JOURNAL HOME CME HOME THIS MONTH PAST ISSUES ETOC COLLECTIONS AUTHORS REVIEWERS EDITORIAL BOARD FEEDBACK RSS HELP
		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a colleague Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (12) Citing Articles via Google Scholar Google Scholar Articles by Procopio, M. A. Articles by Yeager, M. P. Search for Related Content PubMed PubMed Citation Articles by Procopio, M. A. Articles by Yeager, M. P. Related Collections Regional Anesthesia Pharmacology

---

REGIONAL ANESTHESIA

The In Vivo Effects of General and Epidural Anesthesia on Human Immune Function

Marcia A. Procopio, MD^*, Athos J. Rassias, MD^*, Joyce A. DeLeo, PhD^*, Janice Pahl, BA^*, Laurie Hildebrandt, BS^*, and Mark P. Yeager, MD^*

Departments of *Anesthesiology and Pharmacology, Dartmouth Medical School, Hanover, New Hampshire and Dartmouth-Hitchcock Medical Center, Lebanon, New Hampshire

Address correspondence and reprint requests to Marcia A. Procopio, MD, Department of Anesthesiology, Dartmouth-Hitchcock Medical Center, 1 Medical Center Dr., Lebanon, NH 03756. Address e-mail to Marcia.A.Procopio{at}Hitchcock.org

	Abstract

Top Abstract Introduction Methods Results Discussion References

 
Impaired in vivo immunity is often observed after major surgery and is multifactorial. We conducted a randomized clinical study to determine the independent effects of general anesthesia (GA) and of lumbar epidural anesthesia (LEA) on human immune function in the absence of surgical trauma. Nineteen healthy volunteers were randomized to receive GA with thiopental and isoflurane, LEA with lidocaine, or no anesthesia (Control). Serial blood samples were tested for antibody responses to antigen inoculation, neutrophil and mononuclear cell antibody-dependent cell cytotoxicity (ADCC), natural killer cell cytotoxicity, and neutrophil phagocytic activity. Antibody responses were similar in the three groups. Mononuclear cell ADCC increased in the LEA group at the end of the anesthetic (P < 0.05 at effector/target [E/T] ratios of 10:1, 25:1, and 50:1). Natural killer cell cytotoxicity increased at the end of the anesthetic in both the LEA group (P < 0.05 at all E/T ratios) and the GA group (P < 0.05 at an E/T ratio of 5:1 and 10:1). No significant changes were observed for neutrophil ADCC or phagocytosis. General or epidural anesthesia alone, in the absence of surgery, seems to have only transient and minor effects on human immune function.

IMPLICATIONS: General or epidural anesthesia alone, in the absence of surgery, seems to have only transient and minor effects on human immune function.

	Introduction

Top Abstract Introduction Methods Results Discussion References

 
The in vivo response to tissue trauma is a complex integration of events that act to prevent infection and heal injured tissue. The early phase of this process is characterized by the release of proinflammatory cytokines (1), activation of adhesion molecules that attract leukocytes (2), and increased numbers and activity of circulating granulocytes (1,3). Subsequent immune events are often characterized by measurable depression of both acquired (4) and innate immune function (3–6). When immune function is depressed after trauma, there also seem to be associated clinical consequences, including increased rates of nosocomial infection (5,6), systemic sepsis (7), and in experimental settings, enhanced tumor growth (8).

The clinical events that lead to altered immunity after trauma may include tissue injury, ischemia, hypovolemia, blood transfusions, endocrine mediator release, and pain (3,4). Drugs administered to patients may also affect immune function. Opioids in particular may depress immunity (6,8,9), but the immunologic effects of nonopioid anesthetics are less well documented. Human and animal studies have suggested that some IV (10) as well as inhaled anesthetics (11) can depress neutrophil (PMN) and mononuclear cell (MNC) activity, leading to increased mortality from infection and sepsis (12). However, only a few studies have sought to evaluate, in vivo, the independent effect of nonopioid anesthetic drugs on immune function (11–14). Because the independent effects of anesthetics on human immune function are largely unknown, we conducted a clinical study to measure the effects of both general and epidural anesthesia on several tests of immunity in healthy, nonsurgical volunteers. We hypothesized that we would observe a decrement in immune function, as defined by MNC activity, at 24 h after general anesthesia (GA) and no effect from epidural anesthesia on MNC function.

