A Case of Intraoperative Awareness during General Anesthesia in a Patient Taking a Large Dose of Morphine

We experienced a case of intraoperative awareness during general anesthesia in a patient taking a large dose of morphine. A 31-year-old woman underwent general anesthesia for femoral head replacement. She was taking oral morphine sulfate (MS contin^™) 960mg daily for control of cancer pain from a huge sacral chondrosarcoma. She took her regular dose of morphine 3.5 hours before arriving at the operating room, but no sedative drugs preoperatively. Anesthesia was induced with intravenous administration of fentanyl 100μg, droperidol 2.5mg, and propofol 80mg (1.86mg/kg) followed immediately by inhalation of 60% nitrous oxide and 1.0% isoflurane. After facilitation with vecuronium 7mg, the patient was intubated easily with permissible hemodynamic change. Anesthesia was maintained with 60% nitrous oxide and 0.4-1.0% isoflurane in oxygen. The patient remained hemodynamically stable throughout the case. After completion of surgery, she emerged from anesthesia rapidly. She was extubated and returned to the ward thereafter. Thus, intraoperative anesthetic management seemed uneventful. The next day, however, at the postoperative round, she complained voluntarily that she had been aware of being intubated. She recalled no other events during anesthesia. Her postoperative mental state remained unchanged from the preoperative one, and no psychiatric sequelae developed. After the experience of this case, we gave the same induction protocol to other ten patients (aged 18-43) under processed-EEG monitoring. The data obtained suggested adequate depth of anesthesia during tracheal intubation in all cases. Nevertheless the patient recalled the sensation of tracheal intubation clearly. This means that the depth of anesthesia obtained from our induction protocol was sufficient for normal patients but insufficient for her, that is, the induction dose of propofol 80mg was too small for her. Therefore, we extrapolate the possibility that she had developed cross-tolerance to propofol in consequence to a large dose of morphine intake over a long period. Intraoperative awareness is one of the most serious complications of general anesthesia and occasionally results in emotional injuries or medicolegal problems. In performing general anesthesia to a patient taking a large dose of morphine, it is necessary to manage the patient carefully, keeping in mind the possible development of tolerance to opioids and cross-tolerance to other anesthetics.