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*Department of Anesthesiology and
Section of Cardiothoracic Surgery, Department of Surgery, Dartmouth-Hitchcock Medical Center, Lebanon, New Hampshire
Address correspondence and reprint requests to Athos J. Rassias, MD, Department of Anesthesiology, Dartmouth-Hitchcock Medical Center, 1 Medical Center Dr., Lebanon, NH 03756.
| Abstract |
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Implications: A continuous insulin infusion and glucose control during surgery improves white cell function in diabetic patients and may increase resistance to infection after surgery.
| Introduction |
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Patients with diabetes who require coronary artery surgery have an increased susceptibility to perioperative infections (13) and a higher rate of serious morbidity (1,2,4,5). Several factors may contribute to an increased complication rate in diabetics, including poor preoperative nutritional status (6), concurrent obesity (4), and preexisting immune deficiencies (7).
Infectious complications in diabetic patients who undergo coronary artery bypass grafting (CABG) are associated with impaired immunity after surgery (8). The mechanism for impaired postoperative immune function in diabetics is unclear and is probably multifactorial (911). However, hyperglycemia may play an important role in altering leukocyte function, including the function of polymorphonuclear neutrophils (PMNs) (916). There is in vitro evidence that control of glucose concentrations improves immune function (913,15,16), but the clinical implications of glucose control for patients with diabetes mellitus are unknown. We conducted a clinical study to examine the effect of aggressive insulin therapy on PMN function in diabetic cardiac surgery patients.
| Methods |
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Patients were prospectively randomized into one of two treatment groups: standard insulin therapy (SIT group) or aggressive insulin therapy (AIT group). The anesthesia and surgery team was not blinded as to the choice of insulin therapy.
The SIT group patients who were taking insulin were given one half of their usual subcutaneous NPH insulin dosage the morning of surgery and started on an infusion of 5% dextrose with 0.45% sodium chloride solution at 50 mL/h. Oral hypoglycemics were not given the morning of surgery. Glucose levels were checked 1 h before surgery and then repeated intraoperatively using an automated device (AccuDataTMGTS; Boehringer Mannheim Corporation, Indianapolis, IN). IV regular insulin was administered according to the following schedule: For high glucose levels (>450 mg%), an infusion of insulin was started after an IV bolus of 13 U. The infusion was started at 4 U/h and titrated hourly.
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The anesthetic technique included premedication with an H2-blocker for those patients with a history of gastroesophageal reflux and titrated doses of midazolam (15 mg) and fentanyl (50200 µg). General anesthesia consisted of fentanyl 2050 µg/kg, midazolam 210 mg (or diazepam 520 mg), pancuronium 0.10.2 mg/kg, and isoflurane 0.2%1.5% in an oxygen/air mixture. Vasoactive medications and blood product transfusions were administered at the discretion of the anesthesiologist. All patients received prophylactic perioperative antibiotics. Epsilon aminocaproic acid was used for all patients, except for those judged to be at high risk of inadequate hemostasis, for whom aprotinin was used. CPB was conducted at normothermia with a membrane oxygenator. A centrifugal blood pump was used. The pump prime consisted of a mixture of plasmalyte and mannitol with or without 6% hetastarch. Intraoperative red cell scavenging and processing for transfusion was also used. All surgical procedures were performed via a median sternotomy.
Peripheral blood for immunologic studies was drawn through an indwelling arterial catheter immediately before the induction of anesthesia, 1 h after separation from CPB, and at 8:00 AM on the first postoperative day.
PMNs were isolated by density gradient centrifugation of heparinized whole blood layered over PolymorphprepTM per the manufacturer's directions (Gibco BRL, Gaithersburg, MD). Cells were washed once with serum-free RPMI 1640 (Gibco BRL). Red blood cells were lysed with sterile water, followed by suspension reequilibration to 1x phosphate-buffered saline. Cells were washed and resuspended in serum-free RPMI. Suspension concentrations were determined using a model JT Coulter Counter (Beckman Coulter, Inc., Fullerton, CA).
Candida organisms were washed twice in serum-free RPMI 1640 to remove growth medium. Candida concentration was determined by a wet mount count. In duplicate, 1 x 106 PMNs, 5 x 106 Candida, 50 mL of autologous serum, and a balance of serum-free RPMI to a final volume of 500 µL were added to 1.5 mL of polypropylene Eppendorf tubes. The suspensions were vortexed and incubated with constant agitation for 15 min in a 37°C water bath, then placed on ice for 5 min. A wet-mount slide was made and read for percent phagocytosis of at least 200 PMNs per sample (number of PMNs with ingested yeast/total number of PMNs counted x100 = percent phagocytosis). Results were reported as the average of the duplicates.
As previously described (18), chicken red blood cells (cRBCs) were labeled with 51Cr in fetal calf serum (FCS) for 1 h at 37°C and 5% CO2. After washing twice with 10% FCS/RPMI 1640, cells were adjusted to 0.5 x 106 cells/mL and 1 mL of packed ox red blood cells per milliliter of adjusted cRBCs was added. Rabbit anti-cRBC antibody dilutions were prepared in 10% FCS/RPMI 1640 to achieve final assay concentrations of 0.003 to 6 mg/mL. Isolated PMNs were adjusted to 10 x 106 cells/mL in 10% FCS/RPMI 1640. In triplicate, in a 96-well assay plate, the following were added for the appropriate conditions: for experimental samples, 50 µL of antibody dilutions, 50 µL of PMNs, and 50 µL of cRBCs; for maximal lysis, 100 µL of 2% Triton-X and 50 µL of cRBCs; for spontaneous lysis, 100 µL of media and 50 µL of cRBCs; for antibody control, 50 µL of 6 mg/µL, 50 µL of cRBCs, and 50 µL of media. The plate was centrifuged at 100g for 3 min and incubated in 37°C and 5% CO2 for 4 h. After incubation, 75 µL of supernatant was collected from each well and transferred into 6 x 50 mm borosilicate tubes, sealed with paraffin, and counted on a
counter for 1 min. Cytotoxicity was determined as follows:
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Using previously published data (10), we determined that group sizes of 13 patients per study group would give us 84% power to detect a significant (20%) increase in PMN function in the treatment groups after surgery at a 5% level of significance.
