Rapid Response: Your Questions

(formerly Dear SIRS)

Can Soda Lime Canisters Spread MRSA?

Dear Sirs

SIRS


Michael Olympio, MD,
Chair of the APSF Committee on Technology and Co-Founder of the Dear SIRS Initiative.


SIRS

Can Soda Lime Canisters Spread MRSA?

Dear SIRS:

A question has come up that no one seems to be able to answer. It pertains to the possible spread of methicillin-resistant Staphylococcus aureus (MRSA) in the operating room. Should soda lime canisters be changed after use on a MRSA patient? Under certain circumstances? I could not find any literature to answer this and thought you might know.

Thank you.
Laura Stokowski, RN, MS

In Response:

If we change the absorbent, are we also obligated to sterilize the entire breathing apparatus, as the pathways both to and from the absorbent may well be colonized? I asked our circuit filter manufacturing representative (see Martino and Anderson, below) to comment on the effectiveness of the bacterial filters to remove MRSA. We use a double-filter type of circuit, at both the inspiratory and expiratory machine ports, but I am not sure if that is adequate either. This discussion would not be complete without input from infectious disease experts (the CDC), particularly with comment on how few organisms are considered “safe enough.”

Sincerely,
Dr. Michael A. Olympio
Chair, Committee on Technology

In Response:

I do not think that there is any so-called written document stating that the absorbent needs to be changed. However, I do not think that it is a bad idea, especially if you consider the potential for MRSA transmission to another patient. The cost of MRSA versus a bag of sodasorb is self-explanatory. At our hospital, we make it part of the clean-up of the OR suite to change the sodasorb.

Sincerely,
Jonnalee Bill
Immediate Past President
American Society for Anesthesia Technicians and Technologists

In Response:

The environment within the absorbent material is not constant and not uniform. The extent of moisture, the internal temperature, and the pH are all influenced by the relationship between the fresh gas flow, the minute ventilation, and even the I:E ratio. These will alter the probability that organisms may thrive within the absorbent material. I have spent considerable time exploring these variants over the past several years and am amazed at the way these variables alter the environment of the absorber and the absorbent.

While an academic discussion is always exciting, especially to me, Jonnalee makes not only a compelling comment, but also one that is economically sound. Change the absorbent. When in doubt, change the absorbent. This is a good idea for other issues as well. Is the absorbent desiccated? Change the absorbent. How old is the absorbent? Change the absorbent. Is the absorbent exhausted, just “re-colored?” Change the absorbent. It’s cheap. It is readily available. It eliminates the problem.

Sincerely,
Michael Mitton
GE Healthcare (Formerly Datex-Ohmeda)

In Response:

I am no expert in this field; however, I would question the value of changing the absorbent (pH >14) unless the facility also sterilizes the complete breathing system, including all parts coming into contact with patient gas. It seems the soda lime is the least of your worries.

Sincerely, Robert Clark
Dräger Medical, Inc.

In Response:

Organisms have been cultured from the machine breathing circuits, but I’m not sure as to which “side” of the absorber those samples were taken from. Surely, very few if any would find it easy to sterilize the entire breathing circuit for every MRSA patient, as we seem to have many in a single day.

Dr. Olympio

In Response:

While MRSA is not the driving influence, Europeans have demanded anesthesia machines with a breathing system that is autoclavable for some number of years. This is why our newest machines all include systems that can be sterilized. Some Europeans actually do sterilize the breathing system regularly and often, though I don’t know of any who do it for every case. I guess that begs the question, “When should it be sterilized?” Do we always know which patients have “infected” the system? Who is clean and who is dirty? One other point, filters. What does filtering actually mean? I attended an Association for Low Flow Anaesthesia meeting in Bristol, UK, last year where this topic was discussed. To my surprise, filtering does not mean removal of the contaminant, only elimination of SOME contaminant.

Michael Mitton

In Response:

Another comment on MRSA and anesthesia: I read about studies that found that doctors’ ties are the most dominating source of cross-contamination in a hospital. While I believe such is unlikely for the OR, every reasonable action to avoid the risk should be undertaken. Bacterial filters are certainly helpful, but not perfect. Regardless of brand and construction, they just have statistical retaining specifications. I’m not sure whether 99% or 99.9% or 99.99% makes a big difference as a few single bacteria out of the millions or billions in a cluster can form a new colony. With a better filter it just takes longer. Doesn’t MRSA itself result from a single bacterium’s DNA mutation which grows a new stem? As far as the airway is concerned, Wikipedia tells us that the nose is a common home of Staphylococcus aureus, although most of us with an intact immune system don’t suffer from the bacteria. So the airway must be considered. In summary, I would agree: Change the absorbent, and change the filters and tubing wherever possible to protect subsequent patients.

