patients with mechanical valve prosthesis , require long - term anticoagulation and aspirin administration because of the risk of thromboembolism , which is often greatest in the first postoperative year . without anticoagulants and aspirin , the incidence of thromboembolism is three - to six - fold higher than when proper doses of these medications are administered . abnormal flow conditions imposed by the prosthetic heart valves increase risk of thrombosis and embolism . the abnormal flow conditions caused by prosthetic valves are of two types : relative stagnation and high - velocity disturbed flow causing high shear stress . aortic valve prostheses are associated with high - velocity flow , high shear stress and relatively little stagnation . in contrast , mitral valve prostheses are associated with much more stagnation because the velocity of forward flow is much lower , and they are situated facing into the left atrium , which is an area of relative stagnation . these fundamental differences result in a higher incidence of both valve thrombosis and embolism after mitral valve replacement ( mvr ) than after aortic valve replacement ( avr ) . despite design changes and modifications that have been made to improve the longevity , hemodynamics , and thrombogenicity of newer generation mechanical valves , thromboembolism and anticoagulant - related bleeding continue to account for 75% of all complications after mechanical valve replacement . current practice suggests that warfarin therapy should begin about 2 days after prosthetic valve placement [ 3 , 5 ] . after mechanical avr , the goal of antithrombotic therapy is usually to achieve an international normalized ratio ( inr ) of 2.5 to 3.5 for the first 3 months after surgery and 2.0 to 3.0 beyond that time . low - dose aspirin ( 75 to 100 mg per day ) is also indicated in addition to warfarin . at that level of anticoagulation thrombosis and thromboembolism risks are greater with any mechanical valves in the mitral than the aortic position , and , therefore , higher inr levels ( 2.5 to 3.5 ) are generally recommended for mechanical mitral valve prostheses . homografts and most bioprosthetic valves in the aortic position do not require long - term anticoagulation , providing that the patient is in sinus rhythm and has adequate left ventricular systolic function . however , there is evidence that anticoagulant therapy for the first 3 months , until the sewing ring is endothelialized , reduces the higher incidence of embolism at this time . after 3 months , the tissue valve can be treated like native valve , and warfarin can be discontinued in more than two thirds of patients with biological valves . in the remaining patients , who have risk factors for thromboembolism ( table 1 ) , lifelong warfarin therapy , to achieve an inr of 2.0 to 3.0 , risk factors for prosthetic valve thrombosis previous thromboembolism . left ventricular dysfunction ( lvef < 30% ) . older - generation thrombogenic valves ( e.g. starr - edwards , and mechanical disc valves ) . antiplatelet agents without anticoagulants do not provide adequate protection to patients with mechanical valve prosthesis . however , the addition of aspirin , 80 to 160 mg daily , together with warfarin may reduce the risk of thromboembolism and should be given to all patients with mechanical prosthetic valves [ 4 , 8 ] . aspirin alone has been recommended as long - term prophylaxis for patients with bioprostheses to minimize platelet adhesion . risk factors for thrombosis ( table 1 ) , and underlying causes of hypercoagulability should be corrected if possible . atrial fibrillation is associated with a fivefold increase in the risk of stroke and , therefore , it is important to try to restore sinus rhythm . hypertension , obesity , and hyperlipidemia should be treated , cigarette smoking stopped , and advice is given about diet , exercise and the avoidance of potentially thrombogenic drugs . correcting these factors will substantially reduce the risk of thrombosis . in the patient who has a definite embolic episode while undergoing adequate antithrombotic therapy , the dosage of antithrombotic therapy should be increased , when clinically safe ( table 3 ) . in patients with mechanical valve prostheses who have experienced repeated systemic embolization despite optimal anticoagulation therapy , the incidence of embolization might be reduced by replacement with a tissue ( bioprosthetic ) valve . the risk of thrombosis of mechanical prostheses in the tricuspid valve position is quite high and , therefore , bioprostheses are preferred at this site . it must be recognized that the administration of warfarin carries its own mortality and morbidity , i.e. , serious haemorrhage . anticoagulation interruption in a patient with a prosthetic valve is potentially hazardous and may result in prosthetic valve thrombosis and thromboembolism . the risk is much higher for patients with mitral prostheses , for patients with congestive heart failure , and for patients with more thrombogenic prostheses . the risk of increased bleeding during a procedure performed with a patient receiving antithrombotic therapy has to be weighed against the increased risk of thromboembolism caused by stopping the therapy . in patients at low risk of thrombosis , defined as those with a bileaflet mechanical avr with no risk factors , it is recommended that warfarin be stopped 48 to 72 hours before the procedure ( so the inr falls to less than 1.5 ) and restarted within 24 hours after the procedure . bridging anticoagulant therapy is recommended for the higher- risk individuals including those with a mechanical mitral or tricuspid prosthesis or those with a mechanical aortic prosthesis who have risk factors ( table 1 ) . subcutaneous doses of ufh or low - molecular weight heparin lmwh ) may also be considered [ 4 , 10 ] . the decision to provide bridging therapy requires careful consideration of the relative risks of thromboembolism and bleeding in each patient . the acc / aha guidelines recommend bridging therapy in patients with mechanical valves who require interruption of warfarin therapy , as follows : in patients at high risk of thrombosis , defined as those with any mechanical mv replacement or a mechanical avr with any risk factor ( table 1 ) , therapeutic doses of intravenous ufh should be started when the inr falls below 2.0 ( typically 48 hours before surgery ) , stopped 4 to 6 hours before the procedure , restarted as early after surgery as bleeding stability allows , and continued until the inr is again therapeutic with warfarin therapy.in patients at high risk of thrombosis ( table 1 ) , therapeutic doses of subcutaneous ufh ( 15 000 u every 12 hours ) or lmwh ( 100 u per kg every 12 hours ) may be considered during the period of a subtherapeutic inr . lmwh is attractive because it is more easily used outside the hospital . 
