脳死臓器提供者における予期せぬ脳波活動：症例報告

Brain death (BD) is the complete and irreversible loss of brain function, diagnosed primarily through clinical criteria. Electroencephalography (EEG) is a non-invasive tool commonly used to support the diagnosis by showing electrocerebral inactivity. The bispectral index (BIS), a processed EEG parameter, has been proposed as a potential adjunct for the early detection of neurological deterioration and BD. This case provides new insights into BIS monitoring in a brain-dead patient undergoing organ procurement surgery. A 56-year-old woman underwent organ procurement surgery following confirmed BD by clinical and ancillary criteria. Her history included obesity, hypertension, and dyslipidemia. She had suffered an out-of-hospital cardiac arrest with prolonged resuscitation, and no reversible cause was found. Imaging revealed severe cerebral edema and hypoxic-ischemic injury. EEG at 48 hours showed complete electrocerebral silence, and computed tomography angiography of the brain confirmed the absence of intracranial blood flow. Intraoperatively, standard American Society of Anesthesiologists monitoring and BIS™ were used. Hemodynamic stability was maintained with norepinephrine infusion, targeting a mean arterial pressure above 65 mmHg to preserve optimal organ perfusion. Normothermia and lung protective ventilation were ensured. Initially, BIS values remained around 50 with a suppression ratio ranging between 15 and 25, and no electromyographic activity was detected. EEG showed low-amplitude slow-wave activity. Following aortic cross-clamping and circulatory arrest, BIS dropped to zero, and the EEG trace remained isoelectric until the end of the case. Throughout the procedure, the patient remained hemodynamically stable. The surgery was uneventful, and the kidneys, liver, and corneas were successfully retrieved. In this case report, BIS monitoring was used to explore intraoperative EEG patterns in a patient with confirmed BD. BIS values remained around 50 during the procedure, dropping abruptly to zero only after circulatory arrest. As a simple, non-invasive tool, BIS has been proposed as an adjunct for neurologic monitoring in intensive care unit patients with declining values correlating with worsening neurological status and ultimately reaching zero at BD. This has led to the hypothesis that BIS could help detect early cortical inactivity, enabling timely BD diagnosis and ancillary testing, potentially expediting diagnosis and optimizing organ preservation. However, in this case, BIS values compatible with an anesthetized state persisted despite formal BD confirmation, which was consistent with previous reports. BIS readings can be affected by extracranial artifacts such as electrocardiographic activity, arterial pulsation, ballistocardiographic effects, electrocautery, patient handling, and electromyography. In this case, elevated BIS values likely reflected residual signals or artifacts rather than cortical activity. These findings emphasize that, while BIS can aid continuous monitoring and early detection of neurological deterioration, its interpretation in BD requires caution, with careful consideration of potential artifacts and physiological factors. BIS monitoring may support timely BD assessment in comatose patients, but its limitations and the influence of artifacts and physiological factors on BIS readings must be carefully considered.