The Science Journal of the American Association for Respiratory Care

2008 OPEN FORUM Abstracts


Jaime Lynn Tiffin1, Norm Tiffin1, John D. Davies3, Charles Fabyanic1

Introduction: Heated passover humidifiers maintain humidity levels by changing the chamber water temperature. However, the temperature of the chamber water reflects the average flow over time and may not adjust quickly enough to abrupt changes in inspiratory flowrates seen during mechanical ventilation. We tested the response time to abrupt changes in flowrate from a common heated passover humidifier to determine effect on absolute humidity.

Methods: A heated passover humidifier (MR850, Fisher & Paykel, Irvine, CA) with internal water level valve in the chamber (MNR290, Fisher & Paykel) and heated wire circuit were used. We recorded gas temperature and relative humidity (RH) using a hygrometer (SHT-75, Sensirion, Westlake Village, CA) at the patient wye in addition to the chamber water temperature (Fluke, Everett, WA) and the controller display temperature. The flow was increased to 60 L/min from 5 L/min and measurements recorded at 0, 15, 30, 45, 60, 90 seconds and 2 minutes and every minute after for 30 minutes total. An electronic flowmeter (TSI, Shoreview, MN) measured flow proximal to the chamber. Absolute humidity was calculated using this formula:((0.00031243*T3)+(0.0081847*T2)+(0.32321*T)+5.018)*RH

Results: Increasing flowrates from 5 to 60 L/min dropped the absolute humidity at the wye immediately. At 15 seconds the absolute humidity of the gas decreased from 40.9 to 25.8 mg/L with the lowest recording at 3 minutes of 18.7 mg/L. Absolute humidity increased to AARC guideline minimum levels (30 mg/L) at 9 minutes and stabilized at approximately 40.4 mg/L after 21 minutes.

The drop in absolute humidity was a result of lower relative humidity more than temperature decreases. The lowest relative humidity was 50.1% (33.8°C) at 3 minutes from a start of 91.1% (37.4°C).

Gas temperature dropped lowest to 33.7°C at minute 2 from 37.4°C. Chamber water temperature started at 56.1°C (5 L/min) decreased to a low of 50.6oC at minute 3 then increased and held to 82.0°C to 83.0°C at 14 minutes and beyond.

Conclusion: The response time of this passover humidifier is too slow to maintain absolute humidity levels during abrupt changes in flow commonly seen during inspiration. Changes in relative humidity contribute most to this absolute humidity deficit indicating that the chamber water temperature doesn't react quickly enough to compensate.