In order for the heart to pump efficiently, an electrical wiring system is
present in the heart to coordinate the heart's action. An electrical impulse
normally originates in the uppermost part of the heart (SA node) and then
travels in an orderly fashion through the AV node, down toward the apex of
the heart.
When this orderly
flow of current is disturbed, an "arrhythmia" results. This usually is manifest
as a "too slow heart beat" (bradycardia) or a "too fast heart beat" (tachycardia). By placing electrical wires or "catheters" in to the heart and performing a stimulation study,
the normal or abnormal electrical condition of the heart can be assessed.
Patients with heart rhythm disturbances are studied and treated in the
electrophysiology laboratory. The most common diagnostic test conducted
is the electrophysiology study
(EPS). The other major diagnostic test performed is
tilt table-testing for vaso-vagal syncope.
Vasovagal Syncope
(Neurocardiogenic Syndrome or vasodepressor syncope)
This is a relatively common syndrome seen in young people, more commonly in women
. The clinical setting is usually related to standing
for a prolonged period of time in a crowded environment e.g. inside a bus,
train or underground railway system; or following situations such as fright, a dentist's chair or the sight of blood. He or she may feel dizzy, shortness
of breath, a "black screen" may occur in front of the eyes and they then fall to the ground
with a brief period of loss of consciousness. It may be preceded by an awareness of a faster heart beat and nausea and sweatiness. This is usally a benign situation
and the patient will be awake in a few seconds to minutes time. If you
examine the patient during that time, he or she will usually have hypotension (low blood pressure)
and bradycardia (slow heart rate).
The underlying mechanism of this syndrome is still unknown.
It may be due to hypersensitivity of the sympathetic nervous system
that results in a slowing of the heart rate
and/or lowish blood pressure.
Investigation will be mainly to exclude other causes of
syncope (e.g. arrhythmias, drugs, epilepsy, etc.) and the
Tilt Table Test
is useful for the documentation of this syndrome.
Treatment includes beta-blockers, disopyramide, SSRI's, mineralocorticoids or
even permanent pacemaker insertion for those if severe cardioinhibtory
effect (profound bradycardia or asystole during attack)is seen.
It must be emphasized, however, that syncope can be a sign of more sinister heart disease, particularly in patients with structural heart disease. The patients should be considered for investigation with electrophysiological testing.
Fast Heart Rhythms
Problems with electrical signals can make the heart
beat too fast. Below are some common types of fast heart rhythms.
AV Node Reentrant Tachycardia (AVNRT)
With AVNRT, an extra pathway lies in or near
the AV node. Signals traveling through the AV node may get trapped
in this pathway. The trapped signals make the heart beat faster.
Wolff ParkinsonWhite (WPW) Syndrome
With WPW, an extra pathway connects the ventricles
and the atria. Signals passing through the ventricles may travel
along this extra pathway back to the atria. The signals from the
extra pathway make the heart beat faster.
With atrial flutter, an atrium has a circuit
of extra electrical cells. Signals loop around and around inside
this circuit. These signals tell the atria to beat very fast.
Atrial Fibrillation
With atrial fibrillation, the atria have many
circuits that send signals. The extra signals make the atria beat
very quickly and unevenly. The atria may beat so fast and unevenly
that they stop contracting and begin to quiver.
For more information on arrhythmias and mechanisms:
see animated arrhythmia pics
Therapeutic procedures performed
in the EP lab include
transcatheter radiofrequency ablations,
implantations of permanent pacemakers and
defibrillators, overdrive pacing and cardioversions.
