Intracardiac thrombosis can complicate a variety of conditions and lead to devastating consequences as a result of peripheral embolization or blood flow reduction. Long-term therapeutic oral anticoagulation is effective for the prevention of thromboembolism in these conditions but carries attendant risks including major bleeding. Understanding the risks and benefits of long-term oral anticoagulant therapy for various cardiac conditions is essential for ensuring the best clinical outcomes.
These agents affect the coagulation protein cascade to reduce thrombosis.
1. Unfractionated heparin
Unfractionated heparin (UFH) binds to antithrombin III, markedly increasing the effect of antithrombin III in neutralizing thrombin. It also inhibits the activation of factors IX and X. The effectiveness of UFH varies greatly from person to person due to its interactions with a number of plasma proteins and the endothelium. Monitoring the effects of full-dose UFH on hemostasis is mandatory. Monitoring of Xa levels is now recommended for accurate determination of anticoagulant effects of UFH, which is believed to be more reliable, compared to the previous use of activated partial thromboplastin time (aPTT). In certain more dynamic situations, particularly where a higher level of anticoagulation is needed (eg, during coronary interventions), the activated clotting time (ACT) is used to monitor its effect, and the dose of UFH is adjusted to keep the ACT at 250–300 seconds or greater. When administrated intravenously, the effects of UFH are immediate. It is usually administered as a bolus, followed by a continuous intravenous infusion. It may also be administered in divided therapeutic doses subcutaneously. The effects of UFH usually dissipate within 6 hours. A significant advantage of UFH is that its effects can be reversed almost completely with the use of protamine, which makes its use attractive in inpatient situations where bleeding risk is perceived to be high. UFH can be given subcutaneously in smaller doses that do not affect the aPTT for primary prevention of deep vein thrombosis (DVT).
2. Low-molecular-weight heparin
Low-molecular-weight heparins (LMWHs) are breakdown products of UFH. They have a more selective effect on factor X than thrombin. LMWHs bind more selectively to plasma proteins compared to UFH; therefore, the dosing is more predictable and their effects are more selective, and thus, they are associated with a lower bleeding risk. They are more resistant to neutralization by platelet factor 4 than UFH and have less inhibitory effect on platelet function. LMWHs have a more predictable effect on coagulation than UFH, and laboratory monitoring is usually not necessary when dosage is based on body weight. Activated factor Xa levels can be used to monitor their effects but not aPTT or ACT. LMWHs are usually administered subcutaneously twice daily, and their effects are not easily reversed by protamine. LMWH can also be administered in smaller doses for the primary prevention of DVT. LMWHs are contraindicated in the ...