Introduction
To achieve the ideal state of anesthesia, three common and important goals are needed, namely unconsciousness, inactivity and control of the autonomic nervous response to surgical stimuli[1, 2]. Clinically, propofol[3] and remifentanil (REM)[4, 5] are two intravenous maintenance drugs commonly used in balanced anesthesia. Not only can satisfactory hypnotic and analgesic effect achieved, but also smaller concentration needed when the two drugs combined[6]. In the field of anesthesia, it is important to quantification of drug interactions for better drug administration[7]. Studies have reported different concentrations during the combination of propofol and REM under various conditions[8-10]. The concentration of propofol and REM in combination that can achieve the goal of early postoperative recovery is still being explored.
Electroencephalography (EEG) is considered to be an important part of monitoring organs in anesthesia management[11]. The patient state index (PSI, SedLine® Root, Masimo Inc., Irvine, CA, USA)[12] is a reference index provided by the SedLine Brain Monitor, with a reference range of 0-100 and an ideal anesthetic depth of 25-50 (compared to 40-60 recommended by other monitors). It has been shown to be similar to the change of the bispectral index (BIS, Medtronic, Minneapolis, Minnesota, USA) during induction, maintenance and wake-up[13, 14]. Unlike BIS, PSI would test asymmetries of electrical activity in patients’ brains, morever, it improved algorithm[15]. Many studies reported the effect-site concentration of drugs controlling autonomic responses to stimulation under BIS-guided anesthesia[16-18], there is little research on the effects of coadministration of propofol and REM on PSI. Besides, the impact of opioid administration on the BIS during general anesthesia is debatable[19]. In fact, several investigators have pointed out that BIS to be insusceptible to opioid addition, whereas others have reported a hypnotic activity[20]. Therefore, density spectral array (DSA) was introduced to observe the analgesic effect.
Therefore, we designed a study to find a suitable combination concentration of propofol and REM which can not only maintain the appropriate depth with stimulus but also promote early recovery of patients under the guidance of PSI. Besides, to explore whether the use of opioid agents (i.e., remifentanil) could confound function of depth of sedation or anesthesia.