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From the *Department of Anesthesia, McGill University Health Centre, Montreal, Quebec, Canada; Department of Cardiac Surgery, Ospedali Riuniti di Bergamo, Bergamo, Italy; and School of Dietetics and Human Nutrition, McGill University, MacDonald Campus, Montreal, Quebec, Canada.
Address correspondence and reprint requests to Franco Carli, Department of Anesthesia, McGill University Health Centre, 1650 Cedar Ave., Room No. D10.144, Montreal, Quebec, Canada. Address e-mail to franco.carli{at}mcgill.ca.
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Abstract |
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METHODS: Sixteen patients were randomly assigned to receive either general anesthesia with desflurane (control group) or general anesthesia combined with epidural analgesia (EDA group). A primed constant infusion of stable isotope tracers l-[1-13C]leucine and [6,6-2H2]glucose was started after a 32-h fast before surgery, (3 h of fasted state), and continued for 3 h during surgery during which amino acids were infused IV (fed state).
RESULTS: Compared with the fasted state, the endogenous rate of appearance of leucine decreased to a similar extent in both groups, and protein synthesis increased, with no difference between the two groups. Leucine oxidation did not change in either group. After amino acids infusion, endogenous glucose production remained unchanged and glucose clearance decreased in both groups. Blood glucose, plasma cortisol, serum insulin, and glucagon concentrations increased to the same extent in both groups.
CONCLUSIONS: Epidural anesthesia provided no additional benefit beyond the anabolism obtained with amino acids.
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Introduction |
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TopAbstractIntroductionMETHODSRESULTSDISCUSSIONREFERENCES |
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Studies in surgical patients have demonstrated that infusion of amino acids spares protein and induces an anabolic state because it directly stimulates whole-body and muscle protein synthesis, and indirectly attenuates protein breakdown (11). With the understanding of the anabolic properties of infusing amino acids and the greater capacity of epidural blockade in decreasing EGP, this study sought 1) to test the hypothesis that supplementation of amino acids during surgery induces a greater positive protein balance in patients receiving an intraoperative epidural blockade compared with those receiving general anesthesia alone, and 2) to understand the interaction between protein and glucose metabolism during amino acids infusion. To quantify the dynamic changes in protein and glucose metabolism, whole-body protein breakdown, amino acid oxidation, protein synthesis, and EGP and glucose clearance were determined by an isotope dilution technique using stable isotopes l-[1-13C]leucine and [6,6-2H2]glucose.
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METHODS |
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Anesthesia
General anesthesia was induced in all patients with 5 mg/kg thiopental, and 5 µg/kg fentanyl in the control group or 1.5 µg/kg fentanyl in the EDA group. Tracheal intubation was facilitated by 0.6 mg/kg rocuronium, and the lungs were ventilated to normocapnia (35–40 mm Hg) with 30% oxygen enriched with air. In the EDA group, an epidural catheter was inserted between T9–11 before induction of general anesthesia. Neuraxial blockade was established with 15 mL of bupivacaine 0.5% to achieve a bilateral sensory block (to ice and pinprick) from T4-S5 and maintained with intermittent boluses of 5 mL of bupivacaine 0.25% every hour.
General anesthesia in the control group was maintained with desflurane at end-tidal concentrations as required to keep the heart rate within 20% of preoperative values. In the EDA group desflurane was administered at end-tidal concentrations of approximately 3 vol% to achieve tolerance of the endotracheal tube and to prevent awareness. The degree of muscle relaxation was monitored using train-of-four ratio, and supplemental doses of rocuronium were given to achieve complete surgical muscle relaxation throughout the surgery. IV fluid was given as NaCl 0.9% solution at a rate of 8 mL/kg/h. All patients were covered with a warming blanket during surgery to maintain normothermia. Hemodynamic monitoring was performed using a three-lead electrocardiogram monitor and radial artery catheterization for continuous arterial blood pressure measurement.
Amino Acids Infusion
A primed constant infusion of stable isotope tracers l-[1-13C]leucine and [6,6-2H2]glucose was started 3 h before surgery. This period has been defined as "fasted state" because patients continued their state of preoperative fasting. Isotopes infusion continued for another 3 h during surgery during which an infusion of 10% amino acids without electrolytes (TravasolTM Baxter, Montreal, Canada) was infused at a rate of 0.02 mL/kg/min, equivalent to 2.9 g/kg/d over a 3-h period. This second period of the isotope kinetics study has been defined as "fed state" because patients received energy and nitrogen supply by IV infusion.
