Supraventricular tachycardia, SVT, refers to rapid
heart rhythms that originate above the ventricles, in the atria or AV node. It’s also called
paroxysmal supraventricular tachycardia, for its episodic nature – the rhythms
come suddenly, and go spontaneously. In normal conduction, electrical signals
are initiated in the SA node, and travel throughout the atria to reach the AV node.
The AV node is the gateway to the ventricles. It delays the passage of electrical impulses
to the ventricles to ensure that the atria have ejected all the blood into the ventricles
before the ventricles contract. This refractory property of the AV node is essential in limiting
electrical activities that reach the ventricles. It keeps the ventricular rate, hence heart rate,
in the normal range of 60 to 100 beats per minute. SVT occurs when abnormal electrical pathways
bypass or corrupt the AV nodal checkpoint. As a result, heart rate accelerates, and the ventricles
contract before they are properly refilled. These ineffective contractions may cause lightheadedness
because the brain is not getting enough oxygen. By definition, SVT includes all rhythms that
originate above the ventricles. In practice, however, SVT refers only to
AV nodal reentrant tachycardia (AVNRT), atrioventricular reciprocating
tachycardia (AVRT), and atrial tachycardia. AVNRT happens when the AV nodal tissue has
2 pathways with different conductivity, one slow and one fast. If an atrial impulse
arrives at the AV node when one pathway is conductive and the other is refractory, it
will follow the conductive pathway. However, it may circle back through the second pathway
if this becomes excitable again before the signal leaves the node. If this continues, a
self-perpetuating loop, called a re-entrant pathway, may arise. With each cycle, the signal is
transmitted down to the ventricles. The atrial and ventricular rates are identical, regular and fast.
AVNRT is the most common type of SVT in adults, and is twice more common in women than in men.
In patients with AVRT, there is an additional connection, an accessory pathway, between
the atria and ventricles. The pathway acts as a shortcut to the ventricles, bypassing the
AV node. It allows part of electrical impulses to arrive to the ventricles sooner, causing a
so-called “pre-excitation”, which can be seen as a shortened PR interval on an ECG. Because
part of the ventricles depolarize earlier, ventricular depolarization develops in
a more gradual fashion and lasts longer, resulting in a slurring slow rise of the initial
portion of the QRS complex, known as Delta wave, and QRS prolongation. However, the presence
of the accessory pathway alone is not enough to cause tachycardia. AVRT develops when
electrical impulses travel down one pathway, either the normal or accessory, then back up via
the other, creating a loop, or re-entrant circuit. The frequency of this loop determines heart rate
and can be very fast. AVRT can be orthodromic or antidromic depending on the direction of
the loop. AVRT is more common in children. Atrial tachycardia is caused by an ectopic focus
in an atrium, and can arise from any of the three mechanisms: enhanced automaticity, reentry, or
triggered. Ventricular rhythms are regular and fast. P wave morphology varies depending on the
site of origin and tachycardia mechanism. Atrial tachycardia is common in children with congenital
heart disease, but it may also be triggered by environmental factors in healthy people.
Because of the episodic nature of SVT, symptoms come and go and may not be
present during physical examination. ECG findings may be normal, and the condition
is often misdiagnosed as anxiety or panic attacks. Assessment should include continuous
monitoring with a portable ECG recording device. When SVT is caused by an underlying condition, it must be treated. Most people who have no
underlying disease and have an obvious trigger may not require treatment. For others, treatments and
lifestyle changes can often control the condition.
