design and ergonomics of anesthesia machines
Malignant hyperthermia (MH) is “the disease of anesthesia.” We are the only specialty that until recently needed to know anything about it. That is changing with time as more episodes of “awake triggering” not related to anesthesia are being reported. Nevertheless, MH is a condition that the overwhelming majority of us will never see, but we need to be able to recognize and treat it immediately.
In this chapter, we wish not to discuss the diagnosis and treatment of MH as such. You will find that covered thoroughly in the major anesthesia textbooks. What we want to discuss is how to prepare an anesthesia machine preoperatively for a patient who is at risk for MH and what to do with an anesthesia machine during an anesthetic in which a patient develops MH. So, what do we need to do to our machine in such a circumstance?
Anesthesia machines were much simpler in the past. And because they were simpler, it was much easier to make them safe to use on an MH-susceptible patient. All you needed to do was to flush the machine with high fresh gas flow (FGF), usually by running the bellows ventilator at a high tidal volume and FGF, with a reservoir bag on the circuit acting as “lungs.” After doing this for 20 to 30 minutes, you changed the circuit and the carbon dioxide absorbent granules, took off the vaporizers (or emptied them and taped the dials closed), and proceeded. Maybe you also unscrewed the clear bellows dome and put a new bellows piece on as well. Studies showed that halogenated anesthetic agent concentrations were very low, less than 5 ppm, which was considered acceptable.
But, of course, anesthesia machines are more complicated now. More parts and pieces of tubing are made of plastic, which can absorb and slowly release anesthetic agent. More complicated gas flow pathways can mean that there are nooks and crannies that are not blown out well during the high FGF flushing process even after a 30-minute flush. So then, after induction of anesthesia, when the FGF is decreased to a level you would use for a case, these unflushed areas will potentially add more molecules of inhalational agent to the FGF in what you thought was a clean machine, therefore increasing the patient’s exposure to triggers. This has been called a “rebound effect,” when after a long flushing, the concentration of inhalational agent increases because of the release of agent from plastic components and from those areas of the internal circuit that were poorly flushed.
In previous times, the best, safest, and most efficient way to have a machine ready for a nontriggering anesthetic was to have one anesthesia machine that was “dedicated” as your MH machine. So a brand new machine or one that had been thoroughly flushed and prepared was only used for MH-susceptible patients. This machine would not even have vaporizers installed on it. As long as nobody mistakenly used this machine to deliver inhalational agents, everything was fine.
Now, however, the easiest way to prepare a machine for an MH patient is the use of Vapor-Clean filters (Dynasthetics, Salt Lake City, UT) (Figure 14-1). These single-patient-use activated charcoal filters, one on each limb of the circle circuit, reduce agent concentrations to 5 ppm or less after a 90-second flush, according to the manufacturer. They can be used after a diagnosis of MH is made during a case as well, rapidly decreasing the agent concentration after the agent is discontinued. In a situation of a patient who is exhaling agent, the manufacturer recommends changing the filters after one hour of use. Otherwise, in a situation when no agent had been used, the filters are recommended to be changed after 12 hours of use. The cost of a two-filter kit is in the $70 to $100 range. For a small department or hospital, this is much more cost-efficient than having a dedicated MH machine. The Malignant Hyperthermia Association of the United States (MHAUS) has said that Vapor-Clean filters may be used as an alternative or as an addition to their recommended machine preparation guidelines.
Figure 14-1 Set of Vapor-Clean filters (Dynasthetics, Salt Lake City UT). Each kit has two filters, one for the inspiratory and one for the expiratory limbs of anesthesia circuit.