- history of epilepsy, medical treatment that might interfere with the EEG (e.g., tranquilizers, antiepileptic drugs), pregnancy, drug or alcohol abuse, history of neurological disease or a neurosurgical operation that would impair EEG or BIS monitoring, history of pulmonary disease, or the presence of signs predicting difficult mask ventilation or intubation. any pre-existing epileptic EEG patterns in standard 30-minute initial EEG recordings performed in all the patients participating in the study.