Protocols and Ethics
The study protocol was conducted in accordance with the Helsinki declaration (2008) and approved by the Research Ethics Committee of the First Affiliated Hospital, Sun Yat-sen University, China.
Before the trial, enrolled patients were randomized into a sevoflurane group or a propofol group. Randomized treatment allocations with no further stratification were generated using a computer random number generator with a 1:1 allocation using blocks of varying sizes. Allocation details were placed in sequentially numbered sealed envelopes. To standardize technical skills and experience, every case was performed by the same anesthesiologists and surgeons. Treatment allocation was revealed by anesthesiologists on the morning of surgery and supervised by an independent statistician. None of the anesthesiologists participated in the data assessment or analysis. Patients, investigators, surgeons, critical care teams and individuals participating in data analysis were all blind to group allocation. The trial was monitored by an independent data and safety monitoring board. Group allocation was not revealed until the final statistical analysis was completed.
A prospective, single-blind, randomized controlled trial following the CONSORT statement was conducted on patients undergoing elective open infrarenal AAA repair. Written informed consent was obtained from each participant. Inclusion criteria were age greater than 18 years, elective open infrarenal AAA repair surgery and American Society of Anesthesiologists (ASA) physical status III or less. Exclusion criteria included patients older than 80 years, a history of allergy to propofol, previous cardiac surgery, combined surgery, myocardial infarction within the previous 6 weeks, severe chronic obstructive pulmonary disease (forced expiratory volume in 1 second, FEV1<50%), severe hepatic disease (alanine aminotransferase or aspartate aminotransferase >150 U/L), renal failure (creatinine concentration >150 µmol/L), preoperative antioxidant therapy and pregnancy, history of inflammatory bowel disease, history of diarrhea (≥2 liquid stools/day for ≥2 days) within 1 week of surgery and intestinal chronic inflammatory disease. Additionally, Glasgow Aneurysm Score was assessed for each patient before operation in order to ensure that operative risk between groups was similar (19).
Anesthesia and Surgical Procedure
No premedication was given before anesthesia. Routine monitoring for open infrarenal AAA repair surgery included electrocardiograph (ECG), pulse oximetry, capnography, temperature, arterial blood pressure, central venous pressure (CVP) and urine output.
All patients underwent general anesthesia combined with epidural anesthesia. Before induction of general anesthesia, a thoracic epidural catheter was inserted between the T12-L1 or L1-L2 level using a midline or paramedian approach in all patients, through which a test dose of 5 ml of 1% lidocaine was given. After 15 minutes, the quality of the epidural analgesia was assessed using cold discrimination. If pain relief was inadequate, the position of the epidural catheter was adjusted or a new catheter was placed if necessary. The anesthesia staff was instructed not to use the epidural catheters intraoperatively.
In the sevoflurane group, anesthesia was induced with sevoflurane (Sevorane®, Abbott Scandinavia AB, Solna, Sweden) initially started at 8% by deep breathing, followed by 0.2 mg/kg cisatracurium and 3 µg/kg fentanyl. Anesthesia was maintained with sevoflurane 0.8-1.0 minimum alveolar concentration and remifentanil 0.2-0.4 µg/kg/minute. In the propofol group, general anesthesia was induced with a target-controlled infusion of propofol (Diprifusor; AstraZeneca S.p.A, Milan, Italy), set at a plasma target concentration of 3-5 µg/ml, followed by 0.2 mg/kg cisatracurium and 3 µg/kg fentanyl. Anesthesia was maintained with propofol at target concentrations of 2-4 µg/ml and remifentanil 0.2-0.4 µg/kg/minute. Depth of anesthesia was determined with the bispectral index (BIS) (BIS Vista, Aspect Medical Systems, Norwood, MA, USA) with a target range between 40 and 50 during surgery. Cisatracurium was administered for further muscle relaxation, as clinically indicated in two groups. Patients were intubated with a 7.5 mm cuffed endotracheal tube and the ventilation parameters were standardized (respiratory rate 12-15 breaths/minute, tidal volume 8-10 ml/kg, fraction of inspired oxygen 1.0) to achieve 35-45 mm Hg of end-tidal carbon dioxide concentration (ETCO2). Standardized fluid replacement consisted of 10 ml/kg lactated Ringer's solution applied preoperatively and 6 ml/kg/hour of the solution applied perioperatively. Colloid was given to obtain a stable heart rate, CVP of 8-10 cm H2O, a steady mean arterial pressure (MAP) and a urine output >1 ml/kg/hour. Packed red cells were transfused as necessary to maintain a circulating hemoglobin level approximately 8 g/dl. Hemodynamic management was standardized to maintain a steady MAP by inotropic and vasodilator therapy.
