Time-frequency analysis of photoplethysmogram for measuring deepness of anesthesia

One of the most important objectives of anesthesiologists is to ensure an adequate depth of anesthesia for patients during surgeries. The adequate depth of anesthesia does not mean patient's sleeping as deep as possible but it means as deep as necessary to prevent awareness without unintentional overloading the patient with high doses of the anesthesia's drug. Overloading patients that already have history of cardiac illness or other diseases may cause death or complications during recovery stage after the surgery. Therefore monitoring depth of anesthesia will prevent such complications and ensure suitable dose of drugs. Pulse oximeter is one of the standard devices already used in the operation theater. Photoplethysmogram (PPG) is the signal given using pulse oximeter carries information about cardiopulmonary system. The rhythms of both the cardiac and respiratory systems are changed during surgeries. The Autonomic Nervous system (ANS) controls the cardiorespiratory systems during consciousness but this control will have more effects during unconsciousness. We studied the changes of the photoplethysmogram during surgeries to determine level of anesthesia during surgery. Time-Frequency method of analysis was used then system identification to get a mathematical model that represents our system. The results show high correlation up to (95.27%) between the standard methods (BIS Index) with the method developed in this study. The results depend on the processed part of the signal and on the identification methods.