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 ...