0071643192.pdf

HEMATOLOGY, ONCOLOGY, ALLERGY,

AND IMMUNOLOGY

ANTICOAGULATION AND ANTIPLATELET AGENTS

Anticoagulant and antiplatelet therapy are used to treat and prevent thrombosis.

Warfarin

Oral anticoagulant that prevents reduction of vitamin Kto its active form

CONTRAINDICATIONS

■ Hypersensitivity, active bleeding, pregnancy, liver failure

DOSING

■ Start warfarin 5 mg PO daily.

MONITORING

■ For most indications, therapeutic INR is 2.0–3.0.

■ For mechanical valves, therapeutic INR is 2.5–3.5.

■ Table 9.5 shows a list of medications that can interfere with INR.

COMPLICATIONS

■ Major bleeding occurs in about 3% of patients.

■ Many medications can alter the effect of warfarin on clotting factor pro-

duction (see Table 9.10). The most common mechanism is by induction

of hepatic enzymes leading to increased drug clearance.

■ Thrombosis and warfarin-induced skin necrosis can occur between the

third and 10th day after initiating therapy. Risk is greatly reduced by con-

current use of heparin or low-molecular-weight heparin

REVERSAL

See Table 9.11.

Unfractionated Heparin

Inhibits thrombin and other factors in the intrinsic cascade; prevents clot

propagation

TABLE 9.9. Factor Replacement Guidelines

BLEEDINGSITE DESIREDFACTORLEVEL(%)

Deep muscle 40–50

Joint 30–50

Epistaxis 80–100

Oral mucosa 50

GI tract 100

CNS 100

The vitamin K—dependent

cofactors are factors II, VII, IX, X,

protein C, and protein S. Upon

starting warfarin, the patient

may initially be

hypercoagulable because of the

rapid loss of the anticoagulants

protein C and protein S.