- Email: [email protected]
Vitamin K2 for the reversal of warfarin-related coagulopathy
American Journal of Emergency Medicine 33 (2015) 1108.e1–1108.e3
Contents lists available at ScienceDirect
American Journal of Emergency Medicine journal homepage: www.elsevier.com/locate/ajem
Case Report
Vitamin K2 for the reversal of warfarin-related coagulopathy Abstract The American Heart Association/American College of Cardiology Foundation recommends vitamin K1 for warfarin-related coagulopathy. In Japan, vitamin K2 is used more commonly for such purpose. The difference between vitamins K1 and K2 in reversing warfarin-related coagulopathy has not been discussed. Herein, we report a case that was reversed with vitamin K2; alterations in vitamins K1 and K2 levels and coagulation markers are also presented. An 83-year-old woman, who was on chronic warfarin therapy at 2.5 mg/d for cerebral infarction, was admitted to our hospital because of low blood pressure at 60/40 mm Hg. She was noted to have severe anemia (hemoglobin level, 6.5 mg/dL), immeasurable prothrombin time international normalized ratio, abnormally prolonged (64.6 seconds) activated partial thromboplastin time, and a large hematoma on the upper arm and right chest wall. Initial vitamin K1 level was 0.12 ng/mL; vitamin K2 was less than 0.05 ng/mL. In addition to intensive care, she received intravenous administration of 20 mg vitamin K2, 4 U of fresh frozen plasma, and 6 U of packed red blood cells. On the second day, vitamin K1 level decreased to less than 0.05 ng/mL, whereas vitamin K2 increased to 14.36 ng/mL; prothrombin time international normalized ratio slightly prolonged, and activated partial thromboplastin time was normalized. Her hemodynamic status was gradually stabilized, and she was discharged after 7 days. Further studies will be required to determine the best reversal therapy among vitamin K1 alone, vitamin K2 alone, or both vitamins K1 and K2 for warfarin-related coagulopathy. Coagulopathy related to warfarin therapy can be reversed as per the guidelines of American Heart Association/American College of Cardiology Foundation [1]. Rapid reversal of coagulopathy is primarily warranted, and serious bleeding should be treated with 10 mg of vitamin K1 by slow intravenous infusion, with additional doses every 12 hours as needed. Supplemental transfusions of fresh frozen plasma (FFP) or prothrombin complex concentrate may be required, according to the urgency of the situation. Conversely, coagulopathy related to warfarin therapy has been commonly treated with vitamin K2, instead of vitamin K1, in Japan [2]. Compared with vitamin K1, vitamin K2 reduced mortality rate and incidence of coronary heart disease [3] and prevented warfarin-induced arterial calcification [4]. However, no reports have been discussed about the difference between vitamins K1 and K2 in reversing warfarinrelated coagulopathy. We report a case that was reversed with vitamin K2 and present the alterations in levels of vitamin K1 and K2 levels and coagulation markers. An 83-year-old woman was transferred to our hospital from a nursing home because of low blood pressure and consciousness disturbance. On admission, her blood pressure was less than 100 mm Hg, and she did not
0735-6757/© 2015 Elsevier Inc. All rights reserved.
