Up to date, expert answers to frequently asked questions (FAQ) about oxygen supply systems, respiratory care and pulse oximetry written by OCC & collaborators.
Oxygen Delivery Devices
What filters are needed or recommended for a mechanical ventilator?
- It depends on the device setup. Ventilators may require ‘external’ filters (viral, HME, and fan, as well as air intake filters for turbine or compressor ventilators) and internal filters (oxygen inlet filter). Filters can provide three kinds of functions:
- Filtering particulate matter
- HEPA (high-efficiency particulate air) filters rated to 3 microns are generally considered ‘acceptable’ for bacterial and viral filtration. The term HEPA refers to the efficiency of capturing particles with a MPPS (most penetrating particle size) diameter of 0.3 microns.
- Of note, machines that accept 50psi/4bar gas intake usually have internal bronze sintered filters to protect the machine from contaminated gas sources. On occasion additional external filters on the high pressure gas lines are required to prevent damage to the device.
- Preserving heat and moisture
- Heat and Moisture Exchangers (HME) are commonly rated to 3 microns, the HEPA standard (and may be referred to as HME filters – HMEF), but may not be. They are composed of foam, paper or other material that allows moisture to condense.
- Hygroscopic Condenser Humidifiers (HCH) are functionally the same as HMEs, though have a slightly different mechanism as they are impregnated with a salt to aid in moisture condenstation. In the past, HME types were separated based on the additional treatment of the media. An HME relied solely on physical principles (the earliest ones were all aluminum) whereas the HCH included treatment of the media with a hygroscopic salt (LiCl, now mostly CaCl). At present, the reality is that most HME devices also take advantage of the hygroscopic salt (check manufacturer’s specifications to confirm this). In theory, if one were to push the media out of an HME then touch it to the tip of the tongue – it would be very salty (DO NOT TRY THIS).
- The moisture efficiency of an HME or HCH is dependent on its size, media density and salt treatment.
- HMEs may be referred to as type I (adult) and type II (pediatrics), which differ in deadspace and functional tidal volume range.
- Unless specifically designated as having capacity for ‘filtration’ (HMEF), HMEs do not provide adequate filtration of bacteria and viruses.
- Filtering bacteria and viruses
- Bacterial/viral (B/V) filters are defined by the ability to filter particles with a diameter size of 3 microns though may be rated to particles as small as 0.2 microns, and do not necessarily provide heat/moisture preservation.
- The minimum viral filtration efficiency (VFE) that is needed to ensure SARS COV-2 virus can not pass from the patient to the room or machine is unknown.
- B/V filters with 99.97% ASTM efficiency or filters with >95% efficiency for MPPS of 0.3 microns may be recommended to prevent SARS-CoV-2 viral transmission though data and standards continue to evolve.
- Diameter for SARS-CoV-2 virion is ~0.06-0.14 microns, hepatitis C is 0.03 microns and Staph aureus is 1 micron.
- Filtering particulate matter
- Combination filters providing all three functions are available. Filters that conserve heat and moisture and provide B/V filtering are often referred to as HMEF.
- More info on filter types and efficiency testing