	Methods

Top Abstract Introduction Methods Results Discussion References

 
This study was approved by the Dartmouth College Committee for the Protection of Human Subjects (IRB), and all participants provided written, informed consent. Exclusion criteria included age <18 or >60 yr, a chronic medical condition of any kind, use of any chronic medication, pregnancy, anemia, a clinical evaluation that suggested difficult airway management, acute medical illness within 16 wk of study, history of substance abuse or cognitive dysfunction, and any contraindication to general or epidural anesthesia. Potential participants received routine health screening similar to that used for patients scheduled for elective outpatient surgery, including history, physical examination, and hemoglobin determination. Female participants had to have a negative urine pregnancy test.

After a satisfactory preanesthetic evaluation, participants were admitted at 7:00 AM to a same-day surgery unit (SDSU) having had nothing to eat or drink since midnight. As prophylaxis against possible pulmonary aspiration of acid gastric contents, all participants took 150 mg of nizatidine by mouth the evening before surgery. After placement of a peripheral IV catheter, participants were moved to a postanesthesia care unit (PACU) where, after establishment of peripheral IV access, they were monitored with pulse oximetry, noninvasive blood pressure measurement, continuous electrocardiographic display, temperature, and, for patients who received GA, end-tidal carbon dioxide and end-tidal anesthetic measurements. A sealed envelope containing subject group assignment was opened, and subjects were randomly assigned to control, GA, or lumbar epidural anesthesia (LEA).

Control subjects remained horizontal on a hospital stretcher for 1 h, at which time they were moved back to the SDSU, the IV catheter was removed, and they were discharged home.

GA was induced with thiopental 4 mg/kg, followed by inhalation of a 1.5%–2.0% end-tidal concentration of isoflurane in an air/oxygen mixture with spontaneous ventilation via face mask. Endotracheal intubation was not performed, although an oral airway was used in some subjects. GA was maintained for 1 h, at which time the subjects were awakened by termination of isoflurane inhalation and admitted to the PACU as postanesthesia patients. Participants remained in the PACU until they met standard PACU discharge criteria, at which time they were returned to the SDSU and discharged home by use of standardized discharge criteria.

The lumbar epidural space was identified at the L2-3 or L3-4 interspace by using an 18-gauge Hustead needle and the loss-of-resistance to saline technique. A 20-gauge epidural catheter was advanced approximately 3 cm into the epidural space and secured with tape. LEA was induced with 1.5% lidocaine (Astra USA, Westborough, MA) to achieve a sensory level to the fourth thoracic dermatome as tested by pinprick (approximately 10–15 mL, administered in divided doses). After 1 h, subjects were admitted to the PACU as postanesthesia patients until their sensory block resolved, at which time they were returned to the SDSU and discharged home.

One week before the study date and 21 days afterward, serum was obtained by peripheral venipuncture and frozen at -70°C for subsequent analysis. At the end of the 1 h of anesthesia administration, subjects received an IM injection of tetanus toxoid (Lederle Laboratories, Wayne, NJ) and polyvalent pneumococcal vaccine (Pneumovax; Merck, West Point, PA). The paired serum samples were forwarded for commercial enzyme-linked immunoassay analysis of antitetanus (protein) and antipneumococcal (polysaccharide) antibody responses (Specialty Labs, Santa Monica, CA).

As previously described (8,9), peripheral blood MNCs (PBMNCs) and PMNs were isolated by density gradient centrifugation and resuspended in medium at the desired cell concentration. For PMN antibody depression cell cytotoxicity (ADCC), chicken red blood cells (cRBCs) were labeled with ⁵¹Cr (New England Nuclear, Boston, MA) and adjusted to a concentration of 0.5 x 10⁶ cells per milliliter with 1 µL packed ox RBCs per milliliter of adjusted cRBCs. Rabbit anti-cRBC antibody dilutions were prepared to achieve reported assay concentrations. Isolated effector cells were adjusted to 10 x 10⁶ cells per milliliter. Triplicate samples used 50 µL of antibody dilutions, 50 µL of effector cells, and 50 µL of cRBCs. Maximum detergent lysis, spontaneous lysis, and antibody-only controls were tested. After incubation, 75 µL of supernatant was collected from each well and counted on a counter for 1 min. Cytotoxicity was determined asequation

