To the Editor:
With interest we read the article by Gilmour et al. . Both the resolution and sensitivity of the sensor integrated into the humidifiers used do not show the necessary accuracy. Calculation of absolute humidity requires precise determination of the temperature in close vicinity to the humidity sensor. An error of 0.5 degrees C leads to a difference exceeding 1 mg/L calculated absolute humidity at 37.0 degrees C and 100% relative humidity. Furthermore, the sensor was heated outside the circuit. Capacitive sensors require up to 5 min to equilibrate . The reported differences between heated and unheated circuits may be due to different temperatures at the measurement site. "Recalculated" humidity values exceeding 100% should not exist under normal atmospheric conditions and thus indicate temperature deviations.
The main advantage of heated circuits is the maintenance of gas temperature throughout the inspiratory limb, thus preventing humidity rainout and burn injury. When a distal circuit temperature of 37 degrees C is to be achieved using unheated circuits, the humidifier temperature must considerably exceed 37 degrees C to compensate for the temperature decrease in the inspiratory limb. Cooling of gas produces condensation inside the circuit. Especially in pediatric anesthesia, accidental aspiration of condensed water may be dangerous.
How to measure exactly humidity and temperature of gases was reported earlier . Measuring humidity and temperature inside heated and unheated circuits in neonatal anesthesia, we found heated systems to be unambiguously superior regarding water transport .
Jochen M. Strau beta, MD
Stefan Krohn, MD
Robert Sumpelmann, MD
Jurgen Hausdorfer, MD
Department of Anesthesiology III, Medizinische Hochschule, D-30623 Hannover, Germany
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