1995 OPEN FORUM Abstracts
MOVEMENT OF BACTERIAL CONTAMINANTS IS REDUCED IN HEATED VENTILATOR CIRCUITS
R Orec BSc, G Richards MB ChB, B Cornere MNZIMLS, B Dove NZIMLS, A Morris MB ChB. Department of Microbiology, Green Lane Hospital Auckland New Zealand.
Introduction: Several clinical studies have shown that the ventilator circuits of intubated patients rapidly become extensively contaminated with endogenous bacteria. These studies have all been performed with humidified non heated circuits. Bacteria are known to travel in circuits on aerosols created by nebulisers but there is little information on how bacteria move in circuits with passover humidifiers, or the effects of heater wires. Aim To determine the conditions required for movement of bacteria in modern ventilator circuits. Method A typical ventilator circuit, including a Siemens Servo 900C ventilator and a Fisher & Paykel MR 730 humidifier were connected to a rubber "lung". The inspiratory limb between the humidifier and wye piece was comprised of four 30 cm lengths of smooth bore tubing with connectors modified for bacterial sampling. The tubing was fixed at a 20 degree angle down from the humidifier to the mid point then a 20 degree rise to the wye piece. During experiments the tubing was repeatedly inoculated with a known concentration of Pseudomonas aeruginosa and samples were taken regularly from other sites. Two tests of 4 hours duration were performed with an unheated circuit and wye piece temperature of 37 C, the first with inoculation adjacent to the humidifier and sampling downstream (with the airflow), and the second with inoculation at the wye piece and sampling upstream. A third test used the same method as test 2 except that a heated circuit was used with the wye piece setting at 37 C and 2 degrees heating in the circuit. A forth test used the same conditions as test 3 but was of 30 hours duration. Results Condensate formed in the tubing in Test 1 and 2 and bacterial contamination followed the movement of water down to, but not beyond the mid point in the tubing. Bacterial movement could occur against the airflow but not against gravity. With the heated circuit condensate was restricted to a fine mist on the wall of the tubing and there was no movement of water. No movement of bacteria was noted over 4 or 30 hours. Conclusions In this model movement of bacteria in a ventilator circuit only occurred if water was present in sufficient amounts to move within the circuit. Using a heated circuit that reduced condensate to a fine mist prevented movement of bacteria. Contamination of heated circuits is likely to be less extensive than non heated circuits as an important mode of bacterial transport is eliminated.