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*Departments of Anesthesiology and
Surgery, University Hospital Hamburg-Eppendorf, Hamburg, Germany
Address correspondence and reprint requests to Thomas Standl, MD, Department of Anesthesiology, University Hospital Hamburg-Eppendorf, Martini Strasse 52, 20246 Hamburg, Germany. Address e-mail to standl{at}uke.uni-hamburg.de
| Abstract |
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IMPLICATIONS: The effects of three different hydroxyethyl starch (HES) solutions on hemodynamics, rheology, and skeletal muscle tissue tension after acute normovolemic hemodilution were examined in awake volunteers. With HES 130/0.4, increases of tissue oxygen tension in comparison to baseline were larger and more rapid than with HES 70/0.5 or HES 200/0.5.
| Introduction |
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The different HES solutions are characterized by their mean molecular weight (MW), concentration, and their degree and pattern of substitution, which means the number and location of hydroxyethyl groups at the glucose subunits. The hydroxyethyl residues, especially when bound at the C2 carbon position of glucose, hinder the plasma
-amylase to degrade the glucose polymers, hence increasing the intravascular half-life of the HES solution (10,11) .
Earlier studies have demonstrated that the effect of different HES solutions on the changes at the microcirculatory site and tissue oxygenation depends on these features. In a clinical trial in critically ill patients in the intensive care unit, only 500 mL of HES 70/0.5 was able to enhance the tpO2 in the skeletal muscle, whereas HES 200/0.5 resulted in unchanged tpO2, and HES 450/0.5 even decreased the tpO2 (12). The novel HES 130/0.4 differs with respect to MW (130 versus 70, 200, 450, and 470), degree (0.4 versus 0.50.7), and pattern of substitution (C2:C6 = 9:1 versus C2:C6 = 45:1) from often-used HES solutions. These differences in MW, degree, and pattern of substitution are responsible for a different in vivo molecular size after infusion of the respective HES formulation, which should have an impact on blood rheology and tissue oxygenation.
The present prospective, randomized, double-blinded, crossover study compares effects of the molecular design of different HES solutions (HES 130/0.4 versus HES 70/0.5 versus HES 200/0.5) on hemodynamics, rheology, and tissue oxygenation in volunteers undergoing ANH.
| Methods |
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According to the respective actual BW, the intravascular blood volume was calculated with 8% of BW for each individual. All volunteers were allocated once to each of the three HES groups by a special computerized randomization program (crossover design) that guaranteed that each individual received one of the three respective HES solutions no earlier than 8 days after the last treatment to prevent a carry-over effect within this period. All investigators and volunteers were blinded to the infused HES solutions, which were applied in a sterile uniform infusion bag. The following HES solutions were examined: 6% HES 130/0.4 (Voluven®, Fresenius Kabi, Bad Homburg, Germany), 6% HES 70/0.5 (Rheohes®, B. Braun, Melsungen, Germany), and 6% HES 200/0.5 (HAES-steril®, Fresenius Kabi).
Volunteers received one large-bore (2 mm) IV cannula at each forearm using local anesthesia before baseline measurements were started (M0). The left cannula was used for blood donation and collection of blood samples, whereas the right cannula served for the HES infusions exclusively. Within 30 min, each individual donated 18% of his or her own blood (according to the calculated blood volume), which was collected in sterile predonation bags (CPD-A1®, Sarstedt, Germany). In parallel, the respective HES solution was infused through the cannula on the opposite arm in a ratio of 1.2:1 to the donated blood volume. Thirty minutes after the respective HES infusion was completed, the second measurement was performed (M1). Further measurements of all variables were performed every 60 min until 6 h after completion of the HES infusion (M2M7). After the last measurement, volunteers received their donated blood and were discharged 1 h later if all vital variables were stable.
Variables recorded at every time of measurement (M0M7) were heart rate, systolic, diastolic, and mean arterial blood pressure, pulse oximetry, hematocrit (Hct), plasma colloid osmotic pressure (COP) and Vis, and skeletal muscle tpO2.
The Hct was determined after 5-min centrifugation of venous blood (5000 rpm) using a percentage volume scale. The Vis was measured after centrifugation of venous blood using a viscosimeter (Rheomat®, Fresenius Kabi), and COP was measured using an oncometer (BMT 851®, Thomae, Germany).