Experimental Protocol
The kinetics of whole-body leucine metabolism, i.e., rate of appearance (Ra) of leucine, and leucine oxidation were measured by an isotope dilution technique using the stable isotope tracers l-[1-13C]leucine, and NaH13CO3, while the kinetics of whole-body glucose metabolism, i.e., Ra of glucose was measured by an isotope dilution technique using the stable isotope tracer [6,6-2H2]glucose (Cambridge Isotope Laboratories, Cambridge, MA). All isotope solutions were prepared under sterile conditions in the hospital pharmacy. An aliquot of tracer was dissolved in a known volume of sterile water to achieve a solution of 100 mg/mL. The solution was passed through a 0.22-µm filter into injection bottles. The bottles were sealed off, heat sterilized at 121°C for 15 min, and kept at 4°C until administration. Each set of solutions was confirmed to be free of pyrogens.
All patients were studied on the day of surgery between 7:00 and 8:00 am after fasting for approximately 32 h. Only clear fluids were allowed until midnight the day preceding the operation because of bowel preparation as required for colorectal surgery. No premedication was given. A cannula was inserted in the superficial vein in the dorsum of the hand to provide access for the infusion of the isotopes. Under local anesthesia, a 22-gauge catheter was inserted in the artery of the opposite arm for sampling of arterial blood. Blood and expired air samples were collected before the isotope infusion to determine baseline isotope enrichments. Primed doses of 1 µmol/kg NaH13CO3, 4 µmol/kg l-[1-13C]leucine, and 22 µmol/kg [6,6-2H2]glucose were administered and followed immediately by a continuous infusion of 0.06 µmol/kg/min l-[1-13C]leucine and 0.22 µmol/kg/min [6,6-2H2]glucose. Four arterial blood and expired-air samples were collected at 150, 160, 170, and 180 min into the isotope infusion (fasted state), when the tracers were assumed to have reached an isotopic steady state. Thereafter, anesthesia was induced and surgery begun. At the same time, the infusion of amino acids was started and the rate of infusion of l-[1-13C]leucine was doubled to 0.12 µmol/kg/min. Four arterial blood and expired-air samples were taken at 330, 340, 350, and 360 min into the isotope infusion (fed state). Plasma samples for the analysis of blood glucose and plasma hormones (insulin, glucagon, cortisol) were drawn at 150 and 330 min of isotopes infusion. A graphic illustration of the study protocol is presented in Figure 1.
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Analytical Methods and Calculations
Each blood sample was transferred immediately to a heparinized tube and centrifuged at 4°C. The plasma obtained was stored at –70°C until analysis. Expired-air samples were collected through a mouthpiece in a 2-L latex bag and transferred immediately to 20-mL vacutainers to await 13CO2 isotope enrichment analysis. During artificial ventilation, expired gases were collected by means of a one-way valve into a 5-L bag. Production of CO2 was measured by indirect calorimetry (Datex Deltatrac, Helsinki, Finland) over a 20-min period of steady state during both fasted and fed states. Plasma 
-KIC enrichment was determined by positive chemical ionization gas chromatography-mass spectrometry, as previously described (2). Expired 13CO2 enrichment was analyzed by means of isotope ratio mass spectrometry and used to calculate leucine oxidation. In each analysis run, duplicate injections were always performed, and their means were taken to represent enrichment. Plasma glucose was derivatized to its pentaacetate compound and analyzed by electron impact ionization gas chromatography-mass spectrometry, as previously described (2). Isotopic enrichment of [6,6-2H2]glucose was calculated as molecules percent excess on duplicate injections of four samples at isotopic steady state and one baseline sample. The plasma concentration of glucose was measured by a glucose oxidase method using a glucose analyzer (Beckman Instruments, Fullerton, CA). Circulating concentrations of insulin and glucagon were measured by sensitive and specific double-antibody radioimmunoassays (Amersham International, Bucks, UK). The cortisol plasma concentration was measured using the Ciba Corning ACS 180 automated immunoassay (Ciba Corning Diagnostic, East Walpole, MA). Whole-body leucine and glucose kinetics were calculated by conventional isotope dilution practice using a two-pool stochastic model during steady-state conditions, obtained at each phase of the studies, and before and during surgery, as previously described (2).
