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CARDIOVASCULAR ANESTHESIA

Thromboelastography as a Perioperative Measure of Anticoagulation Resulting from Low Molecular Weight Heparin: A Comparison with Anti-Xa Concentrations

Stephen M. Klein, MD^*, Thomas F. Slaughter, MD^*, Parker T. Vail, MD, Brian Ginsberg, MB, ChB, FFA (SA)^*, Habib E. El-Moalem, PhD, Ratan Alexander, MBBS, FRCA, Francine D’Ercole, MD^*, Roy A. Greengrass, MD, FRCP^*, Thangam T. M. Perumal, MBBS, Ian Welsby, MBBS, FRCA^*, and Tong J. Gan, MBBS, FRCA, FFARCS(I)^*

Departments of *Anesthesiology, Surgery-Division of Orthopedics, and Division of Biometry, Duke University Medical Center, Durham, North Carolina

Address correspondence and reprint requests to Tong J. Gan, MD, Box 3094, Duke University Medical Center, Durham, NC 27710.

Low molecular weight heparin (LMWH) is commonly used to prevent postoperative thromboembolism. Currently, there is no convenient test to measure the degree of anticoagulation from LMWH. This prospective study examines the relationship of thromboelastography and serum anti-Xa concentration in patients treated with enoxaparin. Twenty-four adult patients scheduled for orthopedic surgery using epidural anesthesia were enrolled. Epidural catheters were removed the morning after surgery before the commencement of subcutaneous enoxaparin 30 mg twice daily. Venous blood samples were obtained at 1) the induction of anesthesia (baseline), 2) immediately before the third dose of enoxaparin postoperatively (Day 2-trough), 3) 4 h after the third dose postoperatively (Day 2-peak), and 4) immediately before the fifth dose postoperatively (Day 3-trough). Whole blood samples were obtained for thromboelastography, activated clotting time, and anti-Xa level analyses at each of the four time intervals. At the four sample intervals, the r time (mean ± SEM). (20 ± 1, 25 ± 2, 51 ± 6, 31 ± 3 mm) and the k time (9 ± 0.7, 12 ± 1, 27 ± 5, 14 ± 2 mm) of the thromboelastograph were significantly correlated with the expected peak and trough levels of LMWH and serum anti-Xa levels (P < 0.05). At the Day 3-trough, thromboelastograph r times exceeded the normal range in 6 of 25 patients (25%). Prolongation of r time and k time on postoperative Day 3 may indicate an exaggerated response to LMWH. Thromboelastography is a test that could potentially correlate with the degree of anticoagulation produced by low molecular weight heparin.

Implications: Thromboelastography is a test that could potentially correlate with the degree of anticoagulation produced by low molecular weight heparin. The r time from the thromboelastogram correlates with serum anti-Xa concentration.

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