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Heparin and Warfarin: Use of Anticoagulants in the Prevention and Treatment of Venous Thrombosis and Pulmonary Embolism

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Heparin and Warfarin: Use of Anticoagulants in the Prevention and Treatment of Venous Thrombosis and Pulmonary Embolism Heparin and Warfarin: Use of Anticoagulants in the Prevention and Treatment of Venous Thrombosis and Pulmonary Embolism Rich L. Rosen, MD Amherst, Massachusetts The physician frequently encounters the problems of deep vein thrombosis and pulmonary embolism. Recently, a number of studies have been published which are of considerable help in the management of these disorders. It has been shown that in many cases, low-dose heparin is effective in the prevention of both venous thrombosis and pulmonary embolism. However, once venous thrombosis has already occurred, it is necessary to use full-dose heparin, preferably by the continuous intravenous route, with maintenance of the partial thrombo­ plastin time (PTT) at IV2 times the control at all times. Al­ though monitoring the PTT may not prevent hemorrhage, it will help prevent further thrombosis. Heparin is generally continued for seven to ten days. During this time warfarin is generally begun, and it is important to continue the patient on warfarin for five to seven days while the patient is receiving intravenous heparin therapy. After stopping heparin, oral anti­ coagulation with warfarin should be continued for six weeks. Then, in the absence of a previous history of venous throm­ boembolism or a known predisposing condition, it is safe to abruptly discontinue anticoagulation in most patients. Venous thrombosis and pulmonary embolism edge and to address several key questions by are problems frequently encountered by the reference to a number of recent studies. physician. Recently much attention has been fo­ The following questions will be addressed re­ cused on both the prevention and definitive treat­ garding anticoagulant therapy: (1) What is the ment of these disorders. However, there continues mechanism of action of heparin? (2) What is the to be much confusion over low vs high dosage of rationale behind the use of low-dose heparin? (3) heparin, length of therapy, and many other aspects What is the evidence that low-dose heparin is ef­ of heparin and warfarin usage. This brief article is fective in preventing venous thrombosis and pul­ designed to summarize the current state of knowl- monary embolism? (4) Who should receive low- dose heparin therapy? (5) Is continuous heparin om the Department of Family and Community Medicine, preferable to intermittent heparin in full anticoagu­ mversity of Missouri, Columbia, Missouri. Requests for reprints should be addressed to Dr. Rich L. Rosen, 199 Lin­ lation? (6) For prevention of recurrent deep vein coln Avenue, Amherst, MA 01002. thrombosis, how high should the partial thrombo- 0094-3509/79/050923-05$01.25 ® 1979 Appleton-Century-Crofts THE JOURNAL OF FAMILY PRACTICE, VOL. 8, NO. 5: 923-927, 1979 923 HEPARIN AND WARFARIN plastin time (PTT) be? (7) Will control of the PTT surgical patients have demonstrated, with only prevent hemorrhage? (8) What is the correlation one exception, that there is a significant decrease between the PTT, the Lee-White clotting time, of deep vein thrombosis in the treated compared to and plasma heparin assay? (9) After pulmonary the nontreated groups.3 However, acceptance of embolism treated initially with heparin and fol­ the use of low-dose heparin was delayed until it lowed by warfarin, how long should warfarin be could be shown that in addition to reducing deep given before it is safe to discontinue the heparin? vein thrombosis, it significantly reduced the inci­ (10) In a patient being treated with both heparin dence of pulmonary embolism. In 1975, Kakkar and warfarin, how can the anticoagulant effect of and associates organized a large study involving warfarin alone be determined? (11) After acute 4,121 patients over the age of 40 years who were deep vein thrombosis or pulmonary embolism, undergoing a major abdominal or thoracic proce­ how long should oral anticoagulation be contin­ dure.4 In this study, there was a significant de­ ued? (12) Is it safe to abruptly discontinue oral crease in postoperative deaths due to pulmonary anticoagulation? embolism in the group treated with low-dose heparin. Although the treated group had a greater incidence of wound hematomas, there was no in­ creased risk of serious hemorrhage in this group as compared to the group receiving no heparin. In Literature Review conclusion, the authors of this study claimed that The exact mode of action of low dose heparin low-dose heparin prophylaxis “ is highly effective prophylaxis is not completely defined. However, in preventing postoperative fatal pulmonary em­ much information is available regarding the effect bolism. It is well tolerated by the patient, and re­ of heparin on the coagulation mechanism, and quires no laboratory control to regulate the dos­ from this information it can be seen how heparin in age. .This form of prophylaxis against venous low dose could theoretically prevent thrombosis, thromboembolism can now be recommended for whereas much higher doses are necessary once