Sample Size and Statistical Analysis
On the basis of expected difference in protein balance of 10 µmol/kg/min between the two groups (sd = 5 µmol/kg/min; power 80% and P = 0.05) 16 patients were calculated to be sufficient (12). Data are presented as mean ± sd. Differences between the groups were analyzed using the Mann–Whitney U-test. Within-group comparison of variables was made by analysis of variance for repeated measures with post hoc analysis by Student–Newman–Keuls test. A probability of P < 0.05 was considered to be significant.
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RESULTS |
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Hemodynamics and Other Variables
Arterial blood pressure and oxygen saturation taken before surgery and 120 min after the start of surgery were similar in both groups (Table 2). Heart rates, 120 min after the start of surgery, were slower in the EDA than in the control group. The hematocrit decreased in both groups. Body temperature (°C) and amino acids infusion did not change during surgery in either group. Carbon dioxide production decreased to the same extent in both groups during anesthesia and surgery. The end-tidal desflurane concentrations were lower in the EDA group than in the control group at 120 min after skin incision.
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Leucine Kinetics
An isotopic plateau of -[1-13C]KIC, and expired 13CO2 was achieved in all patients allowing steady-state calculations (CV <5%, Fig. 2). The baseline values of Ra of leucine, leucine oxidation, and protein synthesis were statistically different between groups, (Table 3) but the difference between the fasted and fed states was similar in the two groups (Table 4). Endogenous Ra of leucine decreased to a similar extent in both groups (P < 0.05), while protein synthesis increased, with no difference between the two groups (P < 0.05); thus net protein balance became positive to the same extent in both groups (P < 0.05). Leucine oxidation did not change in both groups (P > 0.05) (Tables 3 and 4).
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Glucose Kinetics
An isotopic plateau of [6,6-2H2]glucose was achieved in all patients (CV <5%), (Fig. 3) allowing steady-state calculations. Blood glucose increased from 5.0 to 8.0 mmol/L during the fed state in both groups (P < 0.05). There were no differences in the EGP during the fasted state between the epidural and the general anesthesia groups. After the infusion of amino acids EGP decreased by an average of 6.6% and this was not significant in both groups. Glucose clearance decreased in both groups by 50% after the infusion of amino acids (Table 5).
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Hormones
Basal values of serum insulin and glucagon, and plasma cortisol were similar in both groups, and increased after amino acids infusion to the same extent (Table 6).
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DISCUSSION |
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TopAbstractIntroductionMETHODSRESULTSDISCUSSIONREFERENCES |
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The same lack of influence on body protein economy by the type of anesthesia (general alone versus general plus epidural) has been found in previous intraoperative studies conducted either in a fasting state (4) or during a glucose infusion (5), and the net protein balance was negative in both states, lending support to the contention that during surgery the type of substrate infused is the only factor influencing protein metabolism.
Studies conducted in the postoperative period have shown that the type of anesthesia influences protein metabolism. In fact, epidural blockade, compared with IV opioids, improved nitrogen balance (13), blunted the decrease in muscle fractional synthetic rate (14), attenuated the increase in whole-body protein breakdown and oxidation (15), and enhanced protein synthesis (12). These positive effects were observed only in association with parenteral nutrition or with an infusion of a mixture of amino acids and glucose. If only glucose was provided, the net protein balance became negative, and epidural analgesia was unable to modify protein synthesis and breakdown when compared with IV opioids with patient-controlled analgesia (2).
One would conclude that intraoperative epidural blockade does not affect protein metabolism regardless of the type of nutritional support, while after surgery it exerts an anabolic effect only during nitrogenous nutritional supplementation.
In an attempt to explain the present findings one would hypothesize that the lack of an enhanced anabolic effect by intraoperative epidural anesthesia might be the result of an attenuation of whole-body protein breakdown due to general anesthesia itself. A decrease in protein breakdown and synthesis has been shown to occur in patients undergoing colorectal surgery during inhaled desflurane and IV propofol anesthesia (16). Similarly, a decrease in all aspects of protein metabolism has been reported during enflurane anesthesia for total abdominal hysterectomy (17). In view of these findings, the inhibitory effect of anesthesia, observed in the present study, might have masked the positive effect on protein synthesis that one would otherwise have expected with epidural anesthesia.
