Episode #38 Breathing System Filters: Questions and Considerations

March 30, 2021

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Welcome to the next installment of the Anesthesia Patient Safety podcast hosted by Alli Bechtel.  This podcast will be an exciting journey towards improved anesthesia patient safety.

We are headed back to the February 2021 APSF Newsletter to talk about HEPA Filters.

You can find the article here. https://www.apsf.org/article/hepa-filters-do-we-really-know-enough-breathing-system-filters-in-the-era-of-covid-19/

If you have more questions about protecting patients during anesthesia care and your anesthesia machine, check out the APSF’s FAQ on anesthesia machine use protection and decontamination during the covid-19 pandemic. You can find the page here. The APSF is continually updating this information with the most recent update in February 2021. https://www.apsf.org/faq-on-anesthesia-machine-use-protection-and-decontamination-during-the-covid-19-pandemic/

Here are the important specifications for breathing system filters:

  • bacterial and viral filtration efficiency (%—higher is better),
  • NaCl or salt filtration efficiency (%—higher is better),
  • resistance to flow (pressure drop in Pa or cmH2O at a given airflow rate in L/min—lower is better),
  • how the former specifications are affected by filter conditioning in humidity,
  • internal volume (ml—lower is better), and
  • humidification
    • (moisture loss in mgH2O/L of air­—lower is better), or
    • (moisture output in mgH2O/L of air—higher is better).

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© 2021, The Anesthesia Patient Safety Foundation

Hello and welcome back to the Anesthesia Patient Safety Podcast.  My name is Alli Bechtel and I am your host. Thank you for joining us for another show. We are headed back to the February 2021 APSF Newsletter and this time we are checking out the Rapid Response Section. In order to keep patients safe during anesthesia care throughout the Covid-19 pandemic, we have had to consider risk of transmission of this new respiratory pathogen which has led to changes in our practice and our equipment use.

Before we dive into today’s episode, we’d like to recognize Preferred Physicians Medical Risk Retention Group, a major corporate supporter of APSF. Preferred Physicians Medical Risk Retention Group has generously provided unrestricted support as well as research and educational grants to further our vision that “no one shall be harmed by anesthesia care”. Thank you, Preferred Physicians Medical Risk Retention Group – we wouldn’t be able to do all that we do without you!”

We are talking about HEPA Filters since this is a piece of equipment that we are using more often as a result of the Covid-19 pandemic during anesthesia care. For the show today we will be reviewing the rapid response article, “HEPA Filters. Do We Really Know Enough? – Breathing System Filters in the Era of Covid-19” by Felipe Urdaneta. I will include a link to the article in the show notes as well. To follow along with us, head over to APSF.org and click on the Newsletter heading. First one down is the Current Issue. Then, scroll down until you see the Rapid Response to questions from our readers section in the left hand column. The first article is the one we will be discussing today. I’m going to start by reading the Rapid Response Question that was submitted by Urdaneta. The author writes:

“Dear RAPID Response:

The global crisis due to COVID-19 has permeated every aspect of our health care systems. Concerns about the biohazard of SARS-CoV-2, spread and contact transmission to patients, health care personnel, environment, and equipment have been widespread, especially with regards to procedures that generate aerosols (AGPs).1-3 Transmission of the virus is primarily respiratory in nature. SARS-CoV-2 virion is approximately 120 nanometers in diameter (0.06–0.14 µm), and travels from person to person in biological carrier particles such as droplets or aerosols.2,3 Recommendations regarding adequate levels of PPE, handwashing, surface cleaning, decontamination, and precautions during airway management procedures have been discussed extensively during the pandemic.4-6 As with other respiratory transmissible diseases, we rely on two important filtering systems: circuit filters when artificial breathing systems are used in the operating room and/or intensive care units (ICU) and face-mask respirators.

However, things are a bit complicated:

  1. Anesthesia machines and mechanical ventilators require filters for air quality purification and cross-contamination prevention. The efficiency standard of such filters is termed HEPA for high-efficiency particulate air/ high-efficiency particulate absorbing capacity.7The ASA recommends placement of HEPA filters between the Y-piece of the breathing circuit and the patient’s mask, endotracheal tube or laryngeal mask airway.8
  2. European and U.S standards to determine filter efficiency are not the same: European standards use 99.95% removal of particles with a diameter of 0.3 µm in diameter, while the U.S uses 99.97%.9
  3. Face mask efficiency is determined by the level of particle penetration. An N95 mask, for example, removes at least 95% of 300 nm particles using an airflow rate of 85 liters/min.10Face mask respirators are regulated according to U.S National Institute for Occupational Safety and Health (NIOSH) and internationally recognized standards and testing.
  4. Filters in breathing circuits and anesthesia machines are not regulated. There is no national or international standard test for filters in breathing circuits. Since there is no standard testing, are all manufactures reporting the same when discussing the level of efficiency?11
  5. Are current available filters adequate for COVID-19?
  6. Because many COVID-19 patients require prolonged mechanical ventilation, how often should these filters be changed in the ICU?
  7. What should health care professionals do in case of filter shortages?

