Multimodal Monitoring During General Anesthesia

In addition to the recordings provided by the hybrid diffuse optical setup, ${\mathrm{SpO}}_2$ , heart rate (HR) and mean arterial pressure (MAP) were extracted from either an anesthesia monitor (Datex-Ohmeda Aisys™, GE Healthcare, Little Chalfont, United Kingdom) by the open-source VitalSigns capture (VScapture) program⁶⁰ (link) or from a general monitor (Philips IntelliVue MX800, Koninklijke Philips N.V.). The BIS data were acquired by a BIS sensor (BIS Vista™, Medtronic plc, IRL). These signals were recorded by the same monitor with the same timestamps and were synchronized with the optical signals with a precision of 1 s and according to the beginning of the measurement.
General anesthesia was performed under total intravenous anesthesia with propofol at a concentration of 1% (Propofol Fresenius^®, Fresenius Kabi Deutschland GmbH, Bad Homburg, Germany) using the Schneider model of target-controlled infusion anesthesia⁶¹ (link) based on age, height, weight, and gender of the patient^{3 ,62} (link) implemented in a TIVA system (Alaris Asena^® PK, Becton, Dickinson and Company, Franklin Lakes, New Jersey). After induction and tracheal intubation, pulmonary volume-controlled ventilation was maintained with a fraction of inspired oxygen ( ${\mathrm{FiO}}_2$ ) of 0.5, a tidal volume of 6 to $7\mathrm{mg}/\mathrm{kg}$ , a respiration rate between 12 and 16 breaths per minute, a positive end-expiratory pressure of 4 to 6 mmHg, and a MAP between 60 and 80 mmHg.
It should be noted that the patients received further medications, such as fentanyl for analgesia,⁶³ (link)^–65 (link, link) atropine sulfate, atracurium besylate, or rocuronium bromide to obtain neuromuscular blockade, remifentanil, neostigmine, lidocaine, and midazolam. Some of them are known to influence cerebral hemodynamics (i.e., fentanyl,⁶³ (link)^–65 (link, link) midazolam,⁶⁶ (link) remifentanil,⁶⁷ (link) atropine sulfate,⁶⁸ (link) and lidocaine⁶⁹ (link)). Our study is not designed to investigate these interactions and it is not trivial to consider these effects since they are not well known. Since the primary goal of the study the direct comparison of the optical and BIS signals, we have decided to remove the duration of their administration from all the signals as described in the following section.