Direct dilution sampling, quantitation, and microbial assessment of open-system ventilation circuits in intensive care units

Abstract

In a systematic approach, 37 duplicate samples of open system circuits (Bennett MA-1 ventilators) of patients in medical and surgical intensive care units were processed by direct and serial (APHA guidelines) dilutions. The paired difference test on 15 of the in-use circuitry solution samples indicated no difference between the direct and serial dilution methods (P < 0.001). Seventy-seven additional respiratory therapy circuitry samples from similar intensive care patients were analyzed via a direct dilution method alone and processed microbiologically. The direct dilution procedure was a rapid and accurate means of evaluation of microbial contamination in the range of ≥10 to ≤106 CFU/ml. High densities of organisms frequently were found. Sites of contamination included the proximal or patient end of the circuitry (heaviest), the nebulizer trap, and the distal or humidifier portions of the circuitry. The contaminants found were predominantly gram-negative nonfermenters: Acinetobacter calcoaceticus var. antitratus, Pseudomonas aeruginosa, Pseudomonas maltophilia, and Flavobacterium meningosepticum. Fermenters were Klebsiella pneumoniae, Proteus sp., Enterobacter cloacae, Citrobacter diversus, and Enterobacter agglomerans. Infrequently, gram-positive Streptococcus spp. and Staphylococcus spp. were noted.