[Interference between M-Entropy and Thermistor Probe Fixed on the Forehead during General Anesthesia].

: We report a case in which M-Entropy was affected by the thermistor probe of tissue-core thermometer (Coretemp® TERUMO, Tokyo, Japan) fixed on the forehead. A 26-year-old man underwent osteosynthesis for forearm fracture under general anesthesia com- bined with brachial plexus block. General anesthesia was induced and maintained with fentanyl and target- controlled infusion of propofol. We use Coretemp* to measure core temperature and put the thermistor probe on forehead next to the M-Entropy sensor. Thirty minutes after start of the surgery, state and response entropy rose suddenly to extremely high lev- els. In spite of slow waves and sleep spindles on raw electroencephalogram (EEG) waveform, his vital signs showed no changes after the rise of entropy values. We suspected Coretemp® probe interfering with M- Entropy, and removed the probe from his forehead. M-Entropy returned to the default values immediately and this phenomenon was reproducible by attaching and detaching the probe. We therefore concluded that some electrical noise from the Coretemp® probe affected M-Entropy. The patient awoke from anesthesia smoothly, and had no memory of intraoperative awareness. This case suggests that we should always consider artifacts in EEG monitors and if their values do not correspond with clinical status, we should check raw EEG waveforms to judge the depth of anesthesia.