Factors affecting humidity of inspired gas in a neonatal lung model

The American National Standards Institute recommends an absolute humidity of > or = 30 mg/L of inspired gas in patients receiving mechanical ventilation. This study was designed to assess the absolute humidity of inspired gas in relation to humidifier settings and environmental conditions in a test lung.

Measurements were obtained at the endotracheal tube manifold of an artificial lung in an isolette or radiant warmer with use of a nonheated wired system and two different ventilator circuit lengths. Temperature and relative humidity were measured at three environmental temperatures and various humidifier settings.

We achieved adequate absolute humidity levels only at near maximum humidifier settings. When the artificial lung was placed in an isolette with a standard circuit length inside the isolette, adequate absolute humidity was never reached at an environmental temperature of 30 degrees C. Increasing the length of the ventilator circuit situated inside the isolette or using a radiant warmer improved absolute humidity compared with that obtained with use of a standard circuit length.

Recommended absolute humidity levels may not be reached at the manifold even at high humidifier settings. Thus actual measurements of relative humidity and temperature at the endotracheal tube site are necessary to optimize humidity according to American National Standards Institute recommendations.