The ability of breathing system filters to prevent liquid contamination of breathing systems: a laboratory study

The ability of 29 different breathing system filters (five pleated hydrophobic and 24 electrostatic) to prevent the passage of water was assessed. Five, 10 or 20 ml of water was added on to the patient side of the filter during ventilation of a patient model with the filter layer orientated either horizontally or vertically. Water did not pass through the filter layer during any of the 30 tests on the pleated hydrophobic filters. In contrast, water passed through the filter layer in 39 of the 144 tests on the electrostatic filters (27%; 95% CI for difference between the filter types 14-35%; p = 0.0004). For electrostatic filters, a smaller internal volume, a larger volume of water added, and having the filter layer horizontal rather than vertical all significantly increased the probability of water penetration. Although pleated hydrophobic filters would protect the breathing system against liquid contamination, electrostatic filters can also be used, provided the internal volume is chosen appropriately and the filter layer is vertical.