Filters in the mechanical ventilator circuit

The filters of a mechanical ventilator play several important roles. On one hand, they protect the patient from any sort of airborne filth which might be blowing around in the gas supply systems or the ambient air. On the other hand, these filters protect the delicate innards of the ventilator from the corrosive swamp gas being exhaled by the patient. Lastly, the filters protect the intensivist and their co-workers from exhaled pathogens and clouds of nebulised medications which did not make it into the patient. All of these jobs will be discussed in greater detail in this chapter.

From the point of view of the college requirements (at least those summative ones which matter) the CICM WCA document “Ventilation” expects that the “acceptable”-level trainee at least “describes requirements for filtering / scavenging”, without really going into details of those technologies. Nonetheless, in the spirit of this resource, extensive irrelevant detail will be offered. If the time-poor exam candidate is for some reason in need of this information, they are advised to stop reading after the grey box below, and generally to learn to manage their time better.

In summary:

The best articles for this sort of thing is probably the series by A.R. Wilkes (Part 1 and Part 2 are available, both from 2010). This series of articles deals mainly with HMEs, but contains a discussion of ventilator filter design which is of relevance here. Beyond this, in their study of exhaled particle sizes, Wan et al (2014) have some interesting discussion points. Kramer et al (2010) is representative of society recommendations for breathing system filters (incidentally, if one were in need of a search string, “breathing system filters” performs best).

Filtration of supplied gas

The gas supplied to the patient may be contaminated by multiple possible agents:

• Non-sterile water, condensing in the pipes
• Dust from gas supply pipes
• Bacteria from nonsterile wall faucets and pipes
• Dust from the soda lime canister

Supplied wall gas is not uniformly a perfect representative of a Parisian standard in gaseous purity. The piping system may not be completely devoid of particulate matter or pathogens. Consider that, once commissioned, it remains in use almost perpetually while the hospital is in use. The oxygen whistling through those pipes is a highly reactive corrosive chemical, and well capable of supporting bacterial metabolism.

Some accidental contamination of piped gas supplies is almost inevitable in the process of commissioning the gas supply system. Eichhorn et al (1977) reported on the unexpected contents of medical gas supplies to a new 176-bed extension to a 450-bed general hospital. “A volatile hydrocarbon at an initial concentration of 10 parts per million and a dust of fine gray particulate matter” was found; the authors had no choice but to purge the pipes with continuous flows.

Accidental contamination due to abuse of components is also known, though rare. Hay (2002) described a case of water contaminating the hospital piped gas supply, which occurred when the gas supply was open to atmospheric air during maintenance. Atmospheric air contains water vapour, which condensed when the system was repressurised with cold gas. Liquid water was detected when it started leaking from orthopaedic tools and filling the anaesthetic flow meter valves. That stuff, unfiltered, would have direct access to the patient’s airway, and it is far from sterile. In fact, it’s far from clean. An unpublished anecdote from a colleague recalls a historical incident from the life of an old hospital, where the contaminated pipe system was so ancient and decrepit that the water flushed out of it was brown with rust.

Obviously, these are rare occurrences, and one would probably not be designing a ventilator with them in mind. The most important role of all such filters is to protect the patient (and to a lesser extent the oxygen sensor and pressure transducer) from bacterial contamination of the last few stages of the gas supply, them being the non-sterile wall faucets. These are 600 kPa-rated gas hose connections which are handled constantly by the unwashed hands of nurses doctors and maintenance technicians, making them a  particular bacterial contamination risk. For this reason, generally, mechanical ventilators have gas inlet filters.

A final mention for soda lime canister dust must be made. In a circle circuit, all gas is flushed through this canister at some point. Conceivably, this could flush out some dust particles. Indeed, Lauria (1975) presented a case where exactly this happened. The patient went on to develop bronchospasm. The modern systems are largely safe from this because of the more granular dustless canister design and because of the routine use of HMEs in anaesthesia (the HME also acts as an inspiratory filter, and should catch any stray soda particles).

Gas supply filters

 These filters are swapped out every 5,000 hrs of ventilator operation, which gives one an impression of how long it takes to accumulate a dangerous amount of inlet gas bacteria. They can afford to be quite thick and beefy – the piped gas pressure supplied into them is 400kPa, which means one can sustain quite an impressive cross-filter pressure drop (up to 200kPa, let’s say).

How good should these filters be? It is not clear how universally applicable they are, but there are some published standards. The ISO standard for filters recommends they withstand challenge with tiny salt particles  (1-3 µm). Wilkes (2002) describes an apparatus for doing this, which detects the salt by means of a photometer. Apparently, particles larger than 5 µm will be captured quite effortlessly by most filter fibres, whereas particles smaller than 0.1 µm are small enough to undergo significant Brownian motion, i.e their erratic movement makes their effective diameter much larger and they also end up hitting the filter fibres. Particles in the  1-3 µm range end up being just the right size to penetrate most filters. Most of the usual testing conditions and performance criteria for these filters were borrowed from the American National Institute for Occupational Safety and Health (NIOSH) standards which are used to rate the performance of health worker facemasks, wisely assuming that what is good enough for doctor’s lungs is also good enough for their patients.

Filtration of expired gas

Realistically, apart from water vapour and CO2, what is the patient exhaling? Turns out, volatile organic compounds and microparticles of water. Which are probably loaded with pathogens. Wan et al (2014) investigated this with a portable airborne particle monitor. The patients exhaled up to 2520 particle per breath, of which 80% were in the 0.3-1.0µm range. The main determinant of particle numbers was the PEEP variable – the higher the PEEP, the more exhaled particles there were. Volume and mode of ventilation did not seem to affect the number of particles.

Is the risk of contamination serious? Probably, yes; even with filters. “Contaminated condensate can potentially pass through some filters under typical pressures encountered during mechanical ventilation” warns A. R Wilkes (2011). Without filtration, expired pathogens are totally free to mingle with the air of the ICU or the operating theatre. An example from the distant past is usually offered in these situations; Bishop et al (1964) reported on the case of a ventilator becoming “grossly contaminated” with Pseudomonas, likely by having had a severely infected patient connected to it (now dead). The authors were mainly thinking of other patients and the attending staff; only the expiratory limb was dirty, and they concluded that attaching another patient to the same circuit would not be risky.

Additionally, the gas mixture in the expiratory port may contain medications. It is important to remember that in the circuit there is also bias flow, and that during much of the respiratory cycle the patient is not breathing. Ergo, not all gas in the circuit is exhaled gas; some of it hasn’t even ever been inside the patient. If a nebuliser is being used, there is surely going to be some un-inhaled nebulised agent in the expiratory limb. Ari et al (2016) determined that up to 45% of the prescribed dose finds its way into the expired gas, potentially exposing ICU staff. The use of any filter was associated with significantly less exposure (the post-filter dose was some 160 times lower).

Lastly, if the patient is being ventilated using some exotic gas mixture, the unused or exhaled gas will also be vented out of the machine. Filters won’t help this, so some other way of getting rid of this gas needs to be implemented. This is more of an issue during anaesthesia, where one might use many litres of nitrous oxide and dangerous volatile ethers. In ICU, one might occasionally use helium-oxygen mixtures, which are generally regarded as safe. One is unlikely to fill an ICU cubicle with enough helium to cause serious problems.