Is neonatal inspired gas humidity accurately controlled by humidifier temperature?
OBJECTIVE
To investigate: a) the relationship between humidifier temperature and inspired gas humidity and b) the effect of insulating the inspiratory tube on "rainout" (condensate).


METHODS
Observational study.


METHODS
Regional neonatal unit in a university hospital.


METHODS
Forty-eight infants receiving assisted ventilation, of whom 31 infants were nursed in incubators and 17 under radiant heaters.


RESULTS
Despite always maintaining humidifier temperature greater than 34.7 degrees C, inspired gas humidity decreased below the American National Standards Institution minimum of 30 mg H2O/L on 35 of 479 occasions. At a humidifier temperature of 36 degrees C, inspired gas humidity varied between 17 and 43 mg H2O/L. In incubators set at a temperature of 34.1 +/- 1.3 (SD) degrees C, inspired gas humidity was linearly related to humidifier temperature, but with wide scatter (p less than .001, r2 = .28). In cooler incubators set at 32.9 +/- 1.8 degrees C, inspired gas humidity varied inversely with humidifier temperature. This variation was attributed to condensate due to inspired gas cooling within the incubator. Insulation of the inspiratory tubing reduced condensate by only 15%.


CONCLUSIONS
Inspired gas humidity cannot be predicted reliably from humidifier temperature. Accurate control will require a new generation of humidifiers that measure inspired gas humidity.
Department of Child Health, University of Dundee, Ninewells Hospital and Medical School, UK.
M
M
O'Hagan
E
E
Reid
W O
WO
Tarnow-Mordi
D004867:Equipment Design; D006801:Humans; D006813:Humidity; D007186:Incubators, Infant / Q000592:standards*; D007231:Infant, Newborn; D015931:Intensive Care, Neonatal / Q000379:methods; D016014:Linear Models; D013696:Temperature