuit temperatures greater than that indicated by the humidifier temperature sensor.

Strau beta et al. claim that "the main advantage of heated circuits is the maintenance of gas temperature throughout the inspiratory limb, thus preventing humidity rainout and burn injury." The main point of our article was that, while circuit temperature is generally maintained by heated circuits, humidity is not. This has significant implications for calculations of heat exchange and for the effect of inspired gas on the tracheobronchial tree. In addition, infrared assessments of heated circuits in our laboratory (unpublished observations) demonstrate that gas temperature within heated circuits is not consistent throughout the inspiratory limb, apparently depending on the proximity of the heating wires to the circuit walls. Possibly even more importantly, heated circuits do not prevent burn injury [2]. While we agree that aspiration of circuit condensate is dangerous [3], we maintain that currently available heated circuits provide, at best, half the answer.

Unfortunately, we are unable to comment on the statement that Strau beta et al. have found "heated systems to be unambiguously superior" because their [4] was not available to us. However, from the title of the article, it appears that the system they assessed was not among the ones we tested. Nonetheless, we believe that our data support our conclusion that the widely used heated circuits we tested maintain temperature at the expense of humidity.

Ian J. Gilmour, MD

Cardiopulmonary Services, The University of Minnesota Hospital and Clinic, Minneapolis, MN 55455-0392

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1. Gilmour LJ, Boyle MJ, Rozenberg A, Palahniuk RJ. The effect of heated wire circuits on humidification of inspired gases. Anesth Analg 1994;79:160-4.
2. FDA Safety Alert. Hazards of heated-wire breathing circuits. Rockville, MD: Food and Drug Administration, July 14, 1993.
3. Craven DE, Goularte TA, Make BJ. Concise clinical study. Contaminated condensate in mechanical ventilator circuits: a risk factor for nosocomial pneumonia? Am Rev Respir Dis 1984;129:625.
4. Strau beta JM, Hausdorfer J, Hagemann H, Schroder D. Klimatisierung anasthetischer Gase wahrend Sauglingsnarkosen mit dem "Anasthesiearbeitsplatz CICERO". Anaesthesist 1992;41:534-8.
© 1995 International Anesthesia Research Society