Chapter 021. Syncope (Part 6) docx

Chapter 021. Syncope
(Part 6)

Diagnostic Tests
The choice of diagnostic tests should be guided by the history and the
physical examination. Measurements of serum electrolytes, glucose, and the
hematocrit are usually indicated. Cardiac enzymes should be evaluated if
myocardial ischemia is suspected. Blood and urine toxicology screens may reveal
the presence of alcohol or other drugs. In patients with possible adrenocortical
insufficiency, plasma aldosterone and mineralocorticoid levels should be
obtained.Although the surface electrocardiogram is unlikely to provide a definitive
diagnosis, it may provide clues to the cause of syncope and should be performed
in almost all patients. The presence of conduction abnormalities (PR prolongation
and bundle branch block) suggests a bradyarrhythmia, whereas pathologic Q
waves or prolongation of the QT interval suggests a ventricular tachyarrhythmia.
Inpatients should undergo continuous electrocardiographic monitoring; outpatients
should wear a Holter monitor for 24–48 h. Whenever possible, symptoms should
be correlated with the occurrence of arrhythmias. Continuous electrocardiographic
monitoring may establish the cause of syncope in as many as 15% of patients.
Cardiac event monitors may be useful in patients with infrequent symptoms,
particularly in patients with presyncope. An implantable event monitor may be
necessary for patients with extremely infrequent episodes. The presence of a late
potential on a signal-averaged electrocardiogram is associated with increased risk
for ventricular tachyarrhythmias in patients with a prior myocardial infarction.
Low-voltage (visually inapparent) T wave alternans is also associated with
development of sustained ventricular arrhythmias.
Invasive cardiac electrophysiologic testing provides diagnostic and
prognostic information regarding sinus node function, AV conduction, and
supraventricular and ventricular arrhythmias (Chaps. 225 and 226). Prolongation
of the sinus node recovery time (>1500 ms) is a specific finding (85–100%) for
diagnosis of sinus node dysfunction but has a low sensitivity; continuous

electrocardiographic monitoring is usually more effective for diagnosing this
abnormality. Prolongation of the HV interval and conduction block below the His
bundle indicate that His-Purkinje disease may be responsible for syncope.
Programmed stimulation for ventricular arrhythmias is most useful in patients who
have experienced a myocardial infarction; the sensitivity and specificity of this
technique is lower in patients with normal hearts or those with heart disease other
than coronary artery disease.
Upright tilt table testing is indicated for recurrent syncope, a single
syncopal episode that caused injury, or a single syncopal event in a "high-risk"
setting (pilot, commercial vehicle driver, etc.), whether or not there is a history of
preexisting heart disease or prior vasovagal episodes. In susceptible patients,
upright tilt at an angle between 60° and 80° for 30–60 min induces a vasovagal
episode.
The protocol can be shortened if upright tilt is combined with
administration of drugs that cause venous pooling or increase adrenergic
stimulation (isoproterenol, nitroglycerin, edrophonium, or adenosine). The
sensitivity and specificity of tilt-table testing is difficult to ascertain because of the
lack of validated criteria.
Moreover, the reflexes responsible for vasovagal syncope can be elicited in
most, if not all, individuals given the appropriate stimulus. The specificity of tilt-
table testing has been reported to be near 90%, but it is lower when pharmacologic
provocation is employed. The reported sensitivity of the test ranges between 20
and 74%, the variability due to differences in populations studied, techniques used,
and the absence of a true "gold standard" against which to compare test results.
The reproducibility (in a time ranging from several hours to weeks) is 80–90% for
an initially positive response, but may be less for an initially negative response
(ranging from 30 to 90%).
A variety of other tests may be useful to determine the presence of
structural heart disease that may cause syncope. The echocardiogram with Doppler
examination detects valvular, myocardial, and pericardial abnormalities. The

echocardiogram is the "gold standard" for the diagnosis of hypertrophic
cardiomyopathy and atrial myxoma. Cardiac cine MRI provides an alternative
noninvasive modality that may be useful for patients in whom diagnostic-quality
echocardiographic images cannot be obtained. This test is also indicated for
patients suspected of having arrhythmogenic right ventricular dysplasia or right
ventricular outflow tract ventricular tachycardia. Both are associated with right
ventricular structural abnormalities that are better visualized on MR imaging than
by echocardiogram. Exercise testing may detect ischemia or exercise-induced
arrhythmias. In some patients, cardiac catheterization may be necessary to
diagnose the presence or severity of coronary artery disease or valvular
abnormalities. Ultrafast CT scan, ventilation-perfusion scan, or pulmonary
angiography is indicated in patients in whom syncope may be due to pulmonary
embolus.In cases of possible cerebrovascular syncope, neuroimaging tests may be
indicated, including Doppler ultrasound studies of the carotid and vertebrobasilar
systems, MRI, magnetic resonance angiography, and x-ray angiography of the
cerebral vasculature (Chap. 364). Electroencephalography is indicated if seizures
are suspected.[newpage]