Alila Academy

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 (PSVT), 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. 

AV nodal reentrant tachycardia (AVNRT)

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. 

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