These are some of the pressing questions with regards to HEPA filters that I would like to see discussed.

Thank you.”

This rapid response highlights many important questions about breathing circuit filters and it is important to understand the safety considerations of these additional filters. I would also like to direct you to the page on the APSF website that includes frequently asked questions about anesthesia machine use and decontamination during the Covid-19 pandemic. The page provides important information about protecting the anesthesia machine from a patient who may be infected with SARS-CoV-2. Once you are done listening to the podcast, we hope that you will head over there to check it out. The APSF recently updated the page in February 2021.

Robert Loeb provides a lot of information about filters including some clinical recommendations in his reply to this rapid response. So, here we go. One of the questions that has been highlighted as a result of providing anesthesia to patients infected with coronavirus is, “What is the risk for cross-infection through the breathing system between patients?” Breathing system filters were used more in clinical practice after a case report of cross-infection of Hepatitis C from a contaminated breathing system to multiple patients in the 1990s. For the most part, the evidence is inconclusive about the potential for cross-infection since there are very few documented events, but in laboratory environments it has been demonstrated. Breathing system filters may be recommended when breathing circuits are reused between patients.

The next part of this review includes respiratory virus transmission which is what we see with Covid-19, Severe Acute Respiratory Syndrome, Middle-East Respiratory Syndrome-associated coronavirus and other coronaviruses. The mode of transmission includes droplets which are greater than 20 microns in diameter, aerosolized particles which are less than 5-10 microns in diameter, and intermediate-sized particles which fall in-between. These particles travel differently as well with droplets falling rather quickly due to gravity and the smaller aerosols staying in the air longer and following airstreams. Evaporation can change droplets into smaller droplet nuclei that can follow airstreams as well. Droplets, aerosols, and intermediate-sized particles are formed following coughing, sneezing, and even talking while aerosols are generated from passive breathing. The authors highlight a very important concept about filtering pathogens. Remember, these respiratory viruses are not transmitted as virus particles floating around in the air all alone, but instead the virus particles are contained within larger particles. In addition, after the droplets and intermediate-sized particles fall from the air onto surfaces, this can lead to surface transmission. One study about virus transmission of seasonal coronaviruses revealed that infected patients exhaled and coughed to generate between 0-200,000 virus particles each hour. Keep in mind that there is an increased risk of infection with increased exposure to viral particles in terms of time of exposure and the number of particles.

We have reached the part of the show where we talk about physics, specifically the Physics of Filtration. First up, larger particles can be filtered with sieve filtration which we can see with a strainer since the large particles are unable to pass through smaller holes. Smaller particles interact with the filter material in a number of ways depending on the size. 0.11 micron sized particles take part in inertial impaction and can directly impact filter strands. Smaller particles in the 0.05-1 micron size range can undergo interception with tangential contact with the filter strands. With even smaller particles, we see diffusion as the particles move in an erratic motion which can lead to contact with the filter strands. Finally, very small charged particles may be attracted to the charged filter material which is known as electrostatic attraction. I encourage you to click over to the article and check out Figure 1 which shows a picture of this. This physics is important because it impacts the efficiency of the filter and the toughest particles to trap with a filter are those that are about 0.3 microns in size.

Now, let’s take a closer look at the filters used in anesthesia breathing systems. First up, we have pleated mechanical filters which have a thick sheet of tightly packed bonded hydrophobic fibers arranged in a random orientation that capture particles inside the depth of the filter. The pleats help to increase the surface area while also decreasing resistance to airflow. The benefits of this filter include high filtration efficiency, heat and moisture exchange when placed close to the airway, continuing to filter when damp and not allowing the passage of liquids, but these effects come with the cost of higher price and higher internal volume. The next filter is the electrostatic filter which are made of thinner sheets of looser, woven electrostatic fibers without pleats. These filters also have low resistance to airflow, but they have 1000-fold lower filtration efficiency without the pleats and liquids can pass through this filter. The next filters on our list are the heat and moisture exchange filters which as their name implies provide heat and moisture exchange when placed close to the airway where two-way airflow occurs and filtering when they contain an electrostatic or pleated mechanical filter. The location of these devices is important since absorption of water occurs during exhalation and this moisture is then released during inhalation. The last filter on our list is membrane filters which are used in respiratory gas analyzers to prevent fluid entry into the analyzer chamber. These are found in water traps since gas can pass through the filter when it is dry, but not when the filter is wet. The filters have very small pores through which larger particles cannot pass.