respond to verbal stimuli in the afternoon on the same day. No obvious trauma history or appetite loss was noted. She had a history of hypertension and cerebral infarction, for which she was prescribed amlodipine 5 mg/d, carvedilol 10 mg/d, valsartan 80 mg/d, and warfarin 2.5 mg/d. Warfarin therapy was initiated approximately one and a half years earlier, and prothrombin time international normalized ratio (PT-INR) was well maintained at 1.6 to 2.6 therapeutic level. She had no known drug allergies, no recent antibiotic intake, and unremarkable family history. On presentation, she had a Glasgow Coma Scale of 8/15 (E3V1M4) and was hypotensive with blood pressure at 60/40 mm Hg; her body temperature was at 36.7°C; pulse rate, at 68 beats per minute; respiratory rate, at 18 breaths per minute; and oxygen saturation, at 100% on room air. There was subcutaneous hemorrhage around the right upper arm extending to the right side of the chest wall. Blood results indicated severe anemia (hemoglobin level, 6.5 mg/dL), leukocytosis (11 200/μL), and normal liver function. Activated partial thromboplastin time (APTT) was abnormally prolonged at 64.6 seconds, and PT-INR was immeasurable. Coagulation markers were monitored; serum vitamins K1 and K2 were also determined (Table). Computed tomographic scan showed a large hematoma on the right upper arm and right chest wall, but no extravasation was observed (Figure). With a diagnosis of warfarin-related coagulopathy, she was admitted to the intensive care unit. Intravenous vitamin K2 (20 mg) was administered, and 4 U FFP and 6 U packed red blood cells were transfused. We added another 20 mg of vitamin K2 for normalizing PT-INR. Starting on the second day, PT-INR improved, and APTT was kept within reference range. Subsequently, PT-INR returned to 1.5, and hemodynamic status was stabilized. On day 7, she was discharged and transferred back to the nursing home. In this study, the patient developed absolute vitamin K deficiency and coagulopathy from warfarin therapy; this was corrected by vitamin K2 and FFP, instead of vitamin K1. Absolute vitamin K deficiency primarily occurs in adults with hepatobiliary disease, small bowel resection, and antibiotic use [5,6]. In the current case, no obvious reasons for developing absolute vitamin K deficiency were observed, and warfarinrelated absolute vitamin K deficiency was confirmed. Vitamin K includes 2 natural vitamins: vitamin K1 and vitamin K2. Bacteria in the colon can convert vitamin K1 into vitamin K2, but conversion of vitamin K2 to vitamin K1 remains unknown in humans [7]. In addition, the difference of mechanism between vitamins K1 and K2 on warfarin reversal has not been examined. In the current case, vitamin K1 level had been kept below the reference range, in spite of remarkable elevation in vitamin K2 level, but as a consequence, international normalized ratio had been well controlled. Administration of aggressive vitamin K2 may be warranted for warfarin-related absolute vitamin K deficiency. We have proposed that serum vitamin K levels should be measured and absolute vitamin K deficiency should be initially treated with
1108.e2
T. Hifumi et al. / American Journal of Emergency Medicine 33 (2015) 1108.e1–1108.e3
Table Alterations in vitamin K1 and vitamin K2 levels and coagulation markers Time course (event)
On admission
2 h after admission (after vitamin K2 20 mg administered)
3 h after admission (after vitamin K2 20 mg followed by FFP 4 U administered)
Second day of admission
Vitamin K1 (ng/mL) Vitamin K2 (ng/mL) PT-INR APTT (s)
0.12 b0.05 Immeasurable 64.6
0.10 5627 14.55 62.6
0.12 1122.5 2.08 40.1
b0.05 14.36 1.46 36
Vitamin K1: reference range, 0.15 to 1.25 ng/mL; vitamin K2: reference range, b0.10 ng/mL.
aggressive administration of vitamin K and reversal agents in patients with coagulopathy related to warfarin [8,9]. Upon detailed research on this topic, we recognized that there was a big difference between the United States and Japan in vitamin K usage. The Japanese are more familiar with vitamin K2 than vitamin K1. The efficacy of vitamin K2 in the prevention of hepatocellular carcinoma in Japanese women with viral cirrhosis has been demonstrated [10]. Fermented soybean (natto), which contains high vitamin K2, is a staple consumption in Japanese daily breakfast [11]. However, the reason why vitamin K2 is commonly used to reverse warfarin-related coagulopathy is unclear. There were many international multicenter studies that compared safety among newly released anticoagulants and warfarin [12-14]. In these researches, bleeding events related to warfarin were reversed by vitamin K, but details were not provided. We suppose that a difference between vitamins K1 and K2 may affect outcomes. In this study, coagulopathy was reversed by vitamin K2 followed by FFP. The imbalance between vitamin K1 and vitamin K2 levels may indicate that balanced supplementation of both vitamin K types could lead to better management of bleeding complications. Further studies will be required to determine which is the best reversal therapy among vitamin K1 alone, vitamin K2 alone, and both vitamins K1 and K2.
