Preservation of humidity and heat of respiratory gases in patients with a minute ventilation greater than 10 L/min.

OBJECTIVE: To compare the temperature and humidification output of one heated humidifier system (Bennett Cascade 2 Humidifier) and two heat and moisture exchangers (Pall Ultipor, BB 50, and Humid-Vent Filter) in intensive care unit (ICU) patients submitted to a minute ventilation of > 10 L/min.

DESIGN: Prospective, controlled, randomized, unblinded study.

SETTING: ICU of a university hospital.

PATIENTS: Eleven sedated and paralyzed patients who required controlled mechanical ventilation with a minute ventilation of > 10 L/min for > 3 days.

INTERVENTIONS: After a randomized selection process, the patients were ventilated for 24-hr periods with the humidifier and one of the heat and moisture exchangers. Both heat and moisture exchangers were first tested for 45 mins; then, the heat and moisture exchanger that demonstrated the best performance in terms of temperature and water preservation was tested for 24 hrs.

MEASUREMENTS AND MAIN RESULTS: During the inspiration phase for each patient, the following measurements were performed: mean and minimum values of temperature, relative and absolute humidity of inspired gases. During the 45-min test period, the Humid-Vent Filter had a better temperature and humidification output than the Pall Ultipor Filter and thus was tested for 24 hrs. The Bennett Cascade 2 Humidifier and the Humid-Vent Filter had a better thermic capacity than the Pall Ultipor Filter (p < .001). No difference was ever observed between the Bennett Cascade 2 Humidifier and the Humid-Vent Filter regarding relative humidity. The Pall Ultipor Filter had a lower temperature and humidification output when compared with the other two systems (p < .007). Concerning absolute humidity of inspired gases, the Pall Ultipor Filter achieved a lower performance than any other tested systems (p < .02). A small but significant decrease in temperature and absolute humidity, but not in relative humidity, was seen after 24 hrs of use with the Humid-Vent Filter. However, with this heat and moisture exchanger, all patients had an absolute humidity of > 28 mg H2O/L and a relative humidity of > 93% after 24 hrs of use.

CONCLUSIONS: In patients with a minute ventilation of > 10 L/min (> 10.5 to 16.0 L/min), the Humid-Vent Filter had a temperature and humidification output close to the reference system (the Bennett Cascade 2 Humidifier). The Pall Ultipor Filter had a significantly lower temperature and humidification output in these patients.