THE EFFECTS OF CIRCUIT AND HUMIDIFIER TYPE ON CONTAMINATION POTENTIALDURING MECHANICAL VENTILATION - A LABORATORY STUDY

Background: This study was undertaken because of concerns that ventila tor humidifiers could be exacerbating the problem of nosocomial pneumo nia in patients receiving mechanical ventilation. Methods: Four differ ent brands of humidifiers were used in conjunction with a Siemens Serv o 900B mechanical ventilator (Siemens Life Support Services, Solna, Sw eden). 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 parti cles produced were assessed. In the second part, we measured the effec t of rime and temperature on bacterial survival in humidifier chambers . Because only bubble-through humidifiers were determined to produce i nfectious particles, the speed with which a contaminated bubble-throug h humidifier could infect circuit condensate was also determined. Aliq uots of chamber water and circuit condensate, as well as air samples a nd distal circuit swabs, were cultured. Results: Humidifiers other tha n bubble-through humidifiers did not produce aerosols. Particle produc tion by bubble-through humidifiers varied directly with flow rate (R(2 ) = 0.91). Chamber temperatures did not affect chamber colony counts e xcept in bubble-through humidifiers. Although chamber colony counts in bubble-through humidifiers decreased with time, organisms remained vi able 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 pr oduce aerosols that readily contaminate both circuit condensate and ef fluent gas. Avoiding bubble-through humidifiers should improve patient safety while allowing changes in practice that can result in signific ant cost savings.