“Bridging” and Mechanical Heart Valves

Mechanical heart valves require anticoagulation to prevent valve-associated thrombosis and thromboembolic stroke. Oral vitamin K antagonists such as warfarin are prescribed universally; however, oral agents do not act immediately and usually require at least 5 days to achieve a therapeutic effect.Article p 564 Measurement of the prothrombin time, which is standardized by reporting the result as the international normalized ratio (INR), assesses the anticoagulant effect of warfarin. For most mechanical heart valves, the target INR ranges between 2.0 and 3.5. In the postoperative cardiac surgical setting, patients are usually started on low doses of warfarin because they tend to have impaired hepatic metabolism and suboptimal nutritional status. Even with low initial doses of warfarin, mechanical heart valve replacement patients are susceptible to excessively high INRs.1 This known exaggerated initial response to warfarin after heart valve replacement can lead to the habitual prescription of such low warfarin doses that warfarin as monotherapy may not achieve a stable and therapeutic INR for weeks after its initiation.To minimize the delay in achieving therapeutic anticoagulation, a “bridging” anticoagulant is prescribed. The “bridge” is administered parenterally, thereby providing an immediate anticoagulant effect. Traditionally, the “bridging” agent has been unfractionated heparin (UFH). More recently, physicians tend to select low-molecular-weight heparin (LMWH), even though few studies exist to validate the efficacy and safety of either LMWH or UFH in this setting.The rationale for shunning UFH has been to avoid the known perils and inconveniences of its use as a continuous peripheral intravenous infusion. UFH is rarely administered in an immediately therapeutic dose because of fear of precipitating bleeding complications. Especially in postoperative mechanical valve replacement patients, there is reluctance to follow the high dosing requirements for initial bolus and infusion regimens published in standardized nomograms. Instead, UFH is usually started in cautious small …