To the Editor:
With great interest I read the review by Kim and Nemergut1about the practice of preparation of modern anesthesia workstations for patients susceptible to malignant hyperthermia (MH). Decontamination of anesthesia workstations is a cornerstone in prevention of MH in susceptible patients. Thus, the Malignant Hyperthermia Association of the United States†as well as several review articles2,3have recommended the workstations be cleared before anesthesia by flushing the machine with 100% oxygen with a flow rate of at least 10 l/min for 20 min, replacement of the fresh gas outlet hose, the anesthetic circuit, and the carbon dioxide absorbent. Furthermore, vaporizers should be removed to avoid unintended contamination of the machine.
Recent reports suggest that these procedures may be not sufficient for purging modern anesthesia workstations4–8because it has been shown that reducing residual gas contents within the machines to concentrations less than 5 ppm requires significantly longer time than in older machines. In their review Kim and Nemergut1therefore recommend on the one hand modification of current guidelines and on the other hand a comprehensive study of all anesthesia machines in current use.
However, main problem of all studies concerning patient safety in MH is the lack of a definition for a clear-cut threshold concentration of a trigger agent. Or in other words: how much of a volatile anesthetic is required to trigger a MH crisis? Or: what waste concentration within the anesthesia machine can be assumed to be safe? Unfortunately, no investigations are available to answer these relevant questions. Although those cases might be rare, some susceptible patients have received anesthesia using volatile anesthetics without any pathologic reaction, and later suffer from a severe or fatal crisis. For example, in one case a 41-yr-old man died during his 13th general anesthesia without having any problems during the previous procedures.9Thus, it can be speculated that different concentrations of a trigger agent can induce MH in individual patients. However, the reason for this is unclear.
Furthermore, different anesthetics might have different trigger potencies for MH. Studies in MH-susceptible swine showed that halothane caused MH symptoms after a much shorter time than desflurane or sevoflurane.10Thus, it is tempting to speculate that different waste concentrations of volatile anesthetics have to be defined depending on their trigger potency, and a “one size fits all” concentration of 5 ppm is not acceptable.
For humans no information regarding this issue is available. There are only in vitro investigations at isolated muscle preparations. These studies showed that enflurane, isoflurane, desflurane, and halothane exhibit different effects on skeletal muscle bundles.11The relevance of these studies for the clinical setting is uncertain. Furthermore, different mutations in the ryanodine receptor gene are associated with different phenotypes in the in vitro contracture test setting.12Thus, it might be speculated that acceptable threshold concentrations of volatile anesthetics depend on specific MH causative mutations. However, it is unclear how this problem should be adequately addressed in modification of current guidelines.
To date there are no reports available presenting cases of MH in susceptible patients who received anesthesia with use of a workstation that was decontaminated according to the current guidelines. Thus, one might assume on the one hand that the current guidelines enable a very high degree of safety also in modern anesthesia workstations. On the other hand, because MH is a rare entity there might be a high possibility that MH will occur in the future because of insufficient washout procedures.
Therefore, 100% patient safety may only be established using a completely fresh prepared workstation including a change of all parts that were in contact with volatile gases. After such preparation measurement of gas waste is mandatory to ensure a concentration of 0 ppm. This concept might be advantageous in emergency situations when a long preparation time is not possible. However, this requires an additional anesthesia workstation at the department, which has been realized in our institution especially when a defect machine is in need of a substitute.
In conclusion, I agree with Kim and Nemergut1that Malignant Hyperthermia Association of the United States as well as the European Malignant Hyperthermia Group should discuss the aforementioned problem and make recommendations on how to prepare modern anesthesia workstations to ensure safety in MH-susceptible patients. Until that time, only fresh machines should be used.
University Witten/Herdecke, Clinics of Cologne, Cologne, Germany. firstname.lastname@example.org