Expiratory filter resistance

Author: Ken Hargett, Caroline Brown, Reviewer: Süha Demirakca, Matthias Himmelstoss, Simon Franz, Munir Karjaghli, David Grooms

In these days of COVID-19, hospitals are experiencing increased volumes of mechanically ventilated patients. Additionally, institutions are struggling with equipment and supply shortages. Concerns about spreading COVID-19 between patients and healthcare workers have prompted increased consideration of controlling the exhaled gas from the ventilator by utilizing filters in the expiratory limb of the ventilator circuit.

 Take-away messages

  • Particularly during a pandemic, the use of filters is recommended to prevent contamination of equipment, staff and the surrounding environment; however, filters may become clogged due to condensation, patient secretions and nebulized medication.
  • The use of filters may cause an increase in resistance that can prevent adequate ventilation of the patient and result in adverse events.
  • Close monitoring is required to ensure any signs of obstruction are identified as quickly as possible.
  • Many ventilators today offer several ways of recognizing increasing expiratory filter resistance over time.

As early as the late 1800s, the first objections were voiced in the literature to breathing through the same device previously used by another patient (1). Patients are not only exhaling just water vapor and CO2; expired gas might also contain a range of microbial pathogens and other particulate matter that can contaminate the ventilator and the surrounding environment. In 2014, Wan et al. showed that patients exhaled up to more than 2,500 particles per breath, with the number of particles increasing in relation to the level of positive end-expiratory pressure (PEEP) (2). If a nebulizer is being used, residual medication may also be added to the mixture. Ari et al. (3) demonstrated that not only can up to 45% of the prescribed dose of medication be contained in exhaled aerosol, but that placement of an expiratory filter was associated with significantly less exposure to aerosols released into the atmosphere during exhalation (up to 160 times lower dose after being filtered). Use of an expiratory filter can therefore reduce environmental contamination and protect healthcare workers and other patients.

Although there is conflicting evidence about the effect of filters in reducing the spread of infection, it would seem prudent to use a filter in a highly contagious environment. During the first outbreak of SARS in 2003, a recommendation was made by the Emergency Care Research Institute that breathing-circuit filters be incorporated in the exhalation limb of any ventilator used on an infected patient (4). Subsequent recommendations issued by the ESICM in 2010 for ICU triage for use during an influenza epidemic or mass disaster include the use of a ‘high‐quality bacterial/viral filter’ on the expiratory port of ventilators (5). Use of a filter may not only guard against contamination, but also protect the ventilator and other consumables used in the circuit, such as the expiratory valve. Mojoli et al. (6) reported on the sudden malfunction of an expiratory valve associated with the nebulization of colistin methanesulfonate in a mechanically ventilated patient. It was likely that the significant amount of the drug remaining in the circuit – potentially 90% and more - had affected the expiratory valve’s function and, within a short space of time, caused the ventilator to alarm. Replacement of the expiratory valve membrane and cover, subsequently protected by a filter, resolved the problem.

However, it is also well-known that filters and heat-moisture exchangers can become clogged due to liquids and secretions from the patient (7, 8, 9, 10). In addition, condensation from the breathing circuit may accumulate and eventually block the filter (11, 12), as can the use of nebulized drugs (13). Irrespective of the cause, occlusion may lead to an increase in resistance (14) and eventually prevent the lungs from being ventilated adequately (8, 11), whereby the type of humidification circuit type used may play an important role (14). In extreme cases, complete occlusion will generate an “Exhalation obstructed” alarm in many of today’s ventilators. Any increase in expiratory resistance decreases the speed of exhalation and thus carries with it the risk of dynamic pulmonary hyperinflation and intrinsic positive end-expiratory pressure (PEEP) (15). Many reports of various critical incidents caused by filter obstruction can be found in the literature (16, 17, 18, 19, 20) and in several cases, the reason for the airway obstruction was difficult to diagnose or differentiate from other possible causes (20, 21).  

In a study published in 2011, Lu et al. (22) reported on filter obstructions caused by aerosol particles during nebulization of ceftazidime/amikacin. Despite the filter being removed after each aerosol administration, obstruction was related to severe adverse events in three patients, one of whom experienced a sudden cardiac arrest. As recently as June, 2020, Lee et al. (23) reported on obstruction of the expiratory limb filter in a heated circuit – possibly due to condensation or patient secretions, or a combination of both - and warned against potential fatal complications.

Therefore, it is essential that any obstruction to the filter be detected as early as possible. Sometimes, the filter itself may not be easily or immediately visible or the cause of the obstruction not immediately evident. Whenever a filter is used, not only should it be replaced regularly according to the hospital’s protocol, but staff should be especially vigilant and check for obstruction as soon as a sudden increase in airway pressure or PEEP is evident. While many ventilators offer automatic monitoring and alarming when exhalation is obstructed, healthcare staff still need to be trained in identifying malfunctions and correcting them as quickly as possible. However, particularly with high workloads like those being placed on staff during the current pandemic, the maintenance and monitoring of the ventilator circuit is even more difficult.

There are several ways to recognize increasing resistance in the expiratory filter. One is to monitor the peak expiratory flow and the shape of the expiratory flow curve, which changes as resistance increases. In the absence of a waveform display, exhalation pressure can be checked during an expiratory hold maneuver.

Another is to monitor the expiratory time constant, which describes the speed at which the lung empties after the pressure drop created by opening of the exhalation valve. Under normal conditions, without imposed artificial resistance in the expiratory filter, the RCexp is mostly related to the airway mechanics of the patient. However, when additional resistance is present in the expiratory filter, the RCexp will change with no corresponding changes in compliance or resistance measurements.

