Standard anesthesia practice includes a detailed preanesthetic check and preparation of the anesthesia workstation (the “anesthesia machine”) before the first patient of the day and an abbreviated check before subsequent patients, according to guidelines established by the American Society of Anesthesiologists,a United States Food and Drug Administration,b and other professional societies.1 This “machine check” serves to verify that all components of the anesthesia workstation (including the patient breathing circuit) are present and functional, and that any leaks within the high- or low-pressure portions of the system are within acceptable limits, with the ultimate goal of assuring that the workstation is safe for patient use.
In the case of patients with known or suspected malignant hyperthermia susceptibility (MHS), and perhaps more commonly in some patients with apparent neuromuscular dysfunction of uncertain etiology but in whom MHS is a possibility, such as unexplained hypotonia, there are additional considerations in the preparation of the anesthesia workstation.2,3 If the decision is made to provide the patient a “nontriggering” anesthetic that does not expose the patient to any of the potent inhalational anesthetics (or succinylcholine), there are several alternatives as indicated by the Malignant Hyperthermia Association of the United States (MHAUS).c The first of these alternatives directs the anesthesia provider to “flush and prepare workstation according to manufacturer’s recommendations or published studies.”
In this issue of Anesthesia & Analgesia, Cottron et al.4 report an in vitro bench test evaluation of the sevoflurane washout profiles of 7 different modern anesthesia workstations, including 4 that have not been previously tested. Following a standard priming procedure with 3% sevoflurane that was applied to all 7 of the workstations (subject to some minor device-specific limitations), the workstations were then prepared with preoperative “flushing” according to the current recommendations of the MHAUS on its website. The flush procedure was standardized with a test lung and controlled mechanical ventilation until a sevoflurane concentration of <5 ppm was obtained at the “Y-piece” of the breathing circuit. Following this flush period, the simulation of 2 clinical scenarios was undertaken. In the first, the fresh gas flow was decreased to 10 L/min, while minute ventilation of the test lung was maintained the same as during the flush period. In the second, the minute ventilation was substantially reduced to simulate the mechanical ventilation of an infant. In both scenarios, sevoflurane concentration was measured to detect any “rebound” increase in breathing circuit concentration after these changes and the amount of time required for the sevoflurane concentration to again fall below 5 ppm. In all but 2 of the workstations that were studied, the sevoflurane concentration increased when the fresh gas flow was decreased after the initial flush period. In all but one of the workstations, the sevoflurane concentration increased during the simulated mechanical ventilation of an infant. This report is novel in that not only have 4 of the machines not been previously tested, but it is also the first to simulate mechanical ventilation of small patients in whom the anesthesia workstation and breathing circuit gas flow characteristics, and therefore, potent inhaled anesthetic agent elimination, may be different.
So what is one to make of the increasing evidence that the traditional practice of providing a 20-minute flush of the anesthesia machine at a fresh gas flow rate of at least 10 L/min after removing the carbon dioxide absorbent and vaporizers is insufficient, perhaps markedly so, in preparing a machine that is “safe” for the MHS patient? How should one respond to the apparent wide variation in inhaled anesthetic agent elimination times among different models of modern anesthesia workstations? Indeed, it is possible and perhaps even likely that there would be variation in the elimination time for inhalation agents among different machines of the same model, as shown by the conflicting results in one of the workstations studied by Cottron et al.4
One can never accurately predict the agent absorption characteristics of a specific anesthesia apparatus. The absorption depends on the breathing circuit (make, model, and length), use or nonuse of circuit filters, use or nonuse of a heat and moisture exchanger, use of various types and packaging of carbon dioxide absorbents, as well as the specific anesthesia machine under consideration. It is possible that not every machine of the same make and model is truly identical anyway, as there may have been a change in supplier or formulation of some of the internal rubbers or plastics over time. The same make and model may appear to be sold in the United States, Europe, and other parts of the world; when in reality, the machines may have different components and properties. It is also likely that the absorption of inhalational agents is dependent on the age of the plastics and rubbers in the machines, which may in turn influence the amount of water contained in those components. For these reasons, one will not be able to model with 100% reliability the specific inhaled anesthetic absorption or elimination profile for a specific anesthesia machine with a specific breathing circuit and carbon dioxide absorbent, and even if it could be reliably modeled, clinicians would unlikely be able to remember the preparation and flush specifications for each and every machine.
