Method of Humidifying Inspired Gas Influences the Type of Ventilator-Associated Pneumonia

To the Editor:--The recent article on the histopathologic and microbiologic aspects of ventilator-acquired pneumonia by Fabregas and colleagues was very interesting and provoked much discussion at our recent journal club. ^[1] We noted that these workers reported a very high incidence of Pseudomonas aeruginosa and putida infections in patients whose lungs were mechanically ventilated. Humidification of the inspired gas is necessary because the patients own mechanisms have been bypassed. There are a variety of devices for this, of which the water bath and heat and moisture exchangers are most commonly used. Although the authors do not comment on the method of humidification they used for the ventilator circuits, we assume they are using a water bath device because of the high rates of infection with the above-mentioned organisms. Gallagher and coworkers reported that the use of heat and moisture exchangers in the ventilator circuit reduced the incidence of these infections from 53% to 21%, and, as a consequence, we have abandoned the routine use of water bath humidifiers. ^[2] In addition, Ahlgren and Redding showed that the use of water bath-type humidifiers creates a reservoir for infection, necessitating filters between inspiratory and expiratory ports and between the patient and humidifier. ^[2,3] * The heat and moisture exchanger incorporates a filter that prevents bacterial contamination of the circuit and patient, and is an efficient method of humidification itself. However, if we have supposed wrongly, then the levels of Pseudomonas and similar infections are high, in comparison with Gallagher's article.

H. Stuart, F.R.C.A., A. Rhodes, F.R.C.A., F. J. Lamb, F.R.C.A., Y. Lam, F.R.C.A., Department of Intensive Care Medicine, St George's Hospital, Blackshaw Road, London SW17 0QT, United Kingdom.

(Accepted for publication June 24, 1996.)

*Redding P, McWalter P: Pseudomonas fluorescens cross-infection due to contaminated humidifier water. British Medical Journal 1980; 281:275.