Hiroaki Takada, MD Department of Critical Care Medicine and Trauma National Hospital Organization Disaster Medical Center, Tokyo, 190-0014, Japan E-mail address: [email protected]
Toru Hifumi, MD Kagawa University Hospital, Emergency Medical Center Kita, Kagawa, 761-0793, Japan Corresponding author. 1750-1, Miki, Kita, Kagawa, 761-0793, Japan Tel.: +81 87 891 2392; fax: +81 87 891 2393 E-mail addres: [email protected]
Yasuhiro Kuroda, MD Kagawa University Hospital, Emergency Medical Center Kita, Kagawa, 761-0793, Japan E-mail address: [email protected]
a
Daisuke Ogawa, MD Kenta Suzuki, MD Hideyuki Hamaya, MD Natsuyo Shinohara, MD Yuko Abe, MD Koshiro Takano, MD Kenya Kawakita, MD Masanobu Hagiike, MD Kagawa University Hospital, Emergency Medical Center Kita, Kagawa, 761-0793, Japan E-mail addresses: [email protected], fl[email protected] [email protected], [email protected] [email protected], [email protected] [email protected], [email protected] Yuichi Koido, MD Department of Critical Care Medicine and Trauma National Hospital Organization Disaster Medical Center, Tokyo, 190-0014, Japan E-mail address: [email protected]
http://dx.doi.org/10.1016/j.ajem.2015.01.002
b
c
Figure. Computed tomographic images on admission. Coronal image (A) and axial images (B and C) demonstrate hematoma on the right upper arm and right side of the chest wall.
T. Hifumi et al. / American Journal of Emergency Medicine 33 (2015) 1108.e1–1108.e3
References [1] Hirsh J, Fuster V, Ansell J, Halperin JL. American Heart Association/American College of Cardiology Foundation guide to warfarin therapy. Circulation 2003;107: 1692–711. [2] Tajima T, Kawashima H, Kato Y, Ogo T, Miyao K. Therapeutic effect of menaquinone4 (K2) on hypoprothrombinemia caused by warfarin. Nihon yakurigaku zasshi Folia pharmacologica Japonica 1971;67:412–8. [3] Geleijnse JM, Vermeer C, Grobbee DE, Schurgers LJ, Knapen MH, van der Meer IM, et al. Dietary intake of menaquinone is associated with a reduced risk of coronary heart disease: the Rotterdam Study. J Nutr 2004;134:3100–5. [4] Spronk HM, Soute BA, Schurgers LJ, Thijssen HH, De Mey JG, Vermeer C. Tissuespecific utilization of menaquinone-4 results in the prevention of arterial calcification in warfarin-treated rats. J Vasc Res 2003;40:531–7. [5] Croucher C, Azzopardi D. Compliance with recommendations for giving vitamin K to newborn infants. BMJ 1994;308:894–5. [6] Zipursky A. Prevention of vitamin K deficiency bleeding in newborns. Br J Haematol 1999;104:430–7. [7] Thijssen HH, Vervoort LM, Schurgers LJ, Shearer MJ. Menadione is a metabolite of oral vitamin K. Br J Nutr 2006;95:260–6.
1108.e3
[8] Hifumi T, Takada H, Kiriu N. Letter by Hifumi et al regarding article, “Management and outcomes of major bleeding during treatment with dabigatran or warfarin”. Circulation 2014;130:e94. [9] Takada H, Hifumi T, Bunya N, Kiriu N, Kato H, Koido Y, et al. Acquired absolute vitamin K deficiency in a patient undergoing warfarin therapy. Am J Emerg Med 2014; 32:688.e1–2. [10] Habu D, Shiomi S, Tamori A, Takeda T, Tanaka T, Kubo S, et al. Role of vitamin K2 in the development of hepatocellular carcinoma in women with viral cirrhosis of the liver. JAMA 2004;292:358–61. [11] Tsukamoto Y, Ichise H, Kakuda H, Yamaguchi M. Intake of fermented soybean (natto) increases circulating vitamin K2 (menaquinone-7) and gamma-carboxylated osteocalcin concentration in normal individuals. J Bone Miner Metab 2000;18:216–22. [12] Patel MR, Mahaffey KW, Garg J, Pan G, Singer DE, Hacke W, et al. Rivaroxaban versus warfarin in nonvalvular atrial fibrillation. N Engl J Med 2011;365:883–91. [13] Connolly SJ, Ezekowitz MD, Yusuf S, Eikelboom J, Oldgren J, Parekh A, et al. Dabigatran versus warfarin in patients with atrial fibrillation. N Engl J Med 2009; 361:1139–51. [14] Schulman S, Kearon C, Kakkar AK, Mismetti P, Schellong S, Eriksson H, et al. Dabigatran versus warfarin in the treatment of acute venous thromboembolism. N Engl J Med 2009;361:2342–52.