For these reasons, the other alternative MHAUS recommendations (alternatives 2 to 4 at the MHAUS website) should be preferred. The alternative that provides the use of open-circuit ventilator, intensive care unit-style or transport-style, that has never been exposed to volatile anesthetic agents is best if one is interested in 100% reliability. For cost reasons, most hospitals in the current medicoeconomic environment cannot likely afford a dedicated “vapor-free” or “clean malignant hyperthermia machine” that has never been exposed to volatile anesthetic agents, and if they did, it may be an older, less, familiar machine that anesthesia providers would have difficulty using in an actual emergency. How could the anesthesiologist be absolutely certain that a supposedly vapor-free anesthesia machine had never been used with volatile agents in an emergency or to replace another faulty anesthesia machine as a backup? The use of commercially available charcoal filters may be particularly useful when malignant hyperthermia has been triggered in a patient and the concentration of residual volatile anesthetic agent must be rapidly reduced to below trace levels.5 Nevertheless, depending on activated charcoal filters for anesthesia workstation preparation before anesthesia for a MHS patient has been questioned.6
If the anesthesia provider opts for the first of the 4 MHAUS alternatives (workstation flush and preparation) that may be the most commonly applied choice in clinical practice, it is clear that the various makes and models of modern anesthesia machines do vary significantly in their volatile anesthetic elimination characteristics and potential for rebound increases in the concentration of the anesthetic agent within the breathing circuit. The easily recalled and longstanding anesthesia machine flush mantra of “high fresh gas flow of at least 10 L/min for at least 20 minutes” (in addition to other preparation steps) is no longer adequate or dependable for many modern anesthesia workstations. Because the specific manufacturers’ recommendations (if any) and machine preparation implications of published studies (when available for a specific anesthesia workstation) are not likely to be accurately recalled by individual anesthesia providers, accepted recommendations for the flush and preparation of each workstation should be readily available at the bedside in the form of various “cognitive aids” such as paper checklists or electronic lists.7 The report by Cottron et al.4 also demonstrates that the introduction of typical controlled ventilation settings for infants (relatively small tidal volume and rapid ventilator rate), following a seemingly adequate anesthesia machine flush and preparation protocol, may result in an unexpected rebound of the concentration of volatile anesthetic within the breathing circuit, indicating the need for an extended preuse flush period and continued maintenance of high fresh gas flows throughout the administration of anesthesia.
In conclusion, then, the best approach would be to avoid the use of a lower flow rate even after 20 minutes of flushing at 10 L/min, but rather to continue at least the 10 L/min fresh gas flow rate, or to opt for one of the other 3 alternatives now recommended by MHAUS. If the anesthesiologist chooses to use an anesthesia workstation that has been exposed to volatile agent, it is imperative that the machine be prepared according to the best currently available information for that specific machine.
Name: Timothy W. Martin, MD, MBA.
Contribution: This author helped write the manuscript.
Attestation: Timothy W. Martin approved the final manuscript.
Name: Frank E. Block Jr, MD.
Contribution: This author helped write the manuscript.
Attestation: Frank E. Block Jr, approved the final manuscript.
This manuscript was handled by: Maxime Cannesson, MD, PhD.
a Anesthesia Apparatus Checkout Recommendations, 1993. Available at http://www.apsf.org/newsletter/html/1994/fall (Article #8). Accessed January 25, 2014. Cited Here...
b ASA Recommendations for preanesthesia check-out procedures (2008). Available at http://www.asahq.org/For-Members/Clinical-Information/2008-ASA-Recommendations-for-Preanestehsia-Checkout.aspx. Accessed January 25, 2014. Cited Here...
c Preparation of anesthesia workstations to anesthetize MH susceptible patients—MHAUS. Available at: http://www.mhaus.org/healthcare-professionals/mhaus-recommendations/anesthesia-workstation-preparation. Accessed January 25, 2014. Cited Here...
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