The tpO2 was measured in the M. vastus lateralis of the quadriceps muscle by a microprocessor-controlled fast responding polarographic needle probe of 12.5 µm in diameter (Eppendorf needle®, Helzel Medical Systems, Kaltenkirchen, Germany). After local anesthesia of the skin and subcutis with 5 mL of bupivacaine 0.5%, the needle probe was placed in a depth of 2 cm in the muscle yielding to 200 single tpO2 values in a conical muscular tissue area of 23 cm3 within 5 min. After each of the 20 single tpO2 measurements, the probe was automatically moved 0.7 mm forward followed by a 0.3-mm backward withdrawal within the respective muscular area to avoid tissue compression. After 200 tpO2 measurements, the probe was completely removed out of the leg, calibrated, and placed at a slightly different site in the muscle for the next measurement, resulting in 8 needle insertions in every volunteer per day. In each group, a total number of 12 (volunteers) x 200 (single tpO2) = 2400 tpO2 values were collected at every time of measurement. After completion of the study, the frequency distribution of all 2400 single tpO2 values were plotted as pool histograms, and the 10th, 50th, and 90th percentiles of tpO2 were calculated (Sigma-Po2-Histograph KIMOC 6650®, Helzel Medical Systems).
At M0 and M1, M3 and M6 venous blood samples were collected for measurement of the HES plasma concentrations (Fresenius Kabi laboratory) and for measurement of the platelet aggregation using a standardized test kit with 0.63 µg/mL and 1.25 µg/mL of ristocetin and 2 µg/mL of collagen, respectively (Blood Lumi-Aggregometer®, Chrono-Log, Havertown, PA). This whole blood aggregometry is based on impedance measurements after ristocetin and collagen platelet stimulation (13). For HES plasma concentration measurements, 10 mL of lithium-heparinized monovettes (Sarstedt) were withdrawn, immediately centrifuged, and stored at -30°C until determination. HES was precipitated using acetone hydrolyzed with trifluoracetic acid for the enzymatic determination of glucose from which the amount of HES was calculated.
Volunteers were allocated to each of the three groups by a computerized randomization program. Data are reported as mean ± SD if not otherwise declared. For normally distributed variables, differences within groups were tested by analysis of variance for repeated measurements and post hoc comparison by paired Students t-test. Differences among groups were tested by analysis of variance and post hoc comparison by unpaired Students t-test with Bonferroni correction for
. For the 10th, 50th, and 90th percentiles of each volunteer of each measurement consisting of 200 single measurements, mean and SD of the percentiles of each group was calculated. These data were compared within groups using two-tailed paired t-test to compare values versus baseline. In addition, Friedman test for multiple paired data was used for analysis of overall difference within groups for the 10th, 50th, and 90th percentiles. All differences were considered significant at P < 0.05.
Because no major difference of the absolute tpO2 values among groups were expected, we tested the hypothesis that the mean of the 10th percentile would increase significantly compared with baseline after 3 h after hemodilution in all groups. Based on recent studies, we expected baseline values to be approximately 20 ± 10 mm Hg and an increase of 15 mm Hg in all groups, with no difference larger than 15 mm Hg among groups. We would permit a type I error of
= 0.05, and with the alternate hypothesis, the null hypothesis would be retained with a type II error of ß = 0.1. This analysis reaches a power of 0.9 and indicated that a sample size of at least 11 patients per group was required. To compensate for missing data or potential dropouts, we set the group size to 12 volunteers per group.
| Results |
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| Discussion |
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HES 130 is comparable to HES 200 with regard to hemodynamic efficacy when used as an intraoperative volume substitute in cardiac and major abdominal surgery (1416) . However, coagulation alterations were smaller with HES 130 when compared with HES preparations with larger MW or degree of substitution (DS) (14,17,18) . In contrast, we did not see differences among the three tested HES solutions with respect to platelet aggregation in our study. However, we infused the HES solutions in healthy volunteers who had no intraoperative blood loss, and the applied volume of the respective HES volume of roughly 1.2 L was smaller when compared with these clinical or laboratory investigations. Thus, for less than a daily dose of 2 g/kg of BW (= 33 mL/kg of BW) of 6% HES, which is equivalent to a clinically recommended volume of 2 L/d, we did not expect significant coagulation disorders.
However, our results of the differences in tpO2 with different HES preparations show a significant impact of the HES characteristics MW, DS, and substitution pattern on clinical variables such as microcirculation and tissue oxygenation. These data are in accordance with an earlier study in critically ill patients where only HES 70 provided a significant increase of tpO2 within 90 min after the infusion, and HES 200 or HES 450 did not (12). Because HES 450 even decreased the tpO2 in this study and is also related to the most frequent incidence of coagulation disorders, reticulo-endothelial system uptake, and tissue storage (19), we chose HES 70 and HES 200 to compare the effects of the new HES 130. The HES 130/0.4 preparation was designed to further improve the molecular distribution profile in the intravascular space. The decrease in the DS to 0.4 leads to an increase in metabolic degradation, which is counteracted by the increased C2/C6 ratio (9:1 versus 4:1) of this HES preparation, preventing a too rapid decrease in plasma concentrations. The overall results are reduced tissue storage and absence of plasma accumulation after repetitive application (11,20) while preserving the intravascular volume effect (14,15,18) .