Unadjusted comparisons between treatment groups were performed using the Wilcoxon rank sum statistic for continuous variables or by using the
2 statistic with continuity correction for dichotomous outcomes. Adjusted comparisons were performed using generalized estimating equations that incorporated the dependent nature of the data (the same patient was observed on multiple occasions), adjusted for baseline differences and modeled for other possible confounders.
| Results |
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PMN antibody-dependent cell cytotoxicity (ADCC) activity demonstrated a decrease in both the AIT and SIT groups 1 h after CPB, then a slight increase in both groups 24 h after surgery (Table 2). For both groups, at most antibody concentrations, the change from baseline 1 h after CPB reached statistical significance. The intergroup comparisons showed that, 24 h after surgery, there was a trend toward an increase in activity in the SIT group compared with the AIT group. However, this reached statistical significance only at the high antibody concentration of 1.5 µg/mL (P = 0.046).
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| Discussion |
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The effect of glucose control on the perioperative immune response in diabetic patients is poorly characterized. Kwoun et al. (19) demonstrated that acute hyperglycemia in rats induced a decrease in respiratory burst of alveolar macrophages (but not peripheral PMNs) while the animals were anesthetized for a standardized surgical procedure. They also observed enhanced phagocytosis by alveolar macrophages. Only one large clinical series has addressed the question of glucose control and wound infections in patients with diabetes after cardiac surgery (17). These investigators instituted a protocol with the goal of maintaining postoperative blood glucose <200 mg% with an insulin infusion in diabetic cardiac surgical patients. In the 5 yr before the institution of this protocol, the rate of infection was 2.4%, whereas in the 3 yr after protocol initiation, the infection rate decreased to 1.5%, a statistically significant difference. During the same time period, the mean glucose level on the first postoperative day decreased from 206 to 172 mg% (P < 0.005), a modest difference comparable to that which we observed during surgery. Independent predictors of wound infection included a higher postoperative glucose level and obesity. The authors concluded that improved glucose control through the use of an insulin infusion led to fewer infectious complications in their patients with diabetes.
The main result of this study is improved phagocytic activity in the period immediately after CPB in patients randomized to aggressive glucose control. This is the first randomized trial, with appropriate controls, to demonstrate that impairment in PMN function in diabetics during surgery is at least partially reversible. Consistent with previous in vitro and animal studies, we found glucose control only affected nonspecific neutrophil function. Specifically, antibody-dependent function was unaffected by insulin therapy and returned to normal on the first postoperative day.
Glucose levels were only modestly, but significantly, improved in the AIT group compared with the SIT group through the use of an insulin infusion and frequent blood glucose measurements. These findings are consistent with the hypothesis that improved glucose control leads to improved PMN phagocytic function. In addition, there may be other direct or indirect effects of insulin on the immune system, which may account for some or all of the observed differences. It is possible that endocrinological or cytokine-mediated effects of insulin therapy may have a positive impact on PMN phagocytic function.
CPB induces alterations in PMN function, including altered chemotaxis (20), altered leukotriene generation (21), depressed adhesion (22), and depressed phagocytosis (23). Furthermore, anesthesia and the stress response to surgical trauma can also affect PMN function (24). Given the combined effects of CPB, anesthesia, surgical trauma, stress response, and exogenous as well as endogenous catecholamines, detection of immunologic effects from a brief intraoperative insulin infusion suggests a strong independent influence from this intervention.
Several limitations of this study should be acknowledged. First, peripheral blood PMN function is not the end point of greatest concern. Improvements in PMN activity may translate into decreased rates of infection, but documentation of such an effect awaits appropriate clinical trials. Second, we did not stratify patients according to adequacy of chronic glucose control, as might be measured by hemoglobin A1C (HbA1c). In addition, we included patients with noninsulindependent diabetes mellitus and patients with insulin-dependent diabetes mellitus, creating a slightly heterogeneous patient population. However, evidence implicates hyperglycemia per se as causing impairment of PMN function and not the duration or physiologic etiology for the increased glucose levels (1116,19). This is supported further by work demonstrating that PMNs from patients with diabetes display improved activity when glucose levels are returned to normal (13). Furthermore, Delamaire et al. (25) demonstrated that the type of diabetes, the HbA1C level, and the disease duration do not explain depressed chemotaxis and chemiluminescence observed in PMNs from diabetics. Third, the differences in PMN function may be explained by unobserved differences between the two groups, a possibility that exists for all randomized, controlled trials.
In summary, we demonstrated that a protocol-driven IV infusion of insulin during cardiac surgery leads to improved glucose control. Furthermore, the group of patients randomized to receive aggressive control of glucose with the use of an insulin infusion had improved phagocytic function of peripheral blood PMNs immediately after CPB compared with a standard therapy group.
| Acknowledgments |
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| References |
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