A Suspected Malignant Hyperthermia Episode During Desflurane Anesthesia

Abstract

In Response: We thank Drs. Mayhew and Lowes for their comments regarding our report on a case of suspected malignant hyperthermia (MH) during desflurane anesthesia [1]. Their main concern is that the observed hypercapnia is not per se sufficient to suspect MH. There was absolutely no evidence that the hypercarbia described could have been caused by inadequate ventilation. Minute ventilation was adequate (4.5-5 L/min in a boy of 47 kg) as judged by a stable end-tidal CO2 of 29-32 mm Hg during the first 90 min of desflurane anesthesia. The increase in end-tidal CO2 was sudden (from 29 to 85 mm Hg over 10 min), not gradual over 90 min, as cited by Mayhew and Lowes. Also, the occurrence of vigorous inspiratory efforts paralleled the increase in end-tidal CO2. Had the forced respiratory efforts been caused by diminishing neuromuscular blockade, as suggested, we would have expected them to occur earlier. Neuromuscular blockade after an intubating dose of vecuronium lasts approximately 40 min in peripheral muscles. Respiratory muscles, especially the diaphragm, have a significantly faster recovery from neuromuscular blockade [2]. Furthermore, end-tidal CO2 continued to increase, despite hyperventilating the patient by manual assistance (twofold increase in minute ventilation up to 10-12 L/min). Another symptom of MH, an increase in core temperature, did not occur gradually, as might be expected if it were caused by excessive draping, but started parallel with the increase in CO2 (the temperature remained stable at 36.5 +/- 0.2[degree sign]C for 60 min, followed by an increase to 37.8[degree sign]C in the next 35 min). In the case reported, a short procedure was anticipated, which was only prolonged because of a technical fault. Therefore, the patient was not particularly draped and protected to prevent heat loss. Contrary to the statement of Mayhew and Lowes, it is, in our experience, rather difficult to avoid unwanted cooling of patients in the setting of cerebral angiography. Therefore, there is no doubt that the increase in temperature observed in this patient was unusual. We agree with Mayhew and Lowes that the increase in heart rate was not dramatic in our patient. Nonetheless, an increase in heart rate in the absence of surgical stimulation is another criterion for the diagnosis of MH, also included in the clinical grading scale of MH. Thus, the combination of hypercarbia, temperature increase, and tachycardia yields 33 points on the clinical grading scale [3], which accounts for our scoring the likelihood that the observed event was MH as "somewhat greater than likely." Mayhew and Lowes also criticize our management. As mentioned in our report, the boy's trachea was extubated because he fulfilled standard extubation criteria; he was responsive and demonstrated adequate muscle strength. The angiography room in our neurosurgical department is a remote area to provide anesthesia. Because the boy was stable, we believed that it was safe to transfer him to the intensive care unit for further treatment, instead of waiting for the dantrolene to arrive, which would have taken as long as the transfer. A starting dose of dantrolene therapy, according to textbooks, is 2-3 mg/kg. In that respect, the dose of dantrolene administered to our patient was small. The dose, however, was based on the clinical course of the episode and the patient's stable condition. Because the patient did not show any further signs of increased metabolism, we believed that it was safe to discontinue the dantrolene infusion. We do not believe that the consequences of not being tested immediately for MH are as serious as is feared by Mayhew and Lowes. The patient and his parents have been informed about the nature and the implications of MH. The boy has been issued a MH warning card and will, for the time being, be considered as if he were confirmed to be MH-susceptible. He will be treated accordingly until a muscle biopsy is performed. Because of the genetic nature of MH, we decided to test the mother first, according to the dictum mater semper certa est. The patient's father is also scheduled for muscle biopsy, although a negative result will be of no consequence to the boy. In conclusion, had we not suspected MH in this patient, the boy's family, now proven by in vitro contracture test to be MH-susceptible, would have remained undiagnosed. Andrea Michalek-Sauberer, MD Hermann Gilly, PhD Department of Anesthesia and General Intensive Care; University of Vienna; A-1090 Vienna, Austria