For PBMNC ADCC, isolated effector cells were suspended in medium and adjusted to the desired concentration. Target cells (SKOV3, a human cancer cell line resistant to spontaneous natural killer [NK] lysis and known to express the protein product of the her-2-neau protooncogene) were labeled with ⁵¹Cr. A bispecific antibody designated 2B1 (anti-her-2-neau:anti-CD16; Chiron Corporation, Emeryville, CA) (15) was added at a final concentration of 1 µg/mL. Triplicate assays were performed in 96-well plates by using 50 µL of antibody, 50 µL of PBMNCs, and 50 µL of target cells and control conditions, as defined previously. After 4 h incubation, specific cytotoxicity was calculated as described previously.

PBMNCs were isolated and resuspended. The human myeloid cell line, K-562, was used as a target cell; cells were labeled with ⁵¹Cr, washed, and resuspended to a final concentration of 1 x 10⁵ cells per milliliter. Triplicate aliquots of 100 µL of the resulting suspension (1 x 10⁴ cells) were pipetted into 96-well round-bottomed plates, and effector cells were added in 100-µL aliquots to achieve the desired effector/target (E/T) ratios. After incubation, 70 µL of supernatant was removed from each well to determine cytotoxicity as described previously.

Candida organisms (kept in continuous culture) were washed to remove growth medium, and the cell concentration was determined by wet mount count. In duplicate, 1 x 10⁶ PMNs, 5 x 10⁶ Candida, 50 µL autologous serum, and a balance of serum-free medium to a final volume of 500 µL was added to 1.5-mL polypropylene tubes. The suspensions were incubated with constant agitation at 1 Hz for 15 min in a 37°C water bath and placed on ice for 5 min. A wet mount slide was read immediately to determine the percentage of phagocytosis of 200 PMNs (number of PMNs with ingested yeast per 200 PMNs counted x 100) per sample, and the results were reported as the average of the duplicates.

As our primary measure, we planned to study differences among the three treatment groups in the relative decrease in NK activity between baseline and 24 h after anesthesia. From preliminary data on the effects of opioids on NK activity, we hypothesized the following average relative decrease (±SD) in NK activity at 24 h with reference to baseline: nontreated controls, 1.02 ± 0.18, and GA-treated subjects, 0.58 ± 0.31. Comparing post- with pretreatment NK activity, we computed a 90% power to detect a 40% decrease in NK activity in the GA group with six patients per group at a 5% level of significance. Statistical analysis was performed with SPSS for Macintosh, version 6.1.1 (SPSS Inc., Chicago, IL). A significance level of P < 0.05 was defined for all comparisons. Intragroup comparisons were performed with paired Student’s t-tests (to evaluate differences from baseline for each group), the Wilcoxon’s ranked sum statistic, or ² with continuity correction when appropriate. Intergroup comparisons were performed with one-way analysis of variance. Values are represented as mean ± SE unless otherwise noted.

	Results

Top Abstract Introduction Methods Results Discussion References

 
There were no significant differences among participants in the three groups in age, weight, or sex distribution (Table 1). No adverse events occurred during any of the studies.

View larger version (21K): [in this window] [in a new window]   Figure 1. Percentage of change in mononuclear cell (MNC) antibody-dependent cell cytotoxicity (ADCC) when compared with baseline values (±SE) measured 1 wk before anesthesia. Observations were made at the end of the anesthetic or control period (End Anes/Control) and at 24 h after the anesthetic (24 Hours post). Control = Control group; GA = General Anesthesia group; LEA = Lumbar Epidural Anesthesia group. *P < 0.05 compared with preanesthesia baseline values.

View larger version (22K): [in this window] [in a new window]   Figure 2. Percentage of change in natural killer cell cytotoxicity (NKCC) (±SE) from baseline values (measured 1 wk before anesthesia) in the three groups at the end of anesthesia administration (End Anes/Control), 90 min after anesthesia administration (90 min post Anes), and 24 h after anesthesia administration (24 hrs post Anes) compared with baseline values before anesthesia administration. Control = Control group; GA = General Anesthesia group; LEA = Lumbar Epidural Anesthesia group. *P < 0.05 compared with preanesthesia baseline levels.

