How Do You Know Your Filter Filters?

The following article is written by Steve Koontz, National Training Manager for ARC Medical.

Most clinicians believe that any device marketed as a “bacterial/viral filter” must be capable of capturing any individual bacteria or viruses that might be suspended within inhaled or exhaled gases. We were surprised to discover that this is, by no means, a justifiable assumption [1].

There are two generally accepted methods of sealing plastic materials: ultrasonic welding and a solvent welding process. Both methods are subject to fail points or leaks, which are very undesirable when dealing with patients, especially with undiagnosed diseases such as tuberculosis. How would you know if the welding process was done correctly or incorrectly? Could the gas bypass the filter media and go around the media due to a construction fail point? If it did, how would you know?

A third method of sealing plastic does not use ultrasonic welding or a solvent welding process. Mechanical fittings are used to create air tight seals without fail points, guaranteeing the integrity of the filter media. During the assembly of the filter, an internal wall is created to direct all gas through the filter media. This internal wall is created on both sides of the filter media with the outer edge of the filter media becoming an integral part of this inner wall assembly.  No wings or diffusers are needed to redirect the gas flow over the entire media.

The second ridge and lip molded on the outer wall of each half makes the seal when the two halves are assembled. No gluing or ultrasonic welding is used.  The gas flow cannot bypass the filter media because fail points are not created.  This construction process using mechanical fittings will not compromise the filtration efficiency of the filter media.  

The placement of the filter is as important as the construction of the filter device. When placed at the anesthesia circuit wye, the device becomes a bi-directional filter that protects both the patient and equipment from contamination.  With this placement, the filter protects the patient from equipment contamination and protects the equipment from patient contamination. The use of filters on the inhalation or the exhalation circuit cuff are not needed.  

All filter manufacturers and distributors make claims about their products. To ensure any misleading and ambiguous claims are not interpreted as fact, providers should ask themselves, "How do you know your filter filters?"

Filtration expectations should not be taken for granted. Construction of the filter device is almost never mentioned or explained. Filtration conversations should at least include the following questions:

• Who is the filter media manufacturer?
• What type of media is used?
• Is the media hydrophobic?
• What is the resistance wet and dry?
• Will the media lose filtration efficiencies if it becomes saturated?
• Is independent testing available?
• How is the product constructed?
• Is the device constructed with fail points?
• Is it possible for gas to bypass the media due to fail points?
• What do the 9s mean?
• What type of quality control is used?
• Have any incident reports been reported such as resistance issues?
• Is a shelf life associated with the media?

Clinicians should be aware of the potential for microorganisms to pass through wet filters [2]. Anesthesia circuit filters being used today may become saturated with moisture. This moisture may be created by anesthesia equipment or from patient’s exhaled breath. Scott, et. al. found that large numbers of viable microorganisms of two different sizes pass through anaesthetic breathing system filters when they are wet. As predicted, commonly available breathing filters cannot be relied upon to prevent bacterial transfer [3].

To quickly check for hydrophobic media, pour cold coffee into the device. Hold the device horizontally, apply a little pressure, and blow into the device to simulate clinical use of the filter. Alternately, pour cold coffee into the device when it is sitting on a flat surface. Let it stand for a few minutes and see what happens. Often coffee will pass across the media after saturation occurs, representing a breakthrough and possible lower filtration efficiencies. Coffee is used for a color trail with this test. Under clinical applications, this saturation process would most likely occur without notice.

Heat and moisture exchangers, in combination with a bacterial and viral filter (HMEFs), are widely used during general anesthesia [4]. Although heat and moisture exchanging filters (HMEF) are recommended for use during anesthesia, the criteria for choosing a filter are not clearly defined [5]. The moisture exchange component passively humidifies the inspired air by returning a percentage of the patient's expired moisture [6]. HME/filters are designed to retain exhaled moisture on the patient side of the HMEF. They can only contribute to patient humidification if they are placed in the circuit where gas moves back and forth: at the Y piece [7]. Heat recovery effectiveness should be also considered with these devices.

The use of paper, foam and sponge humidification media for HME properties can create undesirable results.  Moisture from the patient’s exhaled breath may be absorbed into the media, creating resistance and additional weight issues. Resistance and possible airway obstructions may occur, especially during longer procedures.

Spun polyprolene, a type of plastic, is another type of HME media is available on the market today. These fibers are coated with calcium chloride to provide an affinity for moisture. The moisture droplets are adsorbed on the surface of the fibers and are not absorbed. This moisture does not become part of the product. Instead, the moisture droplet will wick along the outside of the fiber, exposing the internal volume of the droplet to the gas flow. This process of capturing the moisture and releasing the moisture helps to reduce resistance issues and gives the patient a warmer gas flow.

Excess patient secretions occluding the HMEF have been responsible for previous case reports of airway obstruction. A previous study suggested that differences in HMEF design might contribute to filter obstruction under wet conditions [8]. Clifford, et al reported an example of this type of obstruction. The HMEF involved was not obviously contaminated.[9]. The FDA Maude database provides end user experiences.

Circuit protection systems are available on the market today. Many practitioners are using these systems and choosing to reuse anesthesia circuits. They have done so without incident for many years. The above aforementioned information would be a prerequisite when making the decision on what product to use. The question all should ask themselves: "How do I know my filter filters?"

[1] [4] [6] Robert R. Demers BS RRT, Bacterial/Viral Filtration - Let the Breather Beware! (CHEST October 2001; Vol. 120; No. 4), 1377-1389.

[ 5] J. Dellamonica, N. Boisseau, B. Goubaux and M. Raucoules-Aimé, Comparison of manufacturers' specifications for 44 types of heat and moisture exchanging filters, (British Journal of Anaesthesia 2004 93-4),532-539

[2] [3] H. T. Scott, S. Fraser, P. Willson, G. B. Drummond, and J. K. Baillie, Passage of pathogenic microorganisms through breathing system filters used in anaesthesia and intensive care: (Anaesthesia July 2010 No. 65), 670-673.

[7] G. Lawes, Hidden hazards and dangers associated with the use of HME/filters in breathing circuits. Their effect on toxic metabolite production, pulse oximetry and airway resistance; (Br J Anaesth 2003; 91), 249-264

[8] D. Turnbull, P. C. Fisher, G. H. Mills, and N. J. Morgan-Hughes, Performance of breathing filters under wet conditions: a laboratory evaluation: (http://bja.oxfordjournals.org/content/94/5/675.full - fn-2Oxford Journals, Medicine, BJA, Volume 94, Issue 5), 675-682.

[9] Clifford J. Peady FANZCA, Another report of obstruction of a heat and moisture exchange filter: (Canadian Journal of Anesthesia 2002 49), 1001

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