In addition, the MW distribution of HES 130 was narrowed by the reduction of the high- and low-MW fraction, with most of the molecules above the renal threshold. This technical process leads to an increase in the medium-size MW fraction. Because the HES dose and total HES plasma levels were not significantly different in our study, the differences in skeletal muscle tpO2 cannot be explained by the presence of a certain HES plasma concentration per se. One explanation for the earlier and larger increase in tpO2 after HES 130 is that the optimal molecular size for improved rheology and increased tissue oxygenation probably lies between 60,000 and 130,000 d and that this molecule size is provided to a greater extent by HES 130 than the other compounds within the first time after the infusion. Thus, HES 130 seems to deliver the most appropriate so-called in vivo MW to provide immediate increase of the tpO2. In contrast, significant parts of HES 70 with a MW <60,000 d are cleared by kidney excretion within the first 30 min. This can be assumed because the plasma concentrations in the HES 70 group tended to be smaller within the first hours after infusion in the HES 70 group. However, HES 200 has to be degraded to smaller molecules to develop its optimal rheologic effect within hours after infusion. Immediately after infusion, the plasma concentrations of HES 130 are similar to HES 200, lie exactly between HES 200 and HES 70 2 hours after infusion, and are comparable to HES 70 6 hours after infusion. Thus, HES 130 seems to be the optimum compromise between immediate effect in terms of improved rheology and increased tpO2 in the first hours after the infusion on the one side and reliable degradation and renal excretion (t1/2
= 1.4 hours and t1/2ß = 12 hours) on the other side, which prevents persisting plasma concentrations and reduces tissue storage of HES (21).
One study has also shown improved tissue oxygenation in the deltoid muscle of patients undergoing major abdominal surgery and volume replacement with HES 130 (22). In contrast, equivalent volumes of lactated Ringers solution decreased the muscular tpO2 within a period of 24 hours. In this study, a flexible polarographic probe was used that was inserted in the muscle and remained exactly at the same place during the hours of tpO2 measurement. In contrast, our device was inserted into the muscle at every time of measurement and slightly moved within this muscular area after every 20 single 200 tpO2 measurements, providing a much larger number of tpO2 readings and more representative tpO2 values from different sites of the respective muscle. Although the skeletal muscle seems to be less important than vital organs in terms of tissue oxygenation, the skeletal muscle mass represents a major part of human tissues, is easily accessible, and can be used for tpO2 measurements in patients (6,7,22) . With an experienced investigator, muscular tpO2 values are highly reproducible and suitable as a trend monitor to detect shifts to higher or lower tissue oxygen tensions and to indicate improved or deteriorated tissue oxygenation. However, no conclusions regarding the whole-body oxygenation or quality of the oxygen delivery of single organs can be drawn by the skeletal muscle tpO2, although animal studies have shown that hepatic and muscular tpO2 correlated during ANH (23).
In our setting with volunteers, we were not able to make statements about differences in stroke volume and cardiac output during ANH with the three different HES solutions because we declined to use invasive hemodynamic monitoring in these healthy young individuals. However, clinical studies in patients undergoing cardiac surgery revealed no differences in cardiac filling pressures and cardiac index between HES 130 and HES 200 (14,15) .
In addition, one might speculate that HES 200 recruited more water because of its higher COP, which could have resulted in a higher degree of hemodilution with improved rheology. However, Hct values were not smaller in the HES 200 group when compared with HES 70 or HES 130, making significant differences in blood Vis unlikely. In contrast to HES 70 and HES 130, the plasma Vis was increased in the HES 200 group, and this might have counteracted insignificant advantages of HES 200 in terms of improved rheology and tissue oxygenation.
In conclusion, the present study in volunteers undergoing ANH shows a larger and more rapid increase in skeletal muscle tpO2 over baseline values with HES 130/0.4 than with HES 70/0.5 or HES 200/0.5. This effect may be explained by the optimum in vivo molecular size for improved rheology and tissue oxygenation that was provided by HES 130 within the first hours after the infusion.
| Acknowledgments |
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The authors would like to thank all volunteers for taking part in this investigation and the staff of the intensive care unit (ANITO) of the Department of Anesthesiology for their help in taking care of the volunteers. We also thank Dr Bittner, O. Beck, and Dr Hildebrand of the Analytic Research Section of Fresenius Kabi for the analyses of the HES plasma concentrations.
| Footnotes |
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| References |
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