Study population and data collection
After obtaining approval from the Institutional Review Board at Seoul National University Bundang Hospital (B-1510/318-04), electronic medical records from January 2012 to July 2017 were reviewed. The requirement for informed consent was waived.
Adult patients aged 20 years or over, who had undergone heart valve or CABG surgery with CPB were included in this study. Patients who had been preoperatively diagnosed with neuropsychological diseases, such as dementia or Parkinson’s disease, were excluded. Patients with visual disturbances, hearing loss, postoperative sedation, or reoperation due to postoperative complications were excluded.
Routine general anesthesia practice
On arrival in the operating room, standard monitoring (pulse oximetry, electrocardiogram, and noninvasive arterial pressure) and invasive arterial monitoring were established. Central vein catheterization was performed after induction of general anesthesia.
During the investigation period, different general anesthetic agents were administered to patients. Based on these different anesthetic agents, patients were divided into two groups: InDex and propofol groups. In the InDex group, anesthesia was induced with intravenous propofol, remifentanil, and sevoflurane. During the intraoperative period, anesthesia was maintained with sevoflurane and dexmedetomidine. In the propofol group, total intravenous anesthesia was performed using propofol and remifentanil via a target-controlled infusion device (Orchestra®, Fresenius Vial, France) from the induction to the end of the surgery. Dexmedetomidine was not used in the propofol group. Rocuronium was administered as a neuromuscular blocking agent in both groups.
Assessment of postoperative delirium
PD was defined as the positive Confusion Assessment Method for the Intensive Care Unit (CAM-ICU) during the ICU stay. According to the standard ICU practice of our institution, CAM-ICU was examined and recorded twice a day by directed bedside nurses, who had been extensively trained. After patients were transferred to the floor, PD was assessed until postoperative day 5 by nurses who had also been trained to evaluate the patient’s cognitive status. The duration of delirium was defined as the total number of days with positive CAM-ICU.
The type of PD was determined according to the Richmond Agitation and Sedation Scale (RASS) score. When the RASS level was positive or negative, PD was classified as hyperactive or hypoactive, respectively. When a patient presented with mixed positive and negative RASS scores, they were diagnosed with mixed-type PD.
Other outcome variables
Data were collected and categorized into the following three variable sets: (1) preoperative factors, including age, sex, weight, height, body mass index, American Society of Anesthesiologist (ASA) physical status classification, and preoperative laboratory findings; (2) intraoperative factors, including the urgency of surgery, surgery time, CPB time, volume of estimated blood loss, and amount of red blood cell (RBC) transfusion; and (3) postoperative factors, including the duration of ICU stay, extubation time, postoperative admission period, postoperative complications, amount of RBC transfusion, and postoperative laboratory findings. Pre- and postoperative laboratory tests including hematocrit, platelet counts, electrolytes, creatinine, and albumin, were performed within 1 month before surgery and within 24 h after surgery, respectively. We investigated whether or not patients developed postoperative complications related to the renal and neurological systems.
Statistical analysis
Data are expressed as median (interquartile range) or number (proportion). All continuous data were assessed for normality using the Shapiro-Wilk test. Incidence was analyzed using the chi-square test or Fisher’s exact test. The Mann-Whitney U test or Wilcoxon signed rank test was performed to compare the numerical data, as appropriate. A binary logistic regression model was used to evaluate the predisposing factors of PD. The independent variables were the main anesthetic drug, age, sex, BMI, surgery type, ASA class, anesthesia time, CPB time, estimated blood loss, intraoperative and postoperative RBC transfusion amount, and preoperative and postoperative hematocrit, electrolytes, creatinine levels, and albumin level.
Propensity-score matching was performed to reduce the risk of confounder effects between the InDex and propofol groups. Propensity scores were calculated using a logistic regression model. Covariate included the sex, age, height, weight, BMI, ASA class, urgency of surgery, operation time, preoperative laboratory findings, estimated blood loss, duration of CPB, intra- and postoperative RBC transfused, extubation time, and type of surgery. The dependent variable was the main general anesthetic agent: inhalation with dexmedetomidine or propofol. We performed nearest-neighbor matching. After propensity-score matching, the paired t-test or Wilcoxon signed-rank test for continuous variables and McNemar or McNemar-Bowker test for categorical variables were performed, as appropriate.
All analyses were carried out using IBM® SPSS® Statistics, version 22.0 (IBM Corporation, NY, USA). P < 0.05 was considered to indicate statistical significance.