Surgical pleth index (SPI) has been widely investigated in assessing the nociceptive level, and tracheal intubation is a noxious stimulus during the induction of anesthesia. This study aims to evaluate the ability of SPI to predict hemodynamic reactivity after tracheal intubation, and find the target value of SPI to guide the optimal timing for tracheal intubation.

The surgical pleth index (SPI)monitored by a oxygen saturation (Spo2) probe, is a dimensionless score which is based on the photoplethysmographic analysis of the pulse wave and the heartbeat interval. SPI score monitored during surgery is the indicator that may reflect a patient's autonomic response to certain noxious stimulus and is correlated to his/her nociception level. Once noxious stimulus occur, sympathetically-mediated vasoconstriction and cardiac autonomic tone increasing can be reflected by SPI. Several studies have since investigated the potential benefits of SPI-guided anesthesia, such as SPI could reduce the intraoperative opioid consumption and facilitate extubation after surgery.Jain N et al. found that the need for postoperative analgesics decreased although fentanyl consumption increased, when SPI guided opioid use Intraoperatively . The level of SPI was also reported to be able to predict postoperative pain, which could guide clinical use of opioids and improve patient postoperative satisfaction.

Recent studies on SPI mainly focus on the maintenance period of anesthesia and postoperative analgesia, and there is few studies on the application of SPI in anesthesia induction period. Hemodynamic stability in induction period has always been the technical key point that anesthesiologists pay attention to, especially for patients with cardiovascular and cerebrovascular diseases, maintaining hemodynamic stability is particularly important. Endotracheal intubation is a common procedure during general anesthesia. The hemodynamic fluctuation caused by intubation stimulation is usually caused by the premature timing of intubation.

At present, the timing of intubation is much depended on the clinical experience of an anesthesiologist, such as according to the degree of blood pressure, heart rate decline or predicted onset of propofol,opioid and neuromascular blocker. This study aims to explore whether the SPI can be used in quantitative prediction of intubation timing or prediction the hemodynamic reactivity after intubation, and to investigate the influence of age and gender on SPI value.

Inclusion Criteria:

• American Society of Anesthesiology physical status (ASA) 1-2
• Patients aged 18-80 years undergoing non-emergency anesthesia

Exclusion Criteria:

• age <18 years
• severe peripheral or cardiac neuropathy
• significant arrhythmia (i.e. atrial fibrillation or atrioventricular block)
• pacemaker
• neurovascular diseases
• uncontrolled hypertension
• treatment with vasoactive medication during the induction of anesthesia
• any intraoperative treatment with beta receptor blockers, clonidine, beta receptor agonists, or any other drug suspected to interact with the sympathovagal balance
• predicted difficult ventilation
• chronic pain requiring long-term analgesics
• addictive to opioids.

American Society of Anesthesiology physical status (ASA) 1-2 and patients aged 18-80 years undergoing non-emergency anesthesia