Contents lists available at ScienceDirect
American Journal of Emergency Medicine journal homepage: www.elsevier.com/locate/ajem
Case Report
Vitamin K2 for the reversal of warfarin-related coagulopathy Abstract The American Heart Association/American College of Cardiology Foundation recommends vitamin K1 for warfarin-related coagulopathy. In Japan, vitamin K2 is used more commonly for such purpose. The difference between vitamins K1 and K2 in reversing warfarin-related coagulopathy has not been discussed. Herein, we report a case that was reversed with vitamin K2; alterations in vitamins K1 and K2 levels and coagulation markers are also presented. An 83-year-old woman, who was on chronic warfarin therapy at 2.5 mg/d for cerebral infarction, was admitted to our hospital because of low blood pressure at 60/40 mm Hg. She was noted to have severe anemia (hemoglobin level, 6.5 mg/dL), immeasurable prothrombin time international normalized ratio, abnormally prolonged (64.6 seconds) activated partial thromboplastin time, and a large hematoma on the upper arm and right chest wall. Initial vitamin K1 level was 0.12 ng/mL; vitamin K2 was less than 0.05 ng/mL. In addition to intensive care, she received intravenous administration of 20 mg vitamin K2, 4 U of fresh frozen plasma, and 6 U of packed red blood cells. On the second day, vitamin K1 level decreased to less than 0.05 ng/mL, whereas vitamin K2 increased to 14.36 ng/mL; prothrombin time international normalized ratio slightly prolonged, and activated partial thromboplastin time was normalized. Her hemodynamic status was gradually stabilized, and she was discharged after 7 days. Further studies will be required to determine the best reversal therapy among vitamin K1 alone, vitamin K2 alone, or both vitamins K1 and K2 for warfarin-related coagulopathy. Coagulopathy related to warfarin therapy can be reversed as per the guidelines of American Heart Association/American College of Cardiology Foundation [1]. Rapid reversal of coagulopathy is primarily warranted, and serious bleeding should be treated with 10 mg of vitamin K1 by slow intravenous infusion, with additional doses every 12 hours as needed. Supplemental transfusions of fresh frozen plasma (FFP) or prothrombin complex concentrate may be required, according to the urgency of the situation. Conversely, coagulopathy related to warfarin therapy has been commonly treated with vitamin K2, instead of vitamin K1, in Japan [2]. Compared with vitamin K1, vitamin K2 reduced mortality rate and incidence of coronary heart disease [3] and prevented warfarin-induced arterial calcification [4]. However, no reports have been discussed about the difference between vitamins K1 and K2 in reversing warfarinrelated coagulopathy. We report a case that was reversed with vitamin K2 and present the alterations in levels of vitamin K1 and K2 levels and coagulation markers. An 83-year-old woman was transferred to our hospital from a nursing home because of low blood pressure and consciousness disturbance. On admission, her blood pressure was less than 100 mm Hg, and she did not
0735-6757/© 2015 Elsevier Inc. All rights reserved.
