Facts on File Encyclopedia of Health and Medicine

and existing clots from getting larger, though can-
not dissolve clots that already exist. Medications
that dissolve existing clots are called thrombolytic
agents, which have different pharmacologic
actions in the body.
The appropriate anticoagulation therapy
depends on the reason for the therapy (health
condition), the person’s overall health situation,
and any other medications the person needs to
take. Doctors may prescribe anticoagulation ther-
apy for noncardiovascular reasons such as after
orthopedic surgery, particularly JOINT REPLACEMENT.
People commonly refer to anticoagulant medica-
tions as “blood thinners,” though this is a mis-
nomer because these medications do not alter the
blood’s viscosity (thickness).

Antiplatelet Agents

Antiplatelet medications, also called PLATELET
inhibitors, slow clot formation by inhibiting
PLATELET AGGREGATION. These medications are espe-
cially effective in people who have increased risk
for CORONARY ARTERY DISEASE(CAD) or thromboem-
bolic stroke. Platelets are the cells in the blood that
are first on the scene of any injury in the body.
They swarm in response to even the slightest of
damage, such as the irritation and INFLAMMATION
atheromas cause to the walls of the arteries. When
they aggregate, or clump together, they release
chemical signals that activate the sequence of
events resulting in clot formation. Antiplatelet
medications interfere with these chemical signals.
The most commonly used antiplatelet therapy
is ASPIRIN THERAPY. Aspirin inhibits PROSTAGLANDINS,
chemicals that platelets require to enable them to
aggregate or stick together. Aspirin delays clotting
by delaying platelet aggregation, which is the first
step in the coagulation process. Platelets may
come together but not stick, drifting away from
each other again before they initiate the clotting
process. Other commonly prescribed antiplatelet
medications include clopidogrel (Plavix), ticlopi-
dine (Ticlid), dipyridamole (Persantine), and
cilostazol (Pletal). These medications may have
serious side effects or interact with other medica-
tions. Ticlopidine may cause a rare but life-threat-
ening condition, thrombotic thrombocytopenic
purpura (TTP), and requires frequent blood tests
to monitor for its development.

Clotting Factor Inhibitors

Other medications act to interfere with the body’s

ability to activate blood proteins essential for clot-

ting (CLOTTING FACTORS). The most commonly used

oral medication, warfarin (Coumadin), works by

blocking one of the steps in the body’s process to

produce VITAMIN K. Vitamin K is essential to the

metabolic processes that activate clotting factors II,

VII, IX, and X. The gastrointestinal tract does not

absorb heparin, which is available only in

injectable form (intravenous or subcutaneous).

Heparin prevents the conversion of prothrombin

(clotting factor II) to thrombin, a crucial and early

step of coagulation. Both of these medications are

NARROW THERAPEUTIC INDEX(NTI) drugs that require

very close monitoring to maintain their doses

within therapeutic range. Internal bleeding, espe-

cially from the gastrointestinal tract, can occur

when doses are too high. Excessive bleeding from

wounds, such as ACCIDENTAL INJURIES, or from rou-

tine dental procedures, such as prophylactic clean-

ing, is also a risk.

Low molecular weight heparin (LMWH), also

only in injectable form, acts similarly to heparin

though without many of heparin’s undesired side

effects. Several kinds of LMWH, also called frac-

tionated heparin, are available. Each has unique

characteristics and though all are LMWH drugs,

they are not interchangeable. LMWH products

include dalteparin (Fragmin), enoxaparin

(Lovenox), and tinzaparin (Innohep). Another

injectable medication, fondaparinux (Arixtra),

inhibits clotting factor X. Proper site selection and

injection technique are important for people who

use injectable forms of anticoagulant medications.

Benefits, Risks, and Lifestyle Modifications

Anticoagulant medications, whether antiplatelet

or inhibitor, are preventive for blood clots and the

health problems blood clots can cause, such as

stroke, HEART attack, pulmonary embolism, and

DVT. The primary risk of anticoagulation therapy

is excessive or prolonged bleeding, which can be

serious or life-threatening in some situations. Doc-

tors carefully monitor blood clotting times and

other measures to maintain an appropriate thera-

peutic balance. Spontaneous nosebleed (EPISTAXIS),

easy bruising, bleeding from the gums when

brushing the teeth, and blood in the stool are signs

14 The Cardiovascular System