  in patients at high risk of thrombosis , defined as those with any mechanical mv replacement or a mechanical avr with any risk factor ( table 1 ) , therapeutic doses of intravenous ufh should be started when the inr falls below 2.0 ( typically 48 hours before surgery ) , stopped 4 to 6 hours before the procedure , restarted as early after surgery as bleeding stability allows , and continued until the inr is again therapeutic with warfarin therapy . in patients at high risk of thrombosis ( table 1 ) , therapeutic doses of subcutaneous ufh ( 15 000 u every 12 hours ) or lmwh ( 100 u per kg every 12 hours ) may be considered during the period of a subtherapeutic inr . all pregnant patients with mechanical prosthetic valves must receive continuous therapeutic anticoagulation . in women with mechanical valves , a detailed discussion of the advantages and disadvantages of the three anticoagulant options ( warfarin , ufh and lmwh ) is indicated . warfarin ( vitamin k antagonist therapy ) is an effective antithrombotic agent in women during pregnancy . warfarin crosses the placenta and has been associated with an increased incidence of spontaneous abortion , prematurity , and stillbirth when the dose is > 5 mg / day . warfarin is probably safe during the first 6 weeks of gestation , but there is a risk of embryopathy if warfarin is taken between 6 and 12 weeks of gestation . warfarin is also relatively safe during the second and third trimesters of pregnancy but must be discontinued and switched to a heparin compound several weeks before delivery . several studies suggest that ufh or lmwh therapy is safe for the foetus [ 913 ] . heparin does not cross the placenta and does not have the potential to cause foetal bleeding or teratogenicity . thus , heparin is generally considered safer than warfarin during pregnancy in terms of the development of embryopathy . however , many experts questioned the efficacy of heparin in prevention of thromboembolic complications during pregnancy . numerous case series and patient registries attest to a high incidence of thromboembolic complications ( 12% to 24% ) , including fatal valve thrombosis , in high - risk pregnant women managed with subcutaneous ufh or lmwh [ 1316 ] . low - molecular - weight heparin ( lmwh ) is considered a recommended anticoagulation option in pregnant women with prosthetic heart valves . potential advantages of lmwhs over ufh during pregnancy cause less heparin - induced thrombocytopenia ; have a longer plasma half - life & a more predictable dose response ; easier to administer , with lack of need for laboratory monitoring ; associated with a lower risk of heparin - induced osteoporosis ; appear to have a low risk of bleeding complications . recommended therapeutic interventions in patients with prosthetic valves who suffered an embolic event while receiving adequate antithrombotic therapy . pregnant patients with mechanical prosthetic valves who elect to stop warfarin between weeks 6 and 12 of gestation should receive continuous intravenous ufh , dose - adjusted ufh ( to maintain aptt at least twice control ) , or dose - adjusted subcutaneous lmwh ( to maintain the anti - xa level between 0.7 and 1.2 units per ml , 4 hours after administration ) . in pregnant patients with mechanical prosthetic valves who receive warfarin , the inr goal should be 3.0 ( range 2.5 to 3.5 ) . in pregnant women with mechanical prosthetic valves , warfarin should be discontinued 23 weeks before planned delivery , and replaced with either a continuous intravenous ufh or a dose - adjusted lmwh . the american college of chest physicians conference on antithrombotic and thrombolytic therapy   concluded that it is reasonable to use one of the following 3 regimens : either lmwh or ufh between 6 and 12 weeks and close to term only , with warfarin used at other times;aggressive dose - adjusted ufh throughout pregnancy ; oraggressive adjusted - dose lmwh throughout pregnancy . either lmwh or ufh between 6 and 12 weeks and close to term only , with warfarin used at other times ; aggressive dose - adjusted ufh throughout pregnancy ; or aggressive adjusted - dose lmwh throughout pregnancy .
patients with mechanical valve prostheses require a lifelong anticoagulant treatment . 
 the combined use of warfarin and low - dose aspirin appears to reduce the risk of valve thrombosis and systemic embolism at a low risk of bleeding . 
 the management of women with prosthetic heart valves during pregnancy poses a particular challenge , as there are no available controlled clinical trials to provide guidelines for effective antithrombotic therapy . 
 oral anticoagulants , such as warfarin , cause foetal embryopathy ; unfractionated heparin and low - molecular - weight heparin have been reported to be ineffective in preventing thromboembolic complications . 
 this article discusses the available data and the most recent guidelines in the antithrombotic management of patients with prosthetic valves , and antithrombotic therapy in various clinical situations such as pregnant women with prosthetic heart valves , and patients with prosthetic heart valves undergoing noncardiac surgery .