respond to verbal stimuli in the afternoon on the same day. No obvious trauma history or appetite loss was noted. She had a history of hypertension and cerebral infarction, for which she was prescribed amlodipine 5 mg/d, carvedilol 10 mg/d, valsartan 80 mg/d, and warfarin 2.5 mg/d. Warfarin therapy was initiated approximately one and a half years earlier, and prothrombin time international normalized ratio (PT-INR) was well maintained at 1.6 to 2.6 therapeutic level. She had no known drug allergies, no recent antibiotic intake, and unremarkable family history. On presentation, she had a Glasgow Coma Scale of 8/15 (E3V1M4) and was hypotensive with blood pressure at 60/40 mm Hg; her body temperature was at 36.7°C; pulse rate, at 68 beats per minute; respiratory rate, at 18 breaths per minute; and oxygen saturation, at 100% on room air. There was subcutaneous hemorrhage around the right upper arm extending to the right side of the chest wall. Blood results indicated severe anemia (hemoglobin level, 6.5 mg/dL), leukocytosis (11 200/μL), and normal liver function. Activated partial thromboplastin time (APTT) was abnormally prolonged at 64.6 seconds, and PT-INR was immeasurable. Coagulation markers were monitored; serum vitamins K1 and K2 were also determined (Table). Computed tomographic scan showed a large hematoma on the right upper arm and right chest wall, but no extravasation was observed (Figure). With a diagnosis of warfarin-related coagulopathy, she was admitted to the intensive care unit. Intravenous vitamin K2 (20 mg) was administered, and 4 U FFP and 6 U packed red blood cells were transfused. We added another 20 mg of vitamin K2 for normalizing PT-INR. Starting on the second day, PT-INR improved, and APTT was kept within reference range. Subsequently, PT-INR returned to 1.5, and hemodynamic status was stabilized. On day 7, she was discharged and transferred back to the nursing home. In this study, the patient developed absolute vitamin K deficiency and coagulopathy from warfarin therapy; this was corrected by vitamin K2 and FFP, instead of vitamin K1. Absolute vitamin K deficiency primarily occurs in adults with hepatobiliary disease, small bowel resection, and antibiotic use [5,6]. In the current case, no obvious reasons for developing absolute vitamin K deficiency were observed, and warfarinrelated absolute vitamin K deficiency was confirmed. Vitamin K includes 2 natural vitamins: vitamin K1 and vitamin K2. Bacteria in the colon can convert vitamin K1 into vitamin K2, but conversion of vitamin K2 to vitamin K1 remains unknown in humans [7]. In addition, the difference of mechanism between vitamins K1 and K2 on warfarin reversal has not been examined. In the current case, vitamin K1 level had been kept below the reference range, in spite of remarkable elevation in vitamin K2 level, but as a consequence, international normalized ratio had been well controlled. Administration of aggressive vitamin K2 may be warranted for warfarin-related absolute vitamin K deficiency. We have proposed that serum vitamin K levels should be measured and absolute vitamin K deficiency should be initially treated with
1108.e2
T. Hifumi et al. / American Journal of Emergency Medicine 33 (2015) 1108.e1–1108.e3
Table Alterations in vitamin K1 and vitamin K2 levels and coagulation markers Time course (event)
On admission
2 h after admission (after vitamin K2 20 mg administered)
3 h after admission (after vitamin K2 20 mg followed by FFP 4 U administered)
Second day of admission
Vitamin K1 (ng/mL) Vitamin K2 (ng/mL) PT-INR APTT (s)
0.12 b0.05 Immeasurable 64.6
0.10 5627 14.55 62.6
0.12 1122.5 2.08 40.1
b0.05 14.36 1.46 36
Vitamin K1: reference range, 0.15 to 1.25 ng/mL; vitamin K2: reference range, b0.10 ng/mL.
aggressive administration of vitamin K and reversal agents in patients with coagulopathy related to warfarin [8,9]. Upon detailed research on this topic, we recognized that there was a big difference between the United States and Japan in vitamin K usage. The Japanese are more familiar with vitamin K2 than vitamin K1. The efficacy of vitamin K2 in the prevention of hepatocellular carcinoma in Japanese women with viral cirrhosis has been demonstrated [10]. Fermented soybean (natto), which contains high vitamin K2, is a staple consumption in Japanese daily breakfast [11]. However, the reason why vitamin K2 is commonly used to reverse warfarin-related coagulopathy is unclear. There were many international multicenter studies that compared safety among newly released anticoagulants and warfarin [12-14]. In these researches, bleeding events related to warfarin were reversed by vitamin K, but details were not provided. We suppose that a difference between vitamins K1 and K2 may affect outcomes. In this study, coagulopathy was reversed by vitamin K2 followed by FFP. The imbalance between vitamin K1 and vitamin K2 levels may indicate that balanced supplementation of both vitamin K types could lead to better management of bleeding complications. Further studies will be required to determine which is the best reversal therapy among vitamin K1 alone, vitamin K2 alone, and both vitamins K1 and K2.
