Long-term anticoagulation is important in the treatment of many cardiac conditions. Intracardiac thrombi can form and lead to devastating consequences as a result of obstruction of blood flow and peripheral embolization.
Treatment for intracardiac thrombi involves the use of anticoagulants
for both primary and secondary prevention of thrombosis and embolization.
There are, however, risks associated with the use of these agents,
and an understanding of the risks and benefits of anticoagulant
therapy for various cardiac conditions is important.
These agents affect the coagulation protein cascade to reduce thrombosis. Their greatest use is in primary and secondary prevention
of intravascular and intracardiac thrombosis and embolization.
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. Routinely,
the activated partial thromboplastin time (aPTT) is used to monitor
the effects of UFH, which should be titrated to 1.5–2.0
times greater than control. In certain situations, a higher level
of anticoagulation is needed, ie, during coronary interventions.
In those instances, the activated clotting time (ACT) is used to
monitor its effect, and the dose of UFH is adjusted to keep the
ACT 250–300 seconds or greater. When given intravenously,
the effects of UFH are immediate. It is usually given as a bolus,
followed by a continuous infusion. It may also be given subcutaneously.
The effects of UFH will dissipate within 6 hours. Protamine can
be given to reverse its effects more quickly. UFH can be given subcutaneously
in smaller doses, which will not affect the aPTT, for primary prevention
of deep venous thrombosis in certain high-risk situations.
Low-molecular-weight heparins (LMWHs) are breakdown products of UFH. They have a greater affect on factor X than on thrombin.
Low-molecular-weight heparins bind less to plasma proteins than UFH
and therefore, the dosing is more predictable. They are more resistant
to neutralization by platelet factor 4 than UFH and have less inhibitory
affect on platelet function than UFH. Low-molecular-weight heparins
have a more predictable affect on coagulation than UFH and laboratory
monitoring is usually not necessary. Monitoring the effects of LMWHs
is difficult, since the commonly used tests for anticoagulation are
not helpful and activated factor Xa levels need to be measured.
Low-molecular-weight heparins are usually administered subcutaneously
twice daily. They are not easily reversed by protamine. Low-molecular-weight
heparin can also be given in smaller doses for the primary prevention of deep venous thrombosis in certain high-risk situations.
Oral Vitamin K Antagonists
Coumarins, or vitamin K antagonists, are the mainstay of oral anticoagulants and have been used for more ...