A total of 80 patients undergoing systemic anesthesia and extracorporeal coronary artery bypass surgery in the People's Hospital of Guangxi Zhuang Autonomous Region (Nanning, China) from January 2015 to August 2017 were selected. Patients were diagnosed by preoperative imaging, clinical manifestations, and direct observations during surgery. Patients with cognitive impairment, mental and psychological illness and chronic alcoholics, drug addicts, abuse of psychotropic substances, illiterate education, intraoperative use of epinephrine and norepinephrine, or allergy to drugs used in this study were excluded. This study was approved by the Ethics Committee of the People's Hospital of Guangxi Zhuang Autonomous Region, and all patients signed informed consent. Patients were randomly divided into two groups, 40 cases in each group. No significant differences in sex, average age, body mass index (BMI), left ventricular ejection fraction, intraoperative cardiopulmonary bypass time and total operation time were found between the groups (P>0.05). Preoperative mini-mental state examination (MMSE) and Montreal Cognitive Assessment (MoCA, Beijing version) were performed and no significant differences in MMSE and MoCA scores were found between the groups (Table I).

All patients underwent surgical treatment under general anesthesia with endotracheal intubation. Patients were fasted and not allowed to drink water before operation. Peripheral vein was opened and connected to the monitor, and patient's noninvasive blood pressure, electrocardiogram, heart rate and oxygen saturation were monitored continuously. Anesthesia machine was used to detect the changes of respiratory function in patients. Anesthesia was induced by intravenous infusion of midazolam, fentanyl, cisatracurium and etomidate. Tracheal intubation was performed under slow induction of anesthesia, and was connected to anesthesia machine for mechanical ventilation, oxygen flow rate was set to 50%, respiratory rate was set to 14 times/min, and tidal volume was set according to patient weight (10 ml/kg). Peripheral arterial puncture, central venous catheterization and pressure measurement were performed. Intraoperative anesthesia was performed by using propofol, remifentanil and cisatracurium. Cardiopulmonary bypass technology was used. Dexmedetomidine (SFDA approval number: H20130027, CISEN), solution (4 µg/ml) was prepared and pumped into patients in the observation group through a vein with a dose of 1 µg/kg during the first 10 min, followed by the dose of 0.2 µg/kg until the end of surgery. Equal volume of 0.9% saline was used at the same speed for patients in the control group, and 1 mg of midazolam was intravenously administered every 30 min. Based on patient circulatory function, vasoactive drug was given through extracorporeal circulation machine during surgery, and vasoactive drugs were intravenously administered after cardiopulmonary bypass. All patients had the same indications for the use of vasoactive drugs, and drugs used here included: popamine, metaxylamine, epinephrine, phenylephrine and norepinephrine.

Circulatory function-related complications, such as arrhythmia, hypertension, and hypotension were compared between the two groups. BIS was recorded at 30 min before induction of anesthesia (T0), immediately after intubation (T1), when incision was made (T2), when chest was closed (T3), when operation was completed (T4) and at 6 h after operation (T5). The levels of cortisol, epinephrine and norepinephrine at the end of operation were recorded, and recovery time and mechanical ventilation time of the two groups were compared. MMSE and MoCA were performed at 1, 3 and 7 days after operation. The incidence rates of intraoperative awareness and postoperative cognitive dysfunction in the two groups were compared.

BIS value was measured by using A-3000 BIS monitor (Aspect Medical System, Covidien, MA, USA) and its ancillary equipment. BIS value fluctuated between 1 and 100. The higher the BIS value is, the higher the sober level is. BIS higher than 95% indicated completely awaken state, and BIS lower than 70% indicated disorder of consciousness. Stress factors, including cortisol (80–550 nmol/l), epinephrine (<480 pmol/l) and norepinephrine (615–3240 pmol/l), were measured by Bio-Rad 450 automatic biochemical analyzer (Bio-Rad Laboratories, Inc., Hercules, CA, USA). MMSE contains seven aspects and 30 questions with a score of 0 to 30, and 27 points was the threshold for cognitive impairment. Lower MMSE score indicated more severe cognitive impairment. MoCA: Test was completed within 10 min, and the highest score was 30 points, and the lowest score was 0. A score over 26 points indicated normal conditions. Lower MoCA score indicated more severe cognitive impairment. MoCA was not performed within 60 min after MMSE, and MMSE was not performed within 24 h after MoCA. Arrhythmias are indicated by electrocardiogram. Hypertension in anesthesia means: blood pressure is greater than 140/95 mmHg or elevated blood pressure is greater than 20%. Hypotension in anesthesia was defined as: blood pressure is less than 90/60 mmHg or decreased blood pressure is greater than 30%.

SPSS13.0 (SPSS, Inc., Chicago, IL, USA) was used. Measurement data are expressed as mean ± standard deviation. Comparisons of mean values between the two groups were performed by using t-test, and intragroup comparisons were performed by χ² test. P<0.05 was considered to indicate a statistically significant difference.

BIS values in the observation group at T0-T5 were 96.8±2.0, 45.8±2.9, 43.3±2.3, 56.9±3.1, 60.9±3.5 and 81.9±3.3, respectively, and BIS values in the control group at T0-T5 were 96.9±1.0, 53.3±2.6, 49.6±1.8, 63.5±3.9, 65.3±4.6 and 78.9±3.6, respectively. BIS values in the observation group were significantly lower than those in the control group at T1 to T4 (t=8.931, 4.980, 8.379 and 4.814, P<0.05), while BIS value in the observation group was significantly higher than that in the control group at T5 (t=3.885, P<0.05) (Fig. 1).

The incidence rates of intraoperative arrhythmia, hypertension and hypotension in the observation group were significantly lower than those in the control group (P<0.05) (Table II).

At the end of surgery, the levels of cortisol, epinephrine and norepinephrine in the observation group were significantly lower than those in the control group (P<0.05) (Table III).

Anesthesia recovery time in the observation group was significantly shorter than that in the control group (P<0.05), and postoperative mechanical ventilation time in the observation group was also significantly shorter than that in the control group (P<0.05) (Table IV).

MMSE scores in the observation group were 20.6±0.5 points, 25.1±0.7 points and 28.6±0.3 points at 1 day, 3 days and 7 days after operation, respectively. MMSE scores in the control group were 17.5±0.9 points, 21.3±1.1 points and 23.8±0.7 points at 1 day, 3 days and 7 days after operation, respectively. MMSE scores in the observation group were significantly higher than those in the control group at the three time points (t=19.043, 18.433 and 39.862, P<0.001) (Fig. 2).

MoCA scores were 18.6±1.5 points, 25.6±1.0 points and 27.4±0.6 points in the observation group at 1 day, 3 days and 7 days after operation, respectively, and MoCA scores in the control group were 16.3±1.3 points, 21.5±0.8 points and 24.3±0.8 points at 1 day, 3 days and 7 days after operation, respectively. MoCA scores in the observation group were significantly higher than those in the control group (t=7.328, 20.248 and 19.606, P<0.001 <0.05) (Fig. 3).

The incidence rates of intraoperative awareness and postoperative cognitive impairment in the observation group were significantly lower than those in the control group (Table V).