The effects of circuit and humidifier type on contamination potential during mechanical ventilation: A laboratory study

Background: This study was undertaken because of concerns that ventilator humidifiers could be exacerbating the problem of nosocomial pneumonia in patients receiving mechanical ventilation. Methods: Four different brands of humidifiers were used in conjunction with a Siemens Servo 900B mechanical ventilator (Siemens Life Support Services, Solna, Sweden). In the first part, the ventilator was operated with humidifiers filled with contaminated water at room temperature. The viability of airborne particles and the effect of flow rates on the number of particles produced were assessed. In the second part, we measured the effect of time and temperature on bacterial survival in humidifier chambers. Because only bubble-through humidifiers were determined to produce infectious particles, the speed with which a contaminated bubble-through humidifier could infect circuit condensate was also determined. Aliquots of chamber water and circuit condensate, as well as air samples and distal circuit swabs, were cultured. Results: Humidifiers other than bubble-through humidifiers did not produce aerosols. Particle production by bubble-through humidifiers varied directly with flow rate (R2 = 0.91). Chamber temperatures did not affect chamber colony counts except in bubble-through humidifiers. Although chamber colony counts in bubble-through humidifiers decreased with time, organisms remained viable throughout the study. When bubble-through humidifiers were heated, both condensate and effluent gas became heavily contaminated within minutes of flow initiation. Conclusions: Bubble-through humidifiers produce aerosols that readily contaminate both circuit condensate and effluent gas. Avoiding bubble-through humidifiers should improve patient safety while allowing changes in practice that can result in significant cost savings.