Malignant hyperthermia-susceptible (MHS) patients must not be exposed to triggering agents when undergoing anesthesia. Part of the strategy for reducing exposure is to prepare the anesthesia machine to prevent the delivery of residual volatile anesthetics. Whereas traditionally the anesthesia machine could be prepared via a 20-minute flush of oxygen at a fresh gas flow of 10 L.min-1, new anesthesia workstation designs require substantially longer flush to eliminate remaining anesthetic vapor from the machine.
We are not certain of the concentration of volatile anesthetic that is safe after flushing. The accepted standard (without supporting evidence) is a sevoflurane concentration of 5 ppm. This standard will be used for the purposes of this discussion.
What is the best way to prepare an anesthesia workstation before anesthetizing an MHS patient? How can one know that the workstation is indeed safe to use? These key questions are addressed by Dr. Delphine Kern, Department of Anesthesia and Intensive Care, University Hospital of Toulouse, Toulouse, IFR, France, and coauthors in their article titled “The Sevoflurane Washout Profile of Recent Anesthesia Workstations for Malignant Hyperthermia-Sensitive Adults and Infants: A Bench Test Study,” published in this month’s edition of Anesthesia & Analgesia.
In their study, the authors add to the growing body of evidence indicating that the flushing procedure to achieve a concentration < 5 ppm varies significantly between anesthesia workstations. They contribute new insight by simulating pediatric ventilation and showing that concentration can rebound when fresh gas flow is reduced, and that the rebound may be more significant in pediatric patients compared to adult patients.
In their accompanying editorial titled “How Will We Ever Know if Our Machine is Adequately Flushed?” Drs. Timothy W. Martin, Department of Anesthesiology, University of Arkansas for Medical Sciences/Arkansas Children’s Hospital, Little Rock, Arkansas, and Frank E. Block, Jr., Department of Anesthesiology, Vanderbilt University, Nashville, Tennessee, recognize that the exact preparation and flush specification of each and every workstation under all conditions are simply not known, and even if they were, clinicians would be unlikely to remember the requirements of each and every machine. The authors suggest that “MHAUS [Malignant Hyperthermia Association of the United States] recommendations (alternatives 2-4 at the MHAUS website) should be preferred.” The suggested alternatives include maintaining a “clean” anesthesia machine for use only with MHS patients, using an ICU-type ventilator, and placing commercially available charcoal filters in the circuit.
Of all of the alternatives, charcoal filters have been shown to be a relatively low cost, highly effective method for rapidly preparing the anesthesia workstation for an MHS patient. Simply changing the disposable aspects of the workstation (circuit, reservoir bag, CO2 absorbent) and adding charcoal filters eliminates the delivery of residual anesthetic vapor to the patient in 2 minutes or less. This alternative is therefore reliable and avoids the complexity and time needed for an adequate flushing protocol.
The charcoal filter solution has been criticized for different reasons. Most notably, the absorptive capacity is limited. Once saturated, the filters will allow vapor into the circuit. There are two scenarios to consider when assessing the charcoal filter solution: 1) preparation of the machine for an MHS patient, and 2) elimination of vapor delivery when an MH reaction occurs. When using the filters to prepare the anesthesia machine for an MHS patient, it is unlikely the filters will become saturated since very little anesthetic exists in the circuit when the filters are applied. In a previous study of this scenario, vapor was eliminated from the circuit for as long as 90 minutes and only reappeared when the filters were removed. The manufacturer, Dynasthetics, indicates that the filters are effective for 12 hours in this scenario.
If an MH reaction occurs, there is typically a significant amount of vapor in the circuit and the patient continues to exhale anesthetic. The charcoal filters can still be used in this case, but they are more likely to become saturated and the manufacturer recommends replacing the filters after 90 minutes to ensure continued protection.
It is clear that flushing the modern anesthesia workstations to prepare for the MHS patient requires significant time and proper flushing procedures have yet to be defined. Since gas analyzers with a resolution of 5 ppm are not typically available at the bedside, clinicians may continue to remain uncertain that the workstation is adequately prepared to protect the patient. Charcoal filters represent a reliable method for preparing the machine that is well studied and grounded in the principles of chemistry. We could spend a great deal of time and effort developing and following flushing protocols and still worry that the patient may be at risk or, better yet, we can place the filters in the circuit and in just a few minutes know that the patient is protected.