	Discussion

Top Abstract Introduction Methods Results Discussion References

 
This study was done to identify potential independent effects of general or of epidural anesthesia on innate and acquired immunity. Several study conditions should be appreciated: First, we studied only healthy volunteers, free of preexisting conditions that might require surgery and could therefore affect immune function. Second, the duration of anesthesia exposure was limited to one hour, because longer anesthesia exposure may have led to different results. A dose-response relationship was not tested. Third, we measured immune responsiveness at preset intervals and may have missed events between sampling times. Fourth, we used blood sampling primarily as a "window" on immune activity within the body. In vivo immune activity was not tested, with the exception of the antibody responses to vaccine inoculation. Fifth, the ex vivo process of immune cell purification used in some of the assays could affect assay results and obscure minor effects from the treatments. Finally, in contrast to most clinical studies, we followed a study protocol that measured the immune effects of general and of epidural anesthesia in the absence of opioid exposure.

The antibody response to foreign antigens is a complicated process that includes antigen presentation by monocytes, B-cell recognition, clonal expansion, and protein synthesis with support from soluble mediators such as cytokines and other cell populations, particularly T cells. Despite the complex nature of this process, which requires many interactions in which anesthetic drugs may have measurable in vitro effects (4,16), neither general nor epidural anesthesia impaired in vivo antibody responses. The baseline and postvaccination antibody levels reported here are in fact similar to those reported for nonsurgical populations (17). Because vaccine formulations provide antigen exposure that is more prolonged than the duration of anesthesia exposure that we tested, a transient in vivo anesthetic exposure would have had to produce a durable disruption in the sequence of events that led to antibody synthesis for us to find an anesthetic effect. In a similar study with rats, Lockwood et al. (16) reported that pentobarbital and chloral hydrate anesthesia, in the absence of surgical trauma, impaired in vivo antibody responses for up to three weeks, whereas a one-hour halothane exposure had no effect. It is possible that more prolonged anesthesia exposure, similar to the duration of anesthetic action of pentobarbital, could have impaired antibody responses.

Antibody-dependent cell cytotoxicity requires binding of an antibody to specific Fc receptors on an effector cell, coupling with target cells, and delivery of a "lethal hit" to the target (15). Although nonspecific, unstimulated NK activity is sensitive to depression by in vivo opioid exposure (8), antibody binding of an effector cell to a target antigen provides a powerful stimulant to the effector cell; this may explain why we found no evidence that clinical anesthesia impairs MNC ADCC. Anesthesia exposure also failed to affect PMN ADCC, which we have previously shown is depressed after in vivo opioid exposure (9). This latter finding of sustained ADCC activity after anesthesia suggests that clinical studies of postoperative immunity should be interpreted with specific reference to the presence or absence of opioids. Vose and Moudgil (18), for example, reported impaired MNC ADCC after minor surgery under GA, but their use of opioids may have affected their results.

Although circulating PMNs usually increase after injury, there may be a decline in PMN activity during postoperative recovery (4,5). In vitro studies of human whole blood also document depression of PMN oxidative and phagocytic activity by exposure to IV anesthetics (19,20). In vivo animal studies provide even more convincing evidence that anesthetics can affect circulating PMN activity. Shayevitz et al. (21) showed that clinical concentrations of halothane and isoflurane, administered for six hours, could significantly reduce lung injury and lung myeloperoxidase activity (a marker of PMN recruitment) in a rat model of zymosan-induced multiorgan failure. Miller et al. (22) reported a similar finding by using a rat mesenteric vein model of PMN adherence induced by cytokine stimulation. These studies used clinical concentrations of the same GAs we studied (thiopental and isoflurane) to show a diminution in PMN-endothelial cell interaction, although they left open the question of whether the observed effect was on the PMN or the endothelial cell, or perhaps both. Our data show that, despite these effects, PMN phagocytic activity remains unaltered in unstimulated humans.

