clot from forming, essentially halting heart attack
or stroke before the event can cause any damage.
However, doctors must administer them within
three to four hours of clot formation. After four
hours the clot has hardened and thrombolytic
agents cannot dissolve them.
Thrombolytic agents are substances, either natu-
ral extracts or recombinant forms, that convert
plasminogen in the blood to plasmin, an enzyme
that dissolves fibrin. Fibrin is the substance in the
blood that forms the webbing of the clot structure
to snare platelets and other substances in the blood
that become the clot. Early in the COAGULATION
process fibrin is a semisolid, stringlike substance
similar to the strands of a spiderweb. As the coagu-
lation process continues, however, the fibrin
strands and the cellular matter they have captured
harden into the solid structure of a blood clot. Once
the fibrin hardens, plasmin has no effect on it.
The most frequently used thrombolytic agents
are tissue plasminogen activators (tPAs). One of
the original thrombolytic agents, streptokinase,
derives from the streptococcus bacterium and
causes the body to develop antibodies against it.
Because of this, doctors cannot administer strep-
tokinase if the person has received streptokinase
within 12 months. However, it takes about five
days for the body to produce antibodies, allowing
multiple administrations within five days of the
initial dose. The tPAs do not seem to have this
limitation, although it is possible for the body to
develop antibodies against them as well.
Doctors administer thrombolytic agents intra-
venously to treat heart attack, stroke, deep vein
thrombosis, and pulmonary embolism. The effect
is rapid and short acting. Excessive and severe
bleeding is a significant risk, particularly when
stroke is hemorrhagic rather than ischemic. Doc-
tors make every effort to determine the nature of
a stroke before administering thrombolytic agents,
though sometimes bleeding occurs even with
ischemic stroke. As well, these agents may disturb
the integrity of clots that have formed within the
previous 10 days, such as from surgery.
COMMON THROMBOLYTIC AGENTSalteplase (Activase) anistreplase (Eminase)
reteplase (Retavase) streptokinase (Streptase, Kabinase)
tenecteplase (TNKase) urokinase (Abbokinase)
Vasoconstrictors
Vasoconstrictors cause the blood vessels to con-
strict, or tighten, to raise blood pressure. Doctors
administer vasoconstrictors to treat cardiovascular
SHOCK and HYPOTENSION. Many bronchodilating
medications prescribed to treat asthma also have
peripheral vasoconstriction action, and may raise
blood pressure at the same time they open the air-
ways. One of the most commonly used vasocon-
strictors is pseudoephedrine, found in cold, flu,
and some allergy medications. Caffeine and NICO-
TINEare also vasoconstrictors. Though doctors do
not prescribe these products for cardiovascular
use, they have the effect of raising blood pressure
as well as increasing heart rate. The most com-
monly used vasoconstrictor for cardiovascular
purposes is midodrine (ProAmatine).Vasodilators
Many medications to treat hypertension are
vasodilators, drugs that cause the blood vessels to
relax so more blood can flow through them with
less resistance. These medications may lower
blood pressure or relieve angina pectoris. Among
the general vasodilators cardiologists might pre-
scribe to treat hypertension are hydralazine and
minoxidil. Both drugs regulate the calcium that
enters the smooth muscle cells of the ARTERYwalls,
slowing their contractility and causing the arteries
to relax (dilate). Minoxidil is an NTI drug that
requires close monitoring because, although it is a
potent peripheral vasodilator, it also increases
heart rate and has other cardiovascular actions
that require additional medications to moderate.
Nitrate vasodilators are especially effective at
relaxing the coronary arteries to relieve angina
pectoris, which is one of the leading reasons doc-
tors prescribe them. Nitrates also dilate the periph-
eral veins, which decreases the heart’s workload.
Nitrates come in sublingual tablets placed under
the tongue at the onset of anginal symptoms, reg-
ular and long-acting oral medications, transdermal
(skin) patches, and topical ointments. Because the
body acquires a tolerance to nitrates, dosing
schedules are particularly important. Other com-
monly prescribed medications that have vasodilat-
ing actions include certain of the beta blockers,
calcium channel blockers, ACE inhibitors, and
ARBs.medications to treat cardiovascular disease 83