Luis G. Alvarez, M.D. |
When a patient presents with
tachycardia, it is important
to evaluate if the rhythm is
a dangerous one, needing
hospital treatment, or if it
can be safely treated as an
outpatient. The first step
is to ensure the patient is
stable. In the presence of
hemodynamic embarrassment,
etiology matters little. The
treatment is swift
conversion, usually by DC
shock. If, on the other
hand, it is established that
the patient is stable, the
second step is to determine
whether the QRS is narrow or
wide. If narrow, it is
almost certain the
arrhythmia is
supraventricular tachycardia
(SVT). If the QRS is wide
(greater than 120 msec) the
arrhythmia can be
ventricular tachycardia
(VT).
The most important
factor is to ensure that the
data used to make decisions
is adequate: a monitor lead
is insufficient and can be
quite misleading. To decide
whether an arrhythmia is
narrow or wide, a 12-lead
ECG is essential. This
question is not merely
academic:
if VT is treated as
SVT, the consequences may be
lethal.
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For years, the diagnosis of VT has
hinged on the morphology of
the recorded rhythm. In the
1970’s and 80’s several
renowned authors published
articles on how to
discriminate between VT and
SVT. These articles were all
complex and depended on
multiple step algorithms to
decide. They also only
applied to particular forms
of tachycardia and therefore
had limited use in the real
world.
In 1988, an article was published
examining the diagnosis of
tachycardia in a different
manner: Similar to the way
we now evaluate the
significance of chest pain
by the presence of
significant risk factors
(rather than the character,
intensity, location,
radiation, etc); this
article looked at the
diagnosis of tachycardia
based on the presence of
cardiac risk factors. In
general, if the presenting
rhythm was wide-complex
tachycardia, 80% of all
patients turned out to have
a ventricular arrhythmia. In
addition, based on history
alone, if the patient
admitted to any heart
condition, the probability
of VT increased to 95%. If
the patient had a history of
myocardial infarction (MI),
the probability further
increased to 98%. Therefore,
in a patient with history of
MI, the diagnosis of SVT in
the face of a wide complex
carried a likelihood of only
2% of being correct.
The presence of symptoms, particularly
syncope, in association with
VT will dictate the need for
evaluation. In the absence
of symptomatology, the need
for further evaluation
hinges on whether cardiac
pathology is also present.
Coronary artery disease and
previous myocardial
infarction are the main
factors usually leading to
further investigation.
In the asymptomatic patient, the
implications of VT depend
mainly on cardiac function:
VT in an otherwise normal
heart is usually benign and
may not require further
evaluation. In contrast, VT
in the presence of
significant cardiac
pathology is almost always a
poor prognostic sign. The
most important piece of
information to be obtained
is the Ejection Fraction
(EF), which can be obtained
by echocardiography, nuclear
imaging or cardiac
catheterization. In the
presence of an EF of less
than 30%, treatment with an
implanted defibrillator is
often indicated. An EF of
between 35 and 45 % should
prompt formal
electrophysiological
evaluation in a patient with
VT. An EF of above 50% will
not likely require further
evaluation.
In summary, VT may often be a sign of
the need for further cardiac
evaluation and therapy,
particularly in the presence
of other cardiac risk
factors. In the otherwise
healthy patient, it can
instead be a benign
condition. The most
efficient way to distinguish
between these two situations
is assessment of the
ejection fraction.
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