Encyclopedia of Health and Medicine

T
tachycardia
See ARRHYTHMIA.
tamponade, cardiac A life-threatening compression
of the HEART that prevents it from expanding
to fill with BLOOD. Cardiac tamponade is most
often a complication of PERICARDITIS and develops
when fluid rapidly accumulates within the layers
of the PERCARDIUM. The pericardium’s fibrous outer
layer does not readily expand, which forces the
fluid to press inward against the heart. Without
immediate action to drain the fluid, the heart will
stop beating. ELECTROCARDIOGRAM (ECG) demonstrates
the characteristic patterns of electrical
changes in the heart’s rhythm that strongly indicate
cardiac tamponade. COMPUTED TOMOGRAPHY
(CT) SCAN or MAGNETIC RESONANCE IMAGING (MRI) can
confirm the diagnosis. Generally the doctor can
aspirate (withdraw) the fluid using a needle and
syringe (pericardiocentesis), which relieves the
pressure and allows the heart to resume normal
function. Surgery to create an opening in
the pericardium may be necessary to manage cardiac
tamponade that occurs with chronic pericarditis.
Penetrating trauma that causes bleeding
into the pericardium may also cause cardiac tamponade.
See also MYOCARDIAL INFARCTION.
thrombolytic therapy Emergency treatment
with medications to dissolve BLOOD clots that are
in the process of forming. When initiated
promptly, thrombolytic therapy can avert MYOCAR-
DIAL INFARCTION or ischemic (nonhemorrhagic)
STROKE, mitigating damage to the HEART or BRAIN
respectively. Doctors also may use thrombolytic
therapy to treat blood clots that form elsewhere in
the body, such as in the leg (DEEP VEIN THROMBOSIS
[DVT]) or the LUNGS (PULMONARY EMBOLISM).
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Thrombolytic agents act by converting plasminogen,
an inactive protein in the blood circulation,
to plasmin, an active protein that breaks
down the key proteins that form blood clots (fibrinogen
and fibrin). These agents can only act
before the clot fully forms and hardens, which
establishes a therapeutic window of about four
hours from the onset of clot formation. Early diagnosis
is therefore essential.
COMMON THROMBOLYTIC AGENTS
alteplase (aPA) anisoylated purified streptokinase
anistreplase
activator complex (APSAC)
tissue plasminogen streptokinase
activator (tPA) urokinase
The oldest of the thrombolytic agents is streptokinase,
which doctors began using in the 1940s.
Streptokinase and anistreplase derive from streptococcal
BACTERIA. When administered, these
agents initiate ANTIBODY production. The antibodies
become active after five days and remain active
for about six months. During that timeframe, doctors
cannot re-administer streptokinase or anistreplase.
Some people also have adverse responses to
these agents, especially streptokinase. Laboratories
manufacture most of the newer thrombolytic
agents using recombinant technology, which
nearly eliminates these immune responses.
The most significant risk of thrombolytic therapy
is uncontrolled bleeding (HEMORRHAGE). For
this reason, it is crucial for doctors to determine
whether a stroke is ischemic (caused by a clot) or
hemorrhagic (caused by bleeding) before administering
a thrombolytic agent. For venous clots, such
as with deep vein thrombosis, the doctor may
directly inject the agent into the clot. For arterial