Abstract
BACKGROUND: During invasive mechanical ventilation, where medical gases are very dry and the upper airway is bypassed, appropriate gas conditioning and humidification are mandatory at all times. Results of in vitro studies suggest that dry gases may improve lung deposition during nebulization, but this has not been confirmed through in vivo studies. The objective of this study was to measure gas humidity under multiple conditions to better describe gas hygrometry when heated humidifiers are turned off. METHODS: We measured, on a bench, the hygrometry of different gases at steady state: medical gases, at the Y-piece without humidifier, with the humidifier switched off, and with humidifier switched on. We measured gas humidity every 10-60 s during dynamic conditions after switching off the heated humidifier and after switching on the heated humidifier. Hygrometry was measured by using the psychrometric method with at least 3 measurements for each tested condition. RESULTS: We performed 287 psychrometric measurements in different situations. The mean ± SD gas absolute humidity at steady state during different conditions were the following: 1.6 ± 0.2 mg H(2)O/L for the medical gases, 4.5 ± 0.9 mg H(2)O/L at the Y-piece without humidifier, 9.1 ± 0.3 mg H(2)O/L at the Y-piece with heated humidifier turned off, and 34.2 ± 2.2 mg H(2)O/L at the Y-piece with the heated humidifier turned on. During the dynamic evaluation, after turning off the humidifier, humidity was < 30 mg H(2)O/L after a few minutes, attained 15 mg H(2)O/L after 15 min, and was below 10 mg H(2)O/L after 1 h but never reached the level of dry medical gases. After turning on the heated humidifier, the gas hygrometry reached 30 mg H(2)O/L after 5 min. CONCLUSIONS: When heated humidifiers are turned off, gas humidity levels are very low but not as low as medical gases. The clinical impact of repeated shutdowns is unknown. As recommended, heated humidifiers should never be turned off during nebulization.