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.