Effect of Expiratory Flow on Moisture Output of Passive Humidifiers as Measured by the ISO 9360 Standard

Abstract

BACKGROUND: Adequate humidification of inspired gases during mechanical ventilation is essential to mucosal structure and function. Passive humidifiers (PH), or artificial noses, have become popular. International Standard Organization (ISO) Standard 9360 describes a laboratory method for testing PHs, specifying that expiratory time should be twice inspiratory time but failing to specify expiratory flow and flow pattern. We compared the moisture output of 2 PHs (one with low and one with high dead space) at 3 expiratory flows. DESCRIPTION OF DEVICES: Two PHs were studied (Portex 600, dead space = 10 mL, and ARC ThermoFlo, dead space = 90 mL). EVALUATION METHODS: Both devices were tested for moisture output (mg H2O/L) according to ISO 9360. Moisture output was measured by determining weight loss from a water bath during a 2-hour period of ventilation at a frequency of 10 breaths/min; tidal volume of 1000 mL; and inspiratory flow of 0.5 L/s, using a square inspiratory flow waveform. Three units of each device were tested with 3 descending ramp (exponential decay) expiratory flow patterns, with mean flows of (1) 0.25 L/s; (2) 0.33 L/s; and (3) 0.5 L/s. These flows resulted in an expiratory flow duration of 4 s, 3 s, and 2 s with the 3-s flow allowing a pause of 1 s, and the 2-s flow a pause of 2 s, prior to the subsequent breath. EVALUATION RESULTS: Moisture output for both devices declined as the expiratory flow increased (reduced duration of expiratory flow). The decrease was significant at 0.5 L/s versus 0.25 L/s for the large dead-space device [mean (SD) = 29.4 (1.4) mg H₂O/L vs 33.9 (0.7) mg H₂O/L, p < 0.05] and at each increase in expiratory flow with the small dead-space device [25.1 (0.9) mg H2O/L vs 23.0 (1.0) mg H2O/L vs 19.7 (0.7) mg H2O/L, p < 0.05]. CONCLUSIONS: The ISO 9360 system allows ranking of devices in terms of moisture output, dead space, and resistance. Our findings suggest that the Standard should specify the expiratory flow. Flow differences may help to explain the differences in results between studies of the same devices using ISO 9360. Our findings also suggest that when expiratory flow is increased (eg, in subjects with reduced lung compliance), moisture output of PH may be lower than stated in the manufacturer's literature.