Influence of some anesthetic agents on muscle responses to transcranial magnetic cortex stimulation: a pilot study in...

The effects of some commonly used intravenous and inhalational anesthetic agents on the motor evoked responses to transcranial magnetic cortex stimulation were assessed in 17 subjects. Compound motor action potentials (CMAPs) of the abductor digiti minimi muscle were recorded. Baseline values (12 stimulations/subject) were established before anesthesia was induced with a single agent administered in steps up to a maximal dose (MaxDose). Cortical stimulation was performed and depth of anesthesia was assessed according to Guedel after each dose. A common feature was the marked intra- and interindividual variability of baseline values of CMAPs in those patients not premedicated, those premedicated, and the anesthetized patients. The average amplitude of CMAPs was related to the depth of anesthesia in a given subject, whereas onset latencies of CMAPs did not systematically change. CMAPs were markedly reduced or abolished after administration of potent sedative drugs such as midazolam (MaxDose, 0.4 mg/kg body weight), pentothal (MaxDose, 8 mg/kg), propofol (MaxDose, 2 mg/kg), and isoflurane (MaxDose, 1.9 and 3.7 vol %), as soon as patients reached Stage II and Stage III anesthesia. When fentanyl (MaxDose, 8 micrograms/kg) or nitrous oxide (MaxDose, 79%) was used, the subjects reached Stages I and II, but not Stage III. With these drugs, reliable recording of CMAPs was possible even with the maximal administered dose.