We would like to thank Drs. Feldman and Wappler for their thoughtful comments regarding our manuscript published recently in Anesthesiology.1The use of carbon filters suggested by Dr. Feldman and previous investigators presents an approach to quickly prepare an anesthesia workstation for a malignant hyperthermia (MH)-susceptible patient and facilitate the reduction of anesthetic gases during an MH crisis.2–4We agree carbon filter devices have proven to be effective in reducing the anesthetic gas concentration. However, the Food and Drug Administration has never approved the use of carbon filters specifically for the purposes of reducing or eliminating the risk of anesthetic gas exposure for MH-susceptible patients. The Vapor-Clean filters (Dynasthetics, LLC, Salt Lake City, Utah) are approved by the Food and Drug Administration with a 510(k) clearance as a general-purpose device meant to scavenge volatile anesthetic gases. Therefore, we are hesitant to make a recommendation on this device without clinical studies in at least patients who are not MH susceptible, with the focus directed specifically to validating the filter's ability to reduce and maintain the residual anesthetic gas concentration at or less than the presumed safe concentration of ≤5 ppm. In addition, we reiterate that making such a recommendation is outside the purview of our review and instead lies with organizations such as the American Society of Anesthesiologists, the Anesthesia Patient Safety Foundation, European Malignant Hyperthermia Group, or the Malignant Hyperthermia Association of the United States (MHAUS).
As mentioned by Dr. Wappler, having a “clean” anesthesia machine in reserve for the sole purpose of caring for MH-susceptible patients was a consideration we shared. However, the additional costs involved in acquiring and maintaining an “extra” anesthesia machine and the time required to exchange the machine were concerning.5,6In fact, MHAUS specifically did not recommend exchanging anesthesia machines or parts of the patient breathing system during a crisis because to do so lacked any benefit and acted as a distraction from patient care. Furthermore, one must question the reliability of the anesthesia machine involved in such a practice. As an example, a colleague had the unfortunate experience of having a machine malfunction between anesthesia cases. He quickly found another anesthesia machine that was infrequently used for patient care but was inspected daily. However, he quickly found that parts had been scavenged and the oxygen sensor needed to be replaced. This situation would be unacceptable for an MH-susceptible patient or a patient having an MH crisis. Therefore, although maintaining a clean anesthesia machine in theory represents an overt means to achieve “100% patient safety,” in practice this method may pose other serious risks.
Lastly, we agree the maximum safe level of exposure to anesthetic gases is unknown. The often-cited cutoff of ≤5 ppm is derived from experiments in MH-susceptible swine.6Unfortunately, the substantial morbidity and mortality associated with MH restricts its study in humans. Therefore, the true safe limit of exposure may never be known. The variability of genetic mutations, incomplete penetrance of these alleles, and the relative potencies of different volatile anesthetic agents to trigger an MH crisis, while serving as a reminder of the heterogeneity of MH, further complicate this issue and underscore the difficulties in both studying MH and recommending changes in general policy. Indeed, the maximum safe level of exposure likely will vary with different RyR1 alleles, and these differences may be further modified by the context unique to each patient's genome as well as the triggering anesthetic agent.
In summary, we would like to express our appreciation to Drs. Feldman and Wappler for sharing our interest in MH. The reduced penetrance and variable expressivity of this inherited disorder and lack of a scientifically defined value in humans for the maximum safe limit of exposure to residual anesthetic gases suggests zero tolerance for any exposure to volatile anesthetic agent would be the best practice. However, as noted by Dr. Wappler, there are no reported cases of MH in susceptible patients who were ventilated by anesthesia machines that were decontaminated according to current MHAUS guidelines. Our review of anesthesia machines was limited to published studies and thus did not include all available anesthesia machines. It was evident from our review that MHAUS recommendations need to be updated to include the newer generation of anesthesia machines.*Finally, the lack of published data or clinical experience with carbon filters and malignant hyperthermia caution the use of such devices for MH-susceptible patients as an alternative to machine preparation. We are conducting research to define the best method for standardizing the preparation of anesthesia machines and the role of carbon filters in reducing the risk of an MH crisis.
†The Johns Hopkins University School of Medicine, The Johns Hopkins Hospital, Baltimore, Maryland. firstname.lastname@example.org