use on a large scale in high-risk patients undergo­ thrombosis has actually occurred. Heparin acts by ing major surgery.” Currently, low-dose heparin binding to a plasma protein, antithrombin III, prophylaxis is recommended for patients over 40 markedly enhancing the ability of this protein to years of age undergoing major surgery with the neutralize various clotting factors.1 In particular, following exceptions: major orthopedic proce­ there is an extremely high affinity of the dures (especially of the hip), open prostatectomy, antithrombin-heparin complex for factor Xa. The and in brain or eye surgery.5'7 Low-dose heparin enzymatic coagulation system acts as a biological has not been proven to be uniformly effective in amplification system, and inhibition of small major orthopedic or prostatic surgery. In amounts of factor Xa blocks the formation of large neurosurgical or ophthalmological procedures, no amounts of later reaction products (Figure 1). For increased oozing of blood either operatively or example, it has been shown that 1 /xg of anti­ postoperatively can be tolerated. thrombin III will inhibit 32 units of Xa, which in The recommended dose for prophylaxis with turn prevents the generation of 1,600 units of low-dose heparin is 5,000 units of heparin adminis­ thrombin.2 Thus, it seems reasonable that in order tered two hours prior to surgery and repeated to interfere with the coagulation system at an ear­ every 8 to 12 hours thereafter, until the patient is lier stage, smaller doses of heparin would be re­ ambulatory.6 Heparin comes in various concen­ quired than if thrombin had already been gener­ trations, from 1,000 units/ml to 40,000 units/ml- ated. Once venous thrombi are present, it be­ The concentrated form of either 20,000 or 40,000 comes necessary to specifically neutralize throm­ units/ml is generally preferred for low-dose bin; for this purpose low-dose heparin is inappro­ prophylaxis, and should be injected subcutane­ priate, and much higher doses of heparin must be ously in the abdominal wall with a small, 25 to used. 26-gauge needle. With this understanding of the impact of small Very few trials of low-dose heparin have been doses of heparin, it is not surprising that since conducted in nonsurgical patients, and all of these 1971, more than 20 trials of low-dose heparin in have been in patients with acute myocardial in- 924 THE JOURNAL OF FAMILY PRACTICE, VOL 8, NO. 5, 1979 HEPARIN AND WARFARIN INTRINSIC PATHWAY EXTRINSIC PATHWAY Figure 1. Simplified diagram of the coagulation system. Heparin acts by binding to a plasma protein, enhancing the ability of this protein to neu­ tralize several of the clotting factors, especially factor Xa. The use of relatively low doses of heparin to inactivate a small amount of Xa pre­ vents the formation of much larger amounts of thrombin farction. These studies indicate a significant de­ generally preferable to intermittent intravenous in­ crease in the incidence of deep vein thrombosis,810 jection.11 In a controlled prospective trial, although but no data are available regarding the effect on there was no significant difference in the number pulmonic or systemic emboli. of thromboembolic complications between the two Low-dose heparin is useful in preventing the groups, major bleeding complications (including initial occurrence of thrombosis. In patients who wound hematoma, intracranial bleeding, hemo­ already have active thrombosis or pulmonary em­ thorax, hemarthrosis, and soft tissue hematoma) bolism, full-dose heparin, usually by the intraven­ were seven times greater in the intermittent than in ous route, is necessary to prevent further throm­ the continuous groups.12 In the continuous treat­ bus formation. Continuous heparin infusion is ment groups, there was a one percent incidence of THE JOURNAL OF FAMILY PRACTICE, VOL. 8, NO. 5, 1979 925 HEPARIN AND WARFARIN major bleeding, while in the intermittent treatment mental studies, the time necessary for this to occur groups, there was a six to ten percent incidence of is seven to ten days, and therefore heparin therapy major bleeding. has rather arbitrarily been continued for this length Continuous heparin therapy is usually initiated of time.16 Following treatment with heparin, with a bolus of 5,000 units of heparin followed by a prophylactic therapy with warfarin is usually be­ continuous infusion of heparin, starting at a rate of gun. After initiating warfarin therapy, a frequent 1,000 units/hr (preferably using a mechanical infu­ error is to assume that as soon as the prothrombin sion pump in order to avoid accidental overmedi­ time (PT) reaches the therapeutic range, heparin cation).13 Blood may then be drawn at any time for can be safely discontinued. The PT is very sensi­ monitoring purposes, and the infusion rate may tive to the level of factor VII, which is the first be altered accordingly. If intermittent heparin clotting factor to fall on warfarin therapy; how­ therapy is used, a standard dose of 5,000 units of ever, it may take up to a week for the other clot­ heparin given as a bolus intravenously every four ting factors affected by oral anticoagulants (factors hours is recommended,13 with adjustments based II, IX, and X) to fall to therapeutic levels.
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