Surgery and anesthesia clearly lead to a decrement in NK activity that persists for several days to weeks after surgery (23). Tnnesen et al. (23) reported a transient increase in NK activity beginning just before orthopedic surgery and continuing during surgery, followed by significant depression after surgery. This increase in NK activity may have been caused by stress-induced recruitment of NK cells into the central circulation, as has been reported (24). Our results were similar to these findings because we showed that spontaneous NK activity and antibody-dependent activity of CD16-positive MNCs (primarily NK cells) were increased transiently in participants who received epidural anesthesia. This effect may have been caused by a psychological stress from the epidural procedure that was unabated by anesthesia and was similar to the increases in NK activity shown in volunteers who receive IV catecholamines (24) or who experience major psychological stress (25). In contrast, Markovic et al. (11) reported an in vivo inhibition of -interferon-stimulated NK activity by inhaled anesthetics that persisted for 10 days when animals were exposed to inhaled anesthesia before NK cell stimulation. These results were reported in a mixed splenocyte cell population in which cell-cell interactions may have affected their results. The results reported here suggest that anesthesia exposure alone does not significantly affect human immune responsiveness.

	Acknowledgments

 
Supported by National Institutes of Health Grant DA-09162 (MPY).

	Footnotes

 
Presented in part at the 71st Clinical and Scientific Congress, International Anesthesia Research Society, San Francisco, CA, March 14–18, 1997.

	References

Top Abstract Introduction Methods Results Discussion References

 

1. Lin E, Calvano SE, Lowry SF. Inflammatory cytokines and cell response in surgery. Surgery 2000; 127: 117–26.[Web of Science][Medline]
2. Wahl SM, Feldman GM, McCarthy JB. Regulation of leukocyte adhesion and signaling in inflammation and disease. J Leukoc Biol 1996; 59: 789–96.[Abstract]
3. Salo M. Effects of anaesthesia and surgery on the immune response. Acta Anaesthesiol Scand 1992; 36: 201–20.[Web of Science][Medline]
4. Faist E, Schinkel C, Zimmer S. Update on the mechanisms of immune suppression of injury and immune modulation. World J Surg 1996; 20: 454–9.[Web of Science][Medline]
5. Simms HH, D’Amico R. Posttraumatic auto-oxidative polymorphonuclear neutrophil receptor injury predicts the development of nosocomial infection. Arch Surg 1997; 132: 171–7.[Abstract/Free Full Text]
6. Tubaro E, Borelli C, Croce C, et al. Effect of morphine on resistance to infection. J Infect Dis 1983; 148: 656–66.[Web of Science][Medline]
7. Adamik B, Zimecki M, Wlaszczyk A. Immunological status of septic and trauma patients. II. Proliferative response and production of interleukin 6 and tumor necrosis factor alpha by peripheral blood mononuclear cells from septic survivor, nonsurvivor and trauma patients: a correlation with the survival rate. Arch Immunol Ther Exp (Warsz) 1997; 45: 277–84.[Medline]
8. Yeager MF, Colacchio TA, Yu CT, et al. Morphine inhibits spontaneous and cytokine-enhanced natural killer cell function in human volunteers. Anesthesiology 1995; 83: 500–8.[Web of Science][Medline]
9. Yeager MF, Yu CT, Campbell AS, et al. Effect of morphine and ß-endorphin on human Fc receptor-dependent and natural killer cell functions. Clin Immunol Immunopathol 1992; 62: 336–43.[Web of Science][Medline]
10. Nashina K, Akamatsu H, Mikawa K, et al. The inhibitory effects of thiopental, midazolam, and ketamine on human neutrophil function. Anesth Analg 1998; 86: 159–65.[Abstract]
11. Markovic SN, Knight FR, Murasko DM. Inhibition of interferon stimulation of natural killer cell activity in mice anesthetized with halothane or isoflurane. Anesthesiology 1993; 78: 700–6.[Web of Science][Medline]
12. Hansbrough JF, Zapata-Sirvent RL, Anderson JK, et al. Alterations in splenic lymphocyte subpopulations and increased mortality from sepsis following anesthesia in mice. Anesthesiology 1985; 63: 267–73.[Web of Science][Medline]
13. Duncan PG, Cullen BF, Calverly R, et al. Failure of enflurane and halothane anesthesia to inhibit lymphocyte transformation in volunteers. Anesthesiology 1976; 45: 661–5.[Web of Science][Medline]
14. Doenicke A, Grote B, Suttmann H, et al. Effects of halothane on the immunological system in healthy volunteers. Clin Res Rev 1981; 1: 23–5.
15. Weiner LM, Homes M, Richeson A, et al. Binding and cytotoxicity characteristics of the bispecific murine monoclonal antibody 2B1. J Immunol 1993; 151: 2877–86.[Abstract]
16. Lockwood LL, Silbert LH, Laudenslager ML, et al. Anesthesia-induced modulation of in vivo antibody levels: a study of pentobarbital, chloral hydrate, methoxyflurane, halothane, and ketamine/xylazine. Anesth Analg 1993; 77: 769–74.[Abstract/Free Full Text]
17. Jackson LA, Benson F, Sneller VP, et al. Safety of revaccination with pneumococcal polysaccharide vaccine. JAMA 1999; 281: 243–8.[Abstract/Free Full Text]
18. Vose BM, Moudgil GC. Post-operative depression of antibody-dependent lymphocyte cytotoxicity following minor surgery and anaesthesia. Immunology 1976; 30: 123–8.[Web of Science][Medline]
19. Stevenson GW, Hall SC, Rudnick S, et al. The effect of anesthetic agents on the human immune response. Anesthesiology 1990; 72: 542–52.[Web of Science][Medline]
20. Heller A, Heller S, Blecken S, et al. Effects of intravenous anesthetics on bacterial elimination in human whole blood. Acta Anaesthesiol Scand 1998; 42: 518–26.[Web of Science][Medline]
21. Shayevitz JR, Varani J, Ward PA, et al. Halothane and isoflurane increase pulmonary artery endothelial cell sensitivity to oxidant-mediated injury. Anesthesiology 1991; 74: 1067–77.[Web of Science][Medline]
22. Miller LS, Rangan MY, Kondo S, et al. Suppression of cytokine-induced neutrophil accumulation in rat mesenteric venules in vivo by general anesthesia. Int J Microcirc Clin Exp 1996; 16: 147–54.[Web of Science][Medline]
23. Tonnesen E, Mickley H, Grunnet N. Natural killer cell activity during premedication, anaesthesia and surgery. Acta Anaesthesiol Scand 1983; 27: 238–41.[Web of Science][Medline]
24. Schedlowski M, Falk A, Rohne A, et al. Catecholamines induce alterations of distribution of human natural killer (NK) cells. J Clin Immunol 1993; 13: 344–51.[Web of Science][Medline]
25. Schedlowski M, Jacobs R, Stratmann G, et al. Changes of natural killer cells during acute psychological stress. J Clin Immunol 1993; 13: 119–26.[Web of Science][Medline]