Previous results have suggested that whole-body and muscle protein synthesis is stimulated almost linearly within the normal physiological range of plasma amino acids, and that a two- to threefold increase in plasma concentrations of amino acids is required to saturate the system (18). In view of this, the rate of infusion of amino acids was selected to be 2.9 g/kg/day, with the intention to achieve a plasma amino acid concentration two- to threefold above basal value (19). Three hours of amino acids infusion is sufficient for maximal incorporation of amino acids into whole-body and tissue compartments. Beyond 3 h, muscle protein synthesis decreases back to basal values despite amino acid infusion continuing at the same rate (20).
It is interesting to observe that, although the real quantity of amino acids administered was 0.36 g/kg/d and less than the daily recommended intake of 1.5 g/kg/d (21), a consistent anabolic effect was shown. This agrees with previous findings demonstrating that amino acids are more efficiently used for maintaining lean body mass when given in divided doses rather than with a continuous infusion (20). This anabolic effect could also be due to the increase in insulin levels determined by the infusion of amino acids.
With regard to the present findings on glucose metabolism, the baseline values of EGP were not modified after the infusion of amino acids, and this was independent of the type of anesthesia used, while glucose clearance decreased to the same extent with both techniques.
The fact that epidural anesthesia had the same effect as general anesthesia alone on EGP is in contrast with previous intraoperative studies using a similar protocol and conducted during either saline infusion or feeding with dextrose (4,5). In those studies, epidural anesthesia decreased EGP more than general anesthesia alone, probably due to the suppression of glucagon secretion exerted only by epidural anesthesia. In contrast, in the present study, serum glucagon increased to the same extent in both groups, and this is related to the infusion of amino acids that stimulated glucagon secretion (22,23).
The small, nonsignificant decrease in EGP (8% on average) after the infusion of amino acids is in agreement with a previous finding in surgical patients undergoing major abdominal surgery in which a perioperative infusion of amino acids mixture at 2 g/kg/d (24) decreased endogenous glucose production by 8%, but failed to reach statistical significance.
In volunteers, on the contrary, amino acids increased EGP. Tappy et al. (23), using a stable isotopes technique, showed an increase in EGP by 84% after an infusion of amino acids at a rate of 4.8 g/kg/d. Such an increase had been explained as a result of a direct effect of amino acids acting as substrate for the gluconeogenic pathway and indirectly by increased plasma glucagon concentration which stimulates gluconeogenesis (23). This contrasting effect of amino acids might be explained by considering that anesthesia by itself decreases EGP (4,16,25,26), and therefore, by inhibiting gluconeogenesis, anesthesia might have increased the available amino acids for synthetic pathways.
Despite the lack of change in EGP in both groups, a significant increase in blood glucose to values around 8 mmol/L was observed, implying that glucose uptake was decreased as reflected by a decrease in glucose clearance. The decrease in glucose uptake can be due either to anesthesia or amino acids infusion or both. Other studies have reported a decreased glucose utilization in brains of anesthetized animals (27,28). This could affect whole-body glucose turnover, as the human central nervous system represents 50%–80% of basal whole-body glucose disposal after an overnight fast (29). A decrease in muscular activity would be also an expected consequence of general anesthesia, and this could contribute to a significant decrease in glucose use because muscle uptake accounts for about 20% of basal glucose consumption in a postabsorptive state (30). Thus, the reduction in glucose clearance could be simply related to the decrease in energy requirements of some tissues as reflected by the decrease in whole-body oxygen consumption observed under anesthesia. Amino acids also could decrease peripheral glucose uptake. In volunteers, amino acids inhibited glucose transport/phosphorylation resulting in a decreased intracellular use of glucose (31).
The present study had some limitations. First, there was an extremely long period of preoperative starvation, approximately 32 h, because of the preoperative bowel preparation. This cannot be said for other major operations in which the fasting state is limited to 10–12 h, and therefore the present findings need to be interpreted with caution. Second, the baseline values for Ra leucine, leucine oxidation, and protein synthesis were statistically different between the groups, which occurred by pure coincidence. For this reason we compared the differences (fast and fed states) between groups.
In conclusion, intraoperative infusion of amino acids induces a positive protein balance, independent of the anesthetic technique. In addition, amino acids supplementation did not influence gluconeogenesis, while whole-body glucose uptake decreased in both groups.
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ACKNOWLEDGMENTS |
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Footnotes |
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REFERENCES |
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TopAbstractIntroductionMETHODSRESULTSDISCUSSIONREFERENCES |
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) = epidural; (
) = control.
