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CARDIOVASCULAR ANESTHESIA

Measurement of Patients' Bivalirudin Plasma Levels by a Thrombelastograph® Ecarin Clotting Time Assay: A Comparison to a Standard Activated Clotting Time

Departments of Anesthesiology and Medicine, University of Tennessee Graduate School of Medicine, Knoxville, Tennessee; Haemoscope Corporation, Niles, Illinois

Address correspondence and reprint requests to Roger C. Carroll, PhD, University of Tennessee Graduate School of Medicine, 1924 Alcoa Highway, Knoxville, TN 37920. Address e-mail to RCarrol1{at}utk.edu.

	Abstract

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Standard activated clotting time (ACT) tests have a poor correlation to bivalirudin levels, leading to uncertainty regarding adequate anticoagulation in percutaneous coronary intervention patients. We tested a Thrombelastograph® (TEG®) ecarin clotting time (ECT) assay for sensitivity to bivalirudin using blood from 80 patients undergoing interventional cardiology procedures with bivalirudin anticoagulation. This was compared to a standard Hemochron ACT assay using diatomaceous earth. With the TEG® assay, the direct thrombin activator, ecarin, was used to initiate coagulation and measured as the reaction time. Plasma samples were evaluated for bivalirudin by a chromogenic assay at an independent hematological laboratory. Linear regression of the standard ACT versus bivalirudin level gave an r² = 0.306 whereas the TEG® ECT gave a much higher r² = 0.746 (both P < 0.0001). The TEG® ECT should prove more useful than the standard ACT for monitoring bivalirudin anticoagulation across the clinically therapeutic range.

	Introduction

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Despite a rather narrow therapeutic dosing range and a lack of a ready antidote, bivalirudin is more widely used in percutaneous coronary interventional procedures in place of heparin because it has a more predictable anticoagulant effect (1). Several studies have shown that standard activated clotting time (ACT) tests based on kaolin do not accurately reflect anticoagulation by bivalirudin at larger doses, raising the possibility of overdosing (2–6). Ecarin-based tests have been suggested as being better than ACT or other standard coagulation tests because ecarin directly activates prothrombin to a meizo-thrombin form that has less feedback procoagulant activity than thrombin (2,5).

The Thrombelastograph® (TEG®) Hemostasis System has been suggested as an alternative method of monitoring bivalirudin anticoagulation during bypass surgery (7). In this study we compared the correlations to plasma bivalirudin concentrations in patients undergoing catheterization to a diatomaceous earth based ACT and an ecarin-based TEG® ecarin clotting time (ECT) test.

	Methods

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This protocol and patient consent forms were approved by an IRB acting in accordance with the Helsinki Declarations. Eighty candidates for interventional cardiology procedures were recruited after obtaining informed consent. Patients were excluded if they had a history of clotting/bleeding problems, hypersensitivity to the drug, or decreased liver or kidney function. Ten mL of blood was drawn into sodium citrate Vacutainer® tubes after hospital admission but before administration of additional anticoagulants. Another 10-mL sample, drawn into sodium citrate Vacutainer® tubes, was taken from the arterial catheter sheath an average of 10 ± 5 min after bolus administration of bivalirudin (dosing was 0.75 mg/kg bolus, followed by 1.75 mg · kg^–1 · h^–1). At the same time the above samples were taken, a small amount of nonanticoagulated blood was drawn for standard ACT assays.

ACT assays were performed by catheterization laboratory personnel using a Hemochron 801 (International Technidyne, Edison, NJ) with FTCA510-4 ACT cartridges containing silica, phospholipids, and diatomaceous earth (kaolin). The TEG® ECT (Hemoscope Corporation, Niles IL) assay required recalcifying the 0.34 mL aliquot of citrated blood with 0.02 mL of 0.2 M calcium chloride that also contained the ecarin and mixing the blood in the TEG® cup by pipet per manufacturer's instruction. The reaction time until detectable clot formation (R) was chosen as the comparative TEG® ECT parameter. Plasma was isolated from the remaining citrated blood sample after completing the TEG® assays by centrifuging twice at 4°C for 15 min at 1000g. Plasma samples were stored at –70°C until shipment on dry ice to the Hemostasis Reference Laboratory (Hamilton, Ontario, Canada) for determination of bivalirudin levels by chromogenic assay. In this assay method, both thrombin (Enzyme Research Laboratories, South Bend IN) and antithrombin III (Affinity Biologicals Inc. Ancaster, ON, Canada) are added in excess. Therefore, residual thrombin activity, measured on a Diagnostica Stago STA Compact coagulation analyzer (Diagnostica Stago, Inc. Parsippany, NJ) using a thrombin-specific chromogenic substrate S-2238, is inversely proportional to the plasma bivalirudin concentration as determined on a standard spiked plasma calibration curve.