This article has been cited by other articles:

				L. M. Backhus, E. Sievers, G. Y. Lin, R. Castanos, R. D. Bart, V. A. Starnes, and R. M. Bremner Perioperative cyclooxygenase 2 inhibition to reduce tumor cell adhesion and metastatic potential of circulating tumor cells in non-small cell lung cancer. J. Thorac. Cardiovasc. Surg., August 1, 2006; 132(2): 297 - 303. [Abstract] [Full Text] [PDF]

				M. P. Yeager, M. A. Procopio, J. A. DeLeo, J. L. Arruda, L. Hildebrandt, and A. L. Howell Intravenous Fentanyl Increases Natural Killer Cell Cytotoxicity and Circulating CD16+ Lymphocytes in Humans Anesth. Analg., January 1, 2002; 94(1): 94 - 99. [Abstract] [Full Text] [PDF]

This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a colleague Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (12) Citing Articles via Google Scholar Google Scholar Articles by Procopio, M. A. Articles by Yeager, M. P. Search for Related Content PubMed PubMed Citation Articles by Procopio, M. A. Articles by Yeager, M. P. Related Collections Regional Anesthesia Pharmacology

Anesthesia & Analgesia® is published for the International Anesthesia Research Society® by Lippincott Williams & Wilkins with the assistance of Stanford University Libraries' HighWire Press®. Copyright 2006 by the International Anesthesia Research Society. Online ISSN: 1526-7598   Print ISSN: 0003-2999