Sincerely,
S. Kästle
Philips

In Response:

Dr. Olympio writes the following to Vital Signs, Inc.:

There is a question circulating through the APSF and that is whether or not the absorbent should be changed for methicillin-resistant Staphylococcus aureus “MRSA” patients. What are the specs on our circuit filters for MRSA filtration? Obviously I’m wondering if those 2 filters keep MRSA out of the ventilator breathing circuit.

I am not an expert but have a good understanding of filters and filtering mechanisms and a little microbiology knowledge. I will confirm with some “expert” colleagues, but feel confident in what I am sharing with you. The BFE or VFE (bacterial/viral filter efficiency) of our filters are dependent on several factors, but the resistance of the organism strain is not one of them; it has more to do with its morphology. Our published BFE is in fact developed by challenging the filters with a high concentration of Staphylococcus aureus (SA). It is a challenge organism widely used and referenced in various filter standards like ASTM F2101 and the Mil STD for surgical masks. It was chosen in part because of its relevancy in the clinical setting, its shape (spherical), and its ability to be cultured. MRSA, simply put, is a Staphylococcus aureus morphologically, but is a “strain” that has developed a resistance to, specifically, the antibiotic methicillin. Based on my review, we are confident that our filter would be as efficient as we currently report, if challenged with the same protocol using a MRSA strain in place of the laboratory cultured version of Staphylococcus aureus ATCC#6538. I hope I have helped.

Tony Martino
Vital Signs, Inc.

Vital Signs Filter Efficiency Table 1: Vital Signs Filter Efficiency

In Response:

This is very helpful information, to know that your reported specs for filtration do indeed refer to things like SA, in particular. I will ask Jeff Anderson to mail or send me the brochure on your company’s various filters, with their specs. Does you
r company have any official opinion as to whether the machine absorbent needs to be changed if subjected to a MRSA patient, if indeed someone were using your filters properly?

Dr. Olympio

In Response:

What is the standard for the filters we use on our circuits? Why would MRSA be treated any differently than other patients with other bacterial infections?

Erv Moss, MD

In Response:

BFE lists the bacterial filter efficiency; VFE lists the viral filter efficiency. With regard to what is being used in your institution, the #5708 is used on the adult circuit; the #303 is used on the pediatric circuits. This may be more information than you need, but VSI and many other manufacturers use Nelson Laboratories to do third-party bacterial and viral challenge testing of anesthesia /respiratory filters. Nelson Laboratories protocol uses Staphylococcus aureus (SA) for the bacterial challenge and a bacteriophage Phi-X174 for the viral challenge. SA is 0.5-1.0 microns and spherical in shape. The Phi-X174 is 0.025-0.027 microns and spherical in shape.

We have discussed this in the past, but when addressing tuberculosis. The Centers for Disease Control (CDC) December 30,2005 / 54(RR17);1-141 Guidelines for Preventing the Transmission of Mycobacterium tuberculosis in Health-Care Settings, 2005 notes that M. tuberculosis is carried in airborne particles called droplet nuclei, and that the particles are approximately 1-5 microns in size. On page 21 of this report it lists procedures for the

Surgical Suites. Here it recommends using filters that filter particles 3 microns in size with an efficiency of greater than or equal to 95%. Both the #5708 and the #303 exceed this recommended efficiency. According to “Bergey’s Manual of Determinative Microbiology, ” M. tuberculosis is a bacteria, rod-shaped, and is sized at 0.3-0.6 x 1.0-4.0 microns. Table 1 above shows Vital Signs’ filter efficiency for bacterial and viruses.

Sincerely,
Jeff Anderson
Vital Signs, Inc.

In Response:

Vital Signs has no scientific data, but I always have an opinion. I have not conferred and therefore this is not a company opinion. I do not know of any studies done by filter companies or machine companies that would answer scientifically about changing the absorbents. It is a question you may want to ask of a machine company, although I suspect they will say you should change out if you have a known “infected” person involved. A common sense approach leads me to say the absorber and absorbent would not need to be changed just because a patient had MRSA. Let’s walk it through. Start with a fresh canister and a circuit with filters on the inspiratory and expiratory machine end. The patient potential MRSA source should not make it to the absorber, but since a filter may not be 100% efficient, one could argue there is the potential that a “bug” could get through, but anything that “makes” it to the absorber needs to pass through another filter to reach the next patient.

