The potential role of respiratory therapy equipment in cross infection. A study using a canine model for pneumonia.

Experimental Pseudomonas aeruginosa pneumonia was induced in 8 dogs that had radiation-induced leukopenia. Three dogs were supported by mechanical ventilation (MV), 3 received continuous heated aerosol therapy (CHAT), and 2 did not receive respiratory therapy and served as control animals. The animals were studied in a carefully controlled environment until they succumbed to infection or they were killed at 24 h. An air sampler was used to collect exhaled P. aeruginosa aerosols at distances as far as 15 feet from the dogs at multiple time intervals. Water condensate in the tubing of MV and CHAT equipment was collected and cultured at the same intervals. Results showed that all dogs had multilobar P. aeruginosa pneumonia at necropsy. Control dogs did not exhale aerosols containing P. aeruginosa. Animals that were supported by MV, exhaled contaminated aerosols, but organisms could not be recovered at distances greater than 2 feet. In contrast, aerosols containing P. aeruginosa were recovered at distances as far as 15 feet from the animals receiving CHAT. Furthermore, as much as 1L of water condensate was collected in a 24-h period from tubing associated with MV and CHAT. Although the nebulizers and humidifiers remained sterile, tubing condensate was contaminated with as much as 10(7) colony-forming units per ml of P. aeruginosa in 5 of the 6 animals receiving either MV or CHAT. Contamination of tubing by P. aeruginosa was present as early as 8 to 12 h. This study identifies potential sources for cross infection through an airborne route for CHAT or from direct contact with contaminated tubing.