Bacterial colonization in humidifying cascade reservoirs after 24 and 48 hours of continuous mechanical ventilation

We evaluated levels of bacterial colonization in the humidifying cascade reservoirs of 466 mechanical ventilators; 326 reservoirs were cultured after 24 hours and 140 were cultured after 48 hours of continuous mechanical ventilation. Bacterial colonization was absent in 284 (87.1%) of the humidifier reservoirs sampled at 24 hours and 125 (89.3%) of the reservoirs cultured at 48 hours. Levels of bacterial colonization in the remaining humidifiers were low (less than 100 organisms/mL). The median temperature recorded in the reservoir fluid of 30 different ventilators was 50 degrees C (range 40 degrees to 60 degrees C). In vitro seeding of reservoir fluid at 50 degrees C with 10(6) organisms/mL of four different species of nosocomial gram-negative bacilli and Staphylococcus aureus demonstrated rapid killing of all five strains over a 6-hour incubation period, and no significant bacterial aerosols were detected. Rates and levels of bacteria in heated humidifier reservoirs are low and nosocomial pathogens survive poorly at the median reservoir temperature of 50 degrees C. We conclude that the heated humidifier reservoir on a mechanical ventilator is an unlikely source of colonization or bacterial aerosols, and therefore it can be changed every 48 hours with the ventilator tubing.