I would think that the absorbent material (i.e., soda lime) may have some “antimicrobial” properties, but on the other hand may support bacterial growth as the absorbent may be wet and warm, which is usually a good environment for the little devils. Again, I would say the likelihood or probability is very low to nonexistent, but this has not been studied or documented in the literature I’ve searched through. I’ll do a little more searching and see if there are any infection control protocols for absorbents/absorbers . . . .

Here is a study (Langevin, et al., 1999) that relates. It does look like this topic has been an area of interest. I believe this article is on the same line as I was going. It certainly showed that under bench testing, if you do nothing to protect the machine, you could move the resistant organisms through the system. I would have thought the “soda lime” would have contributed more, but the answer lies in preventing the bugs from getting down the expiratory limb and “into” the machine, with redundancy at the inspiratory side. It appears that the use of filters on both limbs would support the exchange of the absorbent based on “exhaustion” rather than “contamination.” (Source: http://www.chestjournal.org/cgi/content/full/115/4/1107.)

Tony Martino

Editor’s Note:

The article referenced by Mr. Martino is an important one to read. It is very well referenced and states that, “Despite a half-century of research, the true hostility of the environment within the anesthesia machine and the potential for bacteria to traverse the system remains a matter of debate.” It specifically addresses the issues that we refer to above. Langevin et al. indicate that organisms can indeed pass through the absorbent, but would eventually be killed (presumably by high pH) if left undisturbed for greater than 1 hour within the absorbent. This research (“The potential for dissemination of Mycobacterium tuberculosis through the anesthesia breathing circuit” in Chest 1999;115:1107-1114) also indicates that a 0.22 micron filter “eliminated organisms from the inspired gas flow,” and references other studies demonstrating, “There are now several filters commercially available. . . that can remove 100% of the bacteria, even under considerable bacterial loads.” I recommend that our readers consider the authors’ numerous conclusions and referenced materials in order to draw their own conclusions on this highly debated issue, and to seek more recent literature on the topic.

Dr. Olympio

In Response:

Here is the latest CDC recommendation (2003) as published in MMWR March 26, 2004;Vol. 53: No. RR-3:

8. Anesthesia machines and breathing systems or patient circuits

  1. Do not routinely sterilize or disinfect the internal machinery of anesthesia equipment (IB) (80).
  2. Between uses on different patients, clean reusable components of the breathing system or patient circuit (e.g., tracheal tube or face mask) inspiratory and expiratory breathing tubing, y-piece, reservoir bag, humidifier, and tubing, and then sterilize or subject them to high-level liquid chemical disinfection or pasteurization in accordance with the device manufacturers’ instructions for their reprocessing (IB) (24,26).
  3. No recommendation can be made about the frequency of routinely cleaning and disinfecting unidirectional valves and carbon dioxide absorber chambers (Unresolved issue) (81).
  4. Follow published guidelines or manufacturers’ instructions about in-use maintenance, cleaning, and disinfection or sterilization of other components or attachments of the breathing system or patient circuit of anesthesia equipment (IB) (82,83).
  5. No recommendation can be made for placing a bacterial filter in the breathing system or patient circuit of anesthesia equipment (Unresolved issue) (4,84-89).

The 2003 CDC Guidelines for Preventing Health-Care-Associated Pneumonia is the complete report of the Healthcare Infection Control Practices Advisory Committee. The article by Vezina et al. entitled “Anesthesia breathing circuits protected by the DAR Barrierbac S® breathing filter have a low bacterial contamination rate” (Can J Anaesth 2001;48:748-54) is the most recent reference I can find on this topic. Bottom line—I don’t think we know exactly what to do, and while the risk is low from these ancillary sources, it is not zero, but it is probably higher than some of the risks that anesthesiologists worry about in many of our conferences.

Sincerely,
David L. Bowton, MD, FCCP, FCCM
Professor and Head, Section on Critical Care
Department of Anesthesiology
Wake Forest University School of Medicine


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al advice. Individual or group responses are only commentary, provided for purposes of education or discussion, and are neither statements of advice nor the opinions of the APSF. It is not the intention of the APSF to provide specific medical or legal advice or to endorse any specific views or recommendations in response to the inquiries posted. In no event shall the APSF be responsible or liable, directly or indirectly, for any damage or loss caused or alleged to be caused by or in connection with the reliance on any such information.