How do we measure filter efficiency? There is an international standard for testing the filtration efficiency of breathing system filters which includes the salt test method. This method determines the number of 0.1-0.3 micron airborne sodium chloride particles that pass through the filter during a short-term challenge at airflow rates that will be used in clinical practice. Specifically, the challenge for pediatric and adult filters involves pre-conditioning the filters in humidified air to replicate how the filter may work in the clinical environment and then 0.1mg or 0.2mg of sodium chloride particles are tested at 15L/min or 30L/min respectively. The challenge utilizes non-electrostatically charged dry salt particles and these are quite challenging to trap. Keep in mind that this method does not evaluate filtration of droplets and aerosols or microorganisms. The salt test method provides a way to compare different breathing system filters, but does not have clinical relevance nor does it provide a threshold for minimum performance filter efficiency. The test results provide a percent filtration efficiency which is the percent of particles in the challenge that do not pass through the filter. A simple example is a challenge with 10 million particles and 1000 particles are detected on the other side of the filter leading to a percent filtration efficiency of 99.99%.

Other filters have different standards for testing including the National Institute for Occupational Health and Safety standard for Respiratory Protective Devices which provides testing and rating for non-powered air-purifying respirators. The method for N-series respirators involves a challenge of 200mg non-electrostatically charged 0.1-0.3 micron-sized dry sodium chloride particles at a flow rate of 85 L/min. Another filtration testing standard is for High Efficiency Particulate Air filters and Ultra Low Particulate Air filters which are used in clean air devices and clean rooms. HEPA filters are high efficiency filters and can remove 99.97% of 0.3 micron diameter particles.

So, can breathing system filters work to remove microorganisms? There is no standard test for filtration efficiency of breathing system filters when it comes to bacterial and viral particles. However, there are methods including the Standard Test Method for evaluating the bacterial filtration efficiency (BFE) of medical face mask materials, using a biological aerosol of Staph. aureus. This method uses aerosolized bacterial or viral particles that are 3 microns in size vacuumed though the filter so that anything that passes through the filter lands on a nutrient broth or culture plates. This time  the percent filtration efficiency can be calculated by dividing the number of cultured particles that passed through the filter by the number of particles that started the challenge. When evaluating these different tests of filtration efficiency, filters have a highest percent filtration efficiency for bacteria with viral in the middle and salt particles at the low end.

There is one more test we will talk about, Bubble Point Testing, which is used to rate membrane filters. Hydrophilic 0.22-micron membrane filters are used to sterilize pharmaceuticals and keep epidural infusions sterile. Unfortunately, there is no data for filtering airborne particles for these filters available at this time. But we do know that the 0.2 micron hydrophobic membrane filters have been tested for airborne filtration and have a viral filtration efficiency of 99.99%. These highly efficient filters can be found in the GE D-Fend Pro, Drager WaterLock 2, and Covidien FilterLine Water traps.

While there are no regulations for breathing system filters on anesthesia machines, it is likely a good idea to help prevent cross-infection of patients with Covid019 infection. As the pandemic has unfolded, the APSF and ASA has recommended using breathing system filters at this time, but keep in mind that this is informed by the knowledge that we have at this time with the priority to help keep patients safe during anesthesia care. I will include a link to the recommendations in the show notes. It is also important to understand the risks of these breathing system filters including added dead space with increased carbon dioxide rebreathing and slower inhalation induction and emergence, increased resistance to inspiratory and expiratory gas flow with the resultant increased work of breathing during spontaneous breathing, altered respiratory mechanics, and potential site for disconnections. Care must be taken to ensure that filters do not become obstructed and prevent adequate gas flow.

As we wrap up for today, if you are using or considering using breathing system filters, it is important to know the specifications including the following:

  • bacterial and viral filtration efficiency with a higher percentage is better
  • sodium chloride or salt filtration efficiency and again the higher percentage is better
  • resistance to flow which descibes the pressure drop at a given airflow rate in L/min— and here lower is better,
  • how the former specifications are affected by filter conditioning in humidity,
  • the internal volume described in ml and lower is better, and
  • humidification which includes the
    • moisture loss in mg H2O/L of air­ and here lower is better
    • and the moisture output in mg H2O/L of air and here higher is better

You can find this information on the manufacturer’s website, product literature, journal articles, and online. Unfortunately, we do not know the filtration efficiency necessary to prevent infection from exhaled SARS-CoV-2 or other respiratory viruses through the anesthesia breathing system at this time and we need to continue to learn more going forward.

That’s all the time we have for today. If you have any questions or comments from today’s show, please email us at [email protected].

Visit APSF.org for detailed information and check out the show notes for links to all the topics we discussed today.  Please keep in mind that the information in this show is provided for informational purposes only and does not constitute medical or legal advice. The APSF podcast is intended for anesthesiologists, anesthetists, clinicians and other professionals with an interest in anesthesiology, and patient safety advocates around the world. The Anesthesia Patient Safety Podcast delivers the best of the APSF Newsletter and website directly to you, so you can listen on the go! Visit us at APSF.org/podcast and at @APSForg on Twitter, Facebook, and Instagram.

Until next time, stay vigilant so that no one shall be harmed by anesthesia care.

© 2021, The Anesthesia Patient Safety Foundation