Statistical analysis and graphical representations were done with StatView 4.0 software (SAS Institute, Cary, NC). Power analysis with SPSS SamplePower 2.0 (SPSS Inc., Chicago, IL) determined that a sample size of 80 patients was sufficient for a power of >0.8 to find an r² > 0.30 with an = 0.01 and 2 predictors.

	Results

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The level of plasma bivalirudin after treatment was 6.5 ± 1.5 µg/mL (mean ± sd), with a range of 2 to 11 µg/mL. The corresponding anticoagulant effect on ACT and TEG® ECT before and after treatment is shown in Figure 1. Before treatment the average ACT was 127 ± 27 s and 289 ± 34 s (mean ± sd) after administration of bivalirudin. The TEG® ECT increased from 144 ± 109 s to 557 ± 105 s (mean ± sd) after bivalirudin treatment. A typical set of patient TEG® tracings before and after bivalirudin treatment is shown in Figure 2. The R value shows the greatest change in response to bivalirudin with smaller reductions in the time after the R point to reach a certain level of clot strength or angle of the rate of clot formation. With bivalirudin as the sole anticoagulant drug added between baseline and posttreatment, no significant changes were observed in the maximum amplitude of clot strength.

View larger version (23K): [in this window] [in a new window]   Figure 1. Box plot of patients' activated clotting time (ACT) and ecarin clotting time (ECT) before and after bivalirudin treatment. Box indicates interquartile range with the line inside box indicating the median value. Whiskers indicate 10th to 90th percentile range and observations outside this range are indicated as individual points.

View larger version (21K): [in this window] [in a new window]   Figure 2. Thombelastograph® ecarin clotting time traces before (A) and after (B) bivalirudin treatment.

The linear regression correlation of patients' plasma levels of bivalirudin with ACT after treatment is shown in Figure 3. Correlation of TEG® ECT with bivalirudin level is shown in Figure 4. The r² value for ACT was 0.306 (P < 0.0001) whereas the ecarin-based TEG® ECT gave a better correlation to bivalirudin levels with r² = 0.746 (P < 0.0001).

View larger version (23K): [in this window] [in a new window]   Figure 3. Linear regression correlation of activated clotting time (ACT) versus plasma bivalirudin levels.

View larger version (20K): [in this window] [in a new window]   Figure 4. Linear regression correlation of Thombelastograph® ecarin clotting time versus plasma bivalirudin levels.

	Discussion

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ACT gave a lesser correlation with plasma bivalirudin, likely the result of using an activator of the intrinsic pathway with thrombin feedback on the coagulation cascade. This may not accurately monitor direct thrombin inhibitors such as bivalirudin. A recent modification of the ACT has been correlated with a standard ACT and reported to extend the sensitivity to bivalirudin concentration from 10 to 30 µg/mL (6). This modified ACT test was not evaluated for its correlation with patient plasma levels of bivalirudin. Ecarin is a direct thrombin activator forming miezothrombin that does not as readily activate the coagulation cascade. Other ECT tests have been suggested to be more sensitive to direct thrombin inhibitors over a wider range of plasma levels (3–5). The most recent of these evaluations showed linear regression correlations of ECT and ACT values from intraprocedure samples with therapeutic bivalirudin levels in patients that were very similar to this study (5).

The distribution of plasma bivalirudin levels is very similar to those reported by Cho et al. (3). This may reflect variable consumption of bivalirudin in this patient population. Thus individualized dosing based on the TEG® ECT or a similar test might provide a more therapeutic degree of anticoagulation than standard dosing based solely on patient weight. However, larger clinical studies will have to be performed to determine if the TEG® ECT corresponds to in vivo anticoagulation. In addition, although not examined in this study, the TEG® ECT and other TEG® assays could also provide useful information on the degree of platelet inhibition by nonsteroidal antiinflammatory drugs (8), clopidogrel (9,10), and glycoprotein IIb/IIIa inhibitors (11,12), all of which would be of value to the anesthesiologist during emergent surgery procedures with patients treated with these anticoagulants or for monitoring bivalirudin levels in surgical procedures in which heparin anticoagulation is contraindicated.

	Footnotes

 
Accepted for publication January 9, 2006.

Supported, in part, by the University of Tennessee - Anesthesiology Research Fund and Haemoscope Corporation.

	References

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