Relationship between absolute humidity of inspired and expired gases with an active humidifier

Background and goals

To evaluate the relationship between absolute humidity of inspired (AHi) and expired gases (AHe) during mechanical ventilation using a hot water humidifier (ConchathermIII^®).

Materials and methods

We measured the absolute humidity (AHi) and relative humidity (RHi) of inspired and expired gases in 40 sedated and paralyzed patients (mean age 60.7 ± 16.9 years). The AHi and AHe was measured using a psycrometer (Yokogawa^®). The airflow was separated by a flow separator placed between the Y piece of the ventilator circuit and the catheter mouth. The psycrometer is made of four thermic probes, two of them measure the dry temperature and two of them, covered with a cotton gauze (wet with physiological solution 15 min before every measurement), measure the wet temperature. Dry and wet temperatures were measured on the inspiratory and expiratory lines at the flows separator. We set the active humidifier at 31, 34, 36 and 38°C, and the measurements were made after 30 min for each temperature.

Results

The relationship between AHi and AHe are shown in Fig. 1.

Figure 1

Conclusions

1) AHi is related to AHe (AHi > 38.8 mg/l, positive airway fluid balance; AHi < 38.8 mg/l, negative airway fluid balance). 2) We suggest an AHi of 38.8 mg/l (RHi 100%) with a temperature of 34°C during mechanical ventilation to maintain adequate airway conditioning.