Induction of anesthesia and surgery
All patients received midazolam (7.5 mg, Hoffmann-La Roche, Basel, Switzerland) orally 30 minutes preoperatively for anxiolysis and sedation. Upon arrival in the anesthesia induction room, ECG, pulse oximetry and non-invasive blood pressure were monitored continuously.
Measurement of rSO2 was performed continuously from arrival at the anesthesia induction room until 20 minutes after postoperative declamping of the femoral artery using an INVOS® 5100C Oxymeter (Somanetics, Troy, Michigan, USA). The optodes (Small Adult SomaSensor® SAFB-SM, Somanetics, Troy, Michigan, USA) were fixed above the anterior tibial muscle at both lower legs 20 cm above the ankle after cleaning the skin with 70% ethanol. rSO2 of both legs was measured to exclude an oxygen saturation decrease or increase due to a systemic effect on leg tissue oxygen saturation during clamping.
The anesthetist performing anesthesia was blinded to the measurement of rSO2. Peripheral venous blood samples were obtained from the cubital vein for the measurement of hemoglobine, hematocrit, lactate, glucose, potassium, pO2, pCO2 and base excess immediately before induction of anesthesia.
Anesthesia was induced with intravenous propofol (2 mg.kg-1, Diprivan®, AstraZeneca, London, GB) and a continuous infusion of remifentanil (0.3 μg.kg-1.min-1, Ultiva®, GlaxoSmithKline, Brentford, GB). Muscle relaxation was achieved with rocuronium (0.6 mg.kg-1, Esmeron®, Essex Pharma, Oss, The Netherlands). After endotracheal intubation, ventilation was adjusted to maintain normocapnia (FiO2 = 0.3). Anesthesia was maintained with propofol (4–6 mg.kg-1.h-1) and remifentanil (0.2 - 0.3 μg.kg-1.min-1). Body temperature was kept normal by use of a heating blanket (36.5° - 37.5°C). All patients received cefuroxime (1.5 g) for antibiotic prophylaxis.
As soon as steady state anesthesia was achieved, sevoflurane preconditioning was performed in the sevoflurane group (N = 20) by two periods of sevoflurane application (each lasting 6 minutes) interspersed by 6 minutes washout on the basis of a previously published protocol for interrupted administration of sevoflurane for myocardial preconditioning [
16].
Sevoflurane was administered with high fresh gas flow (10 l.min-1, FiO2 = 0.3). After an endtidal concentration of 1.0 MAC was reached, fresh gas flow was reduced to 4 l.min-1. rSO2 was noted immediately before application of sevoflurane and 1, 3 and 5 minutes thereafter. One minute after the last noted rSO2, the propofol infusion was resumed and sevoflurane was washed out by discontinuation of sevoflurane and increasing the fresh gas flow to 10 l.min-1 to achieve a MAC value below 0.2. rSO2 was noted immediately before washout and 1, 3 and 5 minutes thereafter. One minute after the last noted rSO2 during washout, application of sevoflurane and washout were repeated once. Akrinor® (0.5 ml cafedrine hydrochloride (100 mg.ml-1)/theodrenaline hydrochloride (10 mg.ml-1)) was given intravenously, if the systolic blood pressure decreased below 80% of baseline. In the other patients (IPC only, n = 20), rSO2 was measured during a respective time-matched period without sevoflurane preconditioning. These patients were anesthetized for the same period of time as the sevoflurane group.
After final washout of sevoflurane, IPC was performed in both groups by clamping the femoral artery at the side of surgery for 6 minutes. Clamping was performed using the same clamp (Pilling® Cooley Anastomosis Clamp, Teleflex® Medical, New York, USA) and the same clamping technique, as for definite clamping for surgery. rSO2 was noted immediately before clamping and 1, 3 and 5 minutes thereafter. One minute after the last noted rSO2 during clamping, the reperfusion period was started by declamping. rSO2 was noted immediately before declamping and 1, 3 and 5 minutes thereafter. One minute after the last rSO2 during reperfusion, clamping and reperfusion were repeated once.
Before the first clamping, intravenous heparin (100 units.kg-1, Heparin-Natrium-5000-Ratiopharm®, Ratiopharm, Ulm, Germany) was given to all patients. After the second reperfusion period, the femoral artery was definitely clamped for surgery. rSO2 was noted immediately after clamping for surgery, every 15 minutes during surgery, at declamping and subsequently at 3, 5, 10 and 20 minutes. After declamping, the heparin effect was partially reversed by intravenous protamine (60 – 80% of the initial heparin dose, MEDA Pharma, Bad Homburg, Germany). Immediately after extubation, venous blood samples were obtained from the cubital vein for the measurement of postoperative hemoglobine, hematocrit, lactate, glucose, potassium, pO2, pCO2 and base excess.
Statistical analysis
Data analysis was performed with commercially available software (Graph Pad Prism® for Mac, version 5.03, GraphPad Software, San Francisco, USA).
The data were tested for normality using the Kolmogorov-Smirnov test. The influence of time and an additional factor (the side (left or right) or the medical preconditioning (with or without sevoflurane)) was analyzed by an ANOVA with repeated measurements with time as within subjects factor and side or medical preconditioning as between subjects factor.
The results were corrected for multiple testing. Parametric data are given as mean ± standard deviation. Differences between two groups were analyzed by the t-test for parametric data. P < 0.05 was considered statistically significant. Incidences of diagnoses, medications and localizations of stenoses were compared with Fisher’s exact test. Differences between laboratory values obtained before and after surgery and correlations of age, hemoglobine, ankle brachial index, sex, hyperlipidemia, diabetes mellitus and statins with rSO2 were investigated exploratively without correction for multiple testing.
If a medium effect size of f = 0.25, a medium correlation of 0.5 among repeated measures and a nonsphericity correction of 1 is assumed, the following power estimates apply for a significance level of 0.05 (calculated by G*Power 3.1.6): For all four time periods (sevoflurane preconditioning, ischemic preconditioning, clamping for surgery and postoperative reperfusion) the power for detecting a time effect was >99% and the power for detecting a side effect was about 85%. The power for detecting a medication effect varied between 38% (clamping for surgery), 54% (postoperative reperfusion) and 56% (sevoflurane preconditioning and ischemic preconditioning). The power for detecting an interaction between time and side or medication effect was for all time periods >99%.