The Science Journal of the American Association for Respiratory Care

1996 OPEN FORUM Abstracts

EFFECT OF EXPIRATORY FLOW ON MOISTURE OUTPUT OF PASSIVE HUMIDIFIERS AS MEASURED BY ISO 9360.

Richard Branson RRT, Robert Campbell RRT, Kenneth Davis Jr., MD. Department of Surgery, University of Cincinnati, Cincinnati, Ohio.

Background:ISO 9360 is a test method for determining the moisture output of a passive humidifier (PH). At a tidal volume of 1.0 L and f of 10 breaths/min, I:E is supposed to be set at 1:2 (Insp. Flow = 0.5 L/s & Insp. Time = 2 s). However, expiratory flow pattern and time are not specified. We studied the effects of varying expiratory flow on the measured moisture output of two PH. Methods: The ISO 9360 lung model was constructed from Cascade humidifier, temperature controller, 2 test lungs, and a series of tubing and one way valves to separate inspired and expired gases. Expired temperature entering the PH was set at 34/C. The model was ventilated at a V_{T} of 1.0 L, f of 10 b/min and flow of 0.5 L/s. Expiratory flow was controlled by adjusting compliance of the test lungs and placement of linear resistors in the expiratory gas path. This resulted in 3 expiratory descending flow waveforms: 1) mean expiratory flow - 0.25 L/s; 2) mean expiratory flow - 0.33 L/s; and 3) mean expiratory flow - 0.5 L/s. At these 3 settings the duration of expiratory flow was 4 s, 3s, and 2s. The latter two settings had a pause of 1 s and 2 s respectively prior to the next inspiration. Two PH were used, an AARC Thermoflo filter (deadspace = 90 mL) and a Portex 600 (deadspace = 10 mL). Three of each PH were tested at each of the three expiratory flow conditions for a period of three hours. Results: As mean expiratory flow increased moisture output fell. The decrease was much greater in the PH with a low deadspace. Results are shown below (mean ± SD) and analysis was performed using ANOVA. A p < 0.05 was considered significant.

Moisture Output in mg H_{2}O/L

DEVICE Exp.Flow=0.25 L/s Exp.Flow=0.33 L/s Exp.Flow=0.5 L/s

AARC 33.9 (0.7) 32.7 (1.1) 29.4 (1.4)^{* #}

Portex 25.1 (0.9) 23.0 (1.0)* 19.7 (0.7)^{* #}

* p < 0.05 vs 0.25 L/s, # p < 0.05 vs 0.33 L/s.

Conclusion: Moisture output can be significantly effected by expiratory flow of the lung model. This is probably due to a reduction in the contact time between warm, moist gas and the PH media. This effect is exacerbated by a PH with a low deadspace. Additionally, patients with high expiratory flow (low compliance) may receive gas at a lower moisture content than described by the manufacturer. ISO 9360 should define the shape and duration of expiratory flow to allow fair comparison of different devices.

Reference: OF-96-162

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