Encyclopedia of Health and Medicine

14 The Cardiovascular System
and existing clots from getting larger, though cannot
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 therapy
for noncardiovascular reasons such as after
orthopedic surgery, particularly JOINT REPLACEMENT.
People commonly refer to anticoagulant medications
as “blood thinners,” though this is a misnomer
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 especially
effective in people who have increased risk
for CORONARY ARTERY DISEASE (CAD) or thromboembolic
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), ticlopidine
(Ticlid), dipyridamole (Persantine), and
cilostazol (Pletal). These medications may have
serious side effects or interact with other medications.
Ticlopidine may cause a rare but life-threatening
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 clotting
(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, especially
from the gastrointestinal tract, can occur
when doses are too high. Excessive bleeding from
wounds, such as ACCIDENTAL INJURIES, or from routine
dental procedures, such as prophylactic cleaning,
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 fractionated
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. Doctors
carefully monitor blood clotting times and
other measures to maintain an appropriate therapeutic
balance. Spontaneous nosebleed (EPISTAXIS),
easy bruising, bleeding from the gums when
brushing the teeth, and blood in the stool are signs