Lastly, trending is a good way of tracking increases in expiratory filter resistance over time. On some ventilators, it is possible to plot expiratory flow and RCexp to show the gradual increases in filter resistance over a certain timeframe.

Hamilton Medical ventilators offer monitoring and display of the expiratory flow curve, measurement of the RCexp and a trending window to plot selected variables as a standard or optional feature*.

* Not all features or ventilators available in all markets


  1. Skinner T. Anaesthesia and inhalers. British Medical Journal 1873; 1: 353–4
  2. Wan G-H, Wu C-L, Chen Y-F, Huang S-H, Wang Y-L, Chen C-W (2014) Particle Size Concentration Distribution and Influences on Exhaled Breath Particles in Mechanically Ventilated Patients. PLoS ONE 9(1): e87088
  3. Ari A, Fink J, Pilbeam S. Secondhand aerosol exposure during mechanical ventilation with and without expiratory filters: An in vitro study. Ind J Respiratory Care 2016;(1):677-82
  4. ECRI. Mechanical ventilation of SARS patients. Safety issues involving breathing-circuit filters. Health Devices 2003; 32: 220–2
  5. Sprung CL, Zimmerman JL, Christian MD, et al. Recommendations for intensive care unit and hospital preparations for an influenza epidemic or mass disaster: summary report of the European Society of Intensive Care Medicine's Task Force for intensive care unit triage during an influenza epidemic or mass disaster. Intensive Care Med. 2010;36(3):428-443. doi:10.1007/s00134-010-1759-y
  6. Mojoli, F., Iotti, G.A., Imberti, R. et al. The importance of protecting the mechanical ventilator during colistin methanesulfonate nebulization. Intensive Care Med 39, 535–536 (2013)
  7. Chiaranda, M., Verona, L., Pinamonti, O. et al. Use of heat and moisture exchanging (HME) filters in mechanically ventilated ICU patients: Influence on airway flow-resistance. Intensive Care Med 19, 462–466 (1993)
  8. Williams DJ, Stacey MRW. Rapid and complete occlusion of a heat and moisture exchange filter by pulmonary edema (clinical report). Canadian Journal of Anesthesia 2002; 49:126–31
  9. Kopman AF, Glaser L. Obstruction of bacterial filters by edema fluid. Anesthesiology 1976; 44: 169–70
  10. Mason J, Tackley R. An acute rise in expiratory resistance due to a blocked ventilator filter. Anaesthesia 1981; 36: 335
  11. Schummer W, Schummer C, Fuchs J, Voigt R. Sudden upper airway occlusion due to invisible rain-out in the heat and moisture exchanger. British Journal of Anaesthesia 2002; 89: 335–6
  12. Buckley PM. Increase in resistance of in-line breathing filters in humidified air. British Journal of Anaesthesia 1984;56: 637–43
  13. Stacey MRW, Asai T, Wilkes A, Hodzovic I. Obstruction of a breathing system filter. Canadian Journal of Anaesthesia1996; 43: 1276
  14. Tonnelier A, Lellouche F, Bouchard PA, L'Her E. Impact of humidification and nebulization during expiratory limb protection: an experimental bench study. Respir Care. 2013;58(8):1315-1322
  15. Iotti GA, Olivei MC, Palo A, et al. Unfavorable mechanical effects of heat and moisture exchangers in ventilated patients. Intensive Care Medicine 1997; 23:399–405
  16. McEwan AI, Dowell L, Karis JH. Bilateral tension pneumothorax caused by a blocked bacterial filter in an anesthesia breathing circuit. Anesthesia and Analgesia 1993;76: 440–2
  17. Smith CE, Otworth JR, Kaluszyk P. Bilateral tension pneumothorax due toa defective anesthesia breathing circuit filter. J Clin Anesth 1991;3:229-34
  18. Barnes S, Normoyle D. Failure of ventilation in an infant due to increased resistance of a disposable heat and moisture exchanger. Anesth Analg 1996; 83: 193
  19. Walton JS, Fears R, Burt N, Dorman HB. Intraoperative breathing circuit obstruction caused by Albuterol nebulization. Anesth Analg 1999; 89: 650–1 (cross ref)
  20. Aarhus D, Soreide E, Holst-Larsen H. Mechanical obstruction in the anaesthesia delivery-system mimicking severe bronchospasm. Anaesthesia 1997;52:992-4
  21. Williams DJ, Stacey MR. Rapid and complete occlusion of a heat and moisture exchange filter by pulmonary edema (clinical report). Can J Anaesth 2002; 49: 126–31
  22. Lu Q, Yang J, Liu Z, Gutierrez C, Aymard G, Rouby JJ (2011) Nebulized ceftazidime and amikacin in ventilator-associated pneumonia caused by Pseudomonas aeruginosa. Am J Respir Crit Care Med 184(1):106–115
  23. Lee, Misoon & Ko, Eun & Lee, So & Cho, Ana & Chung, Yang & Koo, Bon & Lee, Joon-Ho. (2020). A Case of Expiratory Limb Filter Obstruction in Heated Circuit Kit. Soonchunhyang Medical Science. 26. 36-37. 10.15746/sms.20.010

Related Articles

filter, expiratory filter, filter resistance, filter obstruction, occlusion, expiratory resistance

Date of Printing: 30.10.2020
The content of this newsletter is for informational purposes only and is not intended to be a substitute for professional training or for standard treatment guidelines in your facility. Any recommendations made in this newsletter with respect to clinical practice or the use of specific products, technology or therapies represent the personal opinion of the author only, and may not be considered as official recommendations made by Hamilton Medical AG. Hamilton Medical AG provides no warranty with respect to the information contained in this newsletter and reliance on any part of this information is solely at your own risk.