The pressure required to force liquid through breathing system filters.

Liquid in the form of sputum and condensation may be forced through a breathing system filter if sufficient pressure is applied to the filter layer, with the potential for cross-infection. Twenty-three different breathing system filters (12 adult, 11 paediatric) were assessed to determine the pressure causing liquid penetration. Five samples of each were tested by connecting the ventilator end of a breathing system filter to a water-filled U-tube and reservoir. The reservoir was positioned so that the water surface was level with the filter layer and raised until liquid appeared through the filter layer on the patient side and the water-column height recorded. There were substantial differences in performance between models. Liquid penetration occurred at lower pressures for adult electrostatic than adult pleated filters: median [IQR] 1.18 [1.08- 1.32] and 8.04 [4.27-12.80] kPa, respectively, p < 0.0001. Liquid penetration occurred at similar pressures in paediatric electrostatic filters (0.98 [0.74-1.32] kPa) compared to adult electrostatic filters.