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Correspondence  |   January 2003
Bispectral Index and Mitochondrial Myopathies
Author Affiliations & Notes
  • Philip G. Morgan, M.D.
    *
  • *Departments of Anesthesiology and Genetics, University Hospitals and Case Western Reserve University, Cleveland, Ohio.
Article Information
Correspondence
Correspondence   |   January 2003
Bispectral Index and Mitochondrial Myopathies
Anesthesiology 1 2003, Vol.98, 283. doi:0000542-200301000-00052
Anesthesiology 1 2003, Vol.98, 283. doi:0000542-200301000-00052
In Reply:—
We appreciate the letter from Dr. Allen and share his concerns for the care of children. It is important to remember that most of these patients were only suspected to have mitochondrial disease and were presenting for diagnostic studies. One of us (P. G. M.) had noticed that some of the children seemed abnormally sensitive to anesthetics and, thus, had started using slow inductions and Bispectral Index® monitoring (BIS®; Aspect Medical Systems, Inc., Newton, MA) as prudent clinical care. Only in retrospect, after diagnostic muscle biopsies were performed, did we note that the apparent increased sensitivity was found in some patients with abnormal mitochondrial function.
Dr. Allen raises two separate points that we will address. The first point questions the validity of the BIS measurement as an end point for central nervous system function in the abnormal brain. The problem is, of course, that any anesthetic end point is questionable when central nervous system function is abnormal. The use of minimum alveolar concentration (MAC), or any of the derivatives of MAC, such as MACawake, is also likely to be a debatable measure of anesthetic concentration. Our point was not that BIS® definitely indicated anesthetic concentration; rather, we noted only that there were differences among patients in responses to sevoflurane when using this monitor. We did not suggest that this measurement necessarily correlated with the MAC in these patients. This is especially true since we did not attempt to reach a true steady state anesthetic concentration.
Having said this, one is left with the desire not to be entirely nihilistic. Clinically, we feel that patients with mitochondrial myopathies are at increased risk from anesthetic exposure. What, then, is a useful end point to guide our care for these patients? In each of our patients, we measured the BIS value with the patient awake and obtained a value of 96–100. Thus, none of the patients started with an extremely low value. In addition, none of these patients clinically appeared somnolent preoperatively. In the absence of a gold standard to guide our anesthetic delivery, it seems prudent to use all of the information we can gather. With our patients, we used all of the usual data (heart rate, blood pressure, arousal, breathing patterns) and added the BIS to help us determine the anesthetic concentration. In each of the patients who exhibited abnormal decreases in BIS at low concentrations of anesthetic, we also noted that the other parameters indicated that they were “asleep.” Since the BIS® readings are objective, we reported the differences between patients with this parameter. We stand by our report that such differences do exist; the interpretation of their implications awaits prospective studies.
This brings us to the second point that Dr. Allen raises. He states that our approach does not reflect “normal care” and represents a research protocol. Most anesthesiologists have cared for unstable or elderly patients in whom a heightened sensitivity to induction agents was suspected. It is common to “go slowly” with the induction agent in such patients in order to gauge their response. Does this represent a departure from “normal care?” In the case of these patients, we merely went slowly so that we could gauge their response. Our department normally uses preoperative sedation in less than half of our pediatric patients, so this omission does not represent a deviation from normal care and was part of our attempt to induce anesthesia slowly, with minimum drug exposure. P. G. M. began noting the relation of the BIS and sevoflurane induction after observing unusual responses in a subset of these patients. We should point out that during this time, some children with mitochondrial myopathies were agitated preoperatively, required sedation, or did not seem appropriate for such a slow induction. They were not considered in this report since the BIS measurements were not obtained under similar conditions. In each case, however, we carefully told the parents what our anesthetic approach would be and described the reasons behind it. Thus, informed parental consent was obtained regarding the administration of the anesthetic technique, as it is in all pediatric cases handled at our institution. However, we did not intend this to be a study, and we did not set it up as one. The data were gathered through a chart review. Institutional Review Board approval was obtained for the chart review, as noted in the report.