At the end of surgery, the patients were routinely transferred to ICU, and all patients received the same routine postoperative care. The epidural catheter infusions were begun immediately upon entry to ICU. The loading dose was 6 ml of 0.25% bupivacaine with 2 mg morphine and during the first 3 postoperative days, all patients received epidural analgesia with a mixture of bupivacaine 0.125% and 0.1 mg/ml morphine with a basal rate of 2 ml/hour, bolus doses of 3-5 ml, and a lockout interval of 20 minutes. To make sure that those patients had a working epidural analgesia, postoperative pain was assessed at rest and movement during postoperative days 1, 2, and 3 by using the visual analog scale rating from 0 (no pain at all) to 10 (worst possible pain). In ICU, crystalloid fluid replacement was infused at 2 ml/kg/hour to maintain a stable heart rate, CVP of 8-10 cm H2O and a steady MAP. Also, blood was given to maintain hemoglobin more than 8 g/dl. Extubation was managed according to the standard ICU protocols by the ICU staff.
The Preparation of Blood Sampling
Blood samples were collected for analysis at the following time points: before the start of surgery (baseline), 30 minutes, 4, 8, 12, and 24 hours after cross-clamping release (reperfusion). Venous blood was sampled from the jugular venous line and centrifuged at 2700 rpm for 15 minutes. Serum samples were stored at -70°C for subsequent analysis. Radial arterial blood was analyzed using a GEM premier 3000 blood gas system (GEM Premier 3000, Instrumentation Laboratory, Bedford, MA, USA).
Assessment of Intestinal Injury
Intestinal injury was assessed by measuring the serum concentrations of I-FABP and endotoxin, as well as the activity of DAO at predetermined time points. The concentration of I-FABP was measured by enzyme-linked immunosorbent assay according to the instruction manual (ADL Co, Mukwonago, WI, USA). The concentration of endotoxin was determined using a quantitative Limulus amoebocyte lysate (LAL) chromogenic assay (Ruicheng Bio-engineering Research Institute, Shanghai, China). DAO activity was assessed using a sandwich enzyme-linked immunoassay with a commercially available kit (HuiJia Bio-engineering Research Institute, Xiamen, China).
To evaluate the patients' intestinal function after aneurysm repair, a modified intestinal dysfunction score based on previously described methods (19) was used. Since patients usually start enteral feeding from the third day after AAA surgery in our center, the recording of intestinal injury score was initiated from 72 hours after operation.
Evaluation of Inflammatory Response and Oxidative Stress
The levels of the inflammatory cytokines including tumor necrosis factor (TNF)-α and interleukin-6 (IL-6) were measured using a sandwich enzyme-linked immunoassay with a commercially available kit (Jiancheng Bio-engineering Research Institute, Nanjing, China). The variables reflecting oxidative stress including malondialdehyde level and superoxide dismutase (SOD) activity in serum were analyzed using methods of thiobarbituric acid reaction and the generation of an artificial chromophore, respectively.
Serum I-FABP concentration served as the primary outcome variable. Sample size was calculated based on differences in I-FABP concentration measured at 30 minutes after cross-clamping release in a pilot study of 10 patients who received either propofol anesthesia (204.0±21.1 pg/ml) and sevoflurane anesthesia (230.1±36.3 pg/ml). The formula N=15.7/ES2+1, wherein ES is the effect size= (difference between groups)/ (mean of the standard deviation [SD] between groups), with α=0.05 and power=0.8 was used to determine that the study required N of 33 per group (20). However, in order to compensate for a 10% of drop-out rate during study period, 70 patients were recruited in this study.
Continuous data were expressed as means ± SD, or median (25% percentile, 75% percentile) of patients and compared with independent t test or Mann-Whitney U test, respectively. Categorical data were expressed as frequency or percentage and compared with Fisher's exact test or the chi-square test where appropriate. The severity of intestinal injury was compared by the Mann-Whitney U test. The hemodynamic data and biochemical serum markers were analyzed using two-way repeated-measures Analysis of Variance (ANOVA) with Bonferroni correction for both within-groups and between group comparisons. A P-value of less than 0.05 was considered statistically significant. All P values were 2-sided and the statistical significant level was 0.05. Statistical analysis was performed using SPSS 16.0 (SPSS Inc., Chicago, IL, USA).