Hiroaki Takada, MD Department of Critical Care Medicine and Trauma National Hospital Organization Disaster Medical Center, Tokyo, 190-0014, Japan E-mail address: [email protected]
Toru Hifumi, MD Kagawa University Hospital, Emergency Medical Center Kita, Kagawa, 761-0793, Japan Corresponding author. 1750-1, Miki, Kita, Kagawa, 761-0793, Japan Tel.: +81 87 891 2392; fax: +81 87 891 2393 E-mail addres: [email protected]
Yasuhiro Kuroda, MD Kagawa University Hospital, Emergency Medical Center Kita, Kagawa, 761-0793, Japan E-mail address: [email protected]
a
Daisuke Ogawa, MD Kenta Suzuki, MD Hideyuki Hamaya, MD Natsuyo Shinohara, MD Yuko Abe, MD Koshiro Takano, MD Kenya Kawakita, MD Masanobu Hagiike, MD Kagawa University Hospital, Emergency Medical Center Kita, Kagawa, 761-0793, Japan E-mail addresses: [email protected], fl[email protected] [email protected], [email protected] [email protected], [email protected] [email protected], [email protected] Yuichi Koido, MD Department of Critical Care Medicine and Trauma National Hospital Organization Disaster Medical Center, Tokyo, 190-0014, Japan E-mail address: [email protected]
http://dx.doi.org/10.1016/j.ajem.2015.01.002
b
c
Figure. Computed tomographic images on admission. Coronal image (A) and axial images (B and C) demonstrate hematoma on the right upper arm and right side of the chest wall.
T. Hifumi et al. / American Journal of Emergency Medicine 33 (2015) 1108.e1–1108.e3
References [1] Hirsh J, Fuster V, Ansell J, Halperin JL. American Heart Association/American College of Cardiology Foundation guide to warfarin therapy. Circulation 2003;107: 1692–711. [2] Tajima T, Kawashima H, Kato Y, Ogo T, Miyao K. Therapeutic effect of menaquinone4 (K2) on hypoprothrombinemia caused by warfarin. Nihon yakurigaku zasshi Folia pharmacologica Japonica 1971;67:412–8. [3] Geleijnse JM, Vermeer C, Grobbee DE, Schurgers LJ, Knapen MH, van der Meer IM, et al. Dietary intake of menaquinone is associated with a reduced risk of coronary heart disease: the Rotterdam Study. J Nutr 2004;134:3100–5. [4] Spronk HM, Soute BA, Schurgers LJ, Thijssen HH, De Mey JG, Vermeer C. Tissuespecific utilization of menaquinone-4 results in the prevention of arterial calcification in warfarin-treated rats. J Vasc Res 2003;40:531–7. [5] Croucher C, Azzopardi D. Compliance with recommendations for giving vitamin K to newborn infants. BMJ 1994;308:894–5. [6] Zipursky A. Prevention of vitamin K deficiency bleeding in newborns. Br J Haematol 1999;104:430–7. [7] Thijssen HH, Vervoort LM, Schurgers LJ, Shearer MJ. Menadione is a metabolite of oral vitamin K. Br J Nutr 2006;95:260–6.
1108.e3
[8] Hifumi T, Takada H, Kiriu N. Letter by Hifumi et al regarding article, “Management and outcomes of major bleeding during treatment with dabigatran or warfarin”. Circulation 2014;130:e94. [9] Takada H, Hifumi T, Bunya N, Kiriu N, Kato H, Koido Y, et al. Acquired absolute vitamin K deficiency in a patient undergoing warfarin therapy. Am J Emerg Med 2014; 32:688.e1–2. [10] Habu D, Shiomi S, Tamori A, Takeda T, Tanaka T, Kubo S, et al. Role of vitamin K2 in the development of hepatocellular carcinoma in women with viral cirrhosis of the liver. JAMA 2004;292:358–61. [11] Tsukamoto Y, Ichise H, Kakuda H, Yamaguchi M. Intake of fermented soybean (natto) increases circulating vitamin K2 (menaquinone-7) and gamma-carboxylated osteocalcin concentration in normal individuals. J Bone Miner Metab 2000;18:216–22. [12] Patel MR, Mahaffey KW, Garg J, Pan G, Singer DE, Hacke W, et al. Rivaroxaban versus warfarin in nonvalvular atrial fibrillation. N Engl J Med 2011;365:883–91. [13] Connolly SJ, Ezekowitz MD, Yusuf S, Eikelboom J, Oldgren J, Parekh A, et al. Dabigatran versus warfarin in patients with atrial fibrillation. N Engl J Med 2009; 361:1139–51. [14] Schulman S, Kearon C, Kakkar AK, Mismetti P, Schellong S, Eriksson H, et al. Dabigatran versus warfarin in the treatment of acute venous thromboembolism. N Engl J Med 2009;361:2342–52.