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AV nodal reentrant tachycardia (AVNRT) is a type of reentrant tachycardia (fast rhythm) of the heart. It is a supraventricular tachycardia, meaning that it originates from a location within the heart above the bundle of HIS. In AVNRT, the fast pathway and the slow pathway are usually both in the right atrium. The slow pathway (which is usually targeted for ablation) is located inferiorly and slightly anterior to the AV node, often following the anterior margin of the coronary sinus. The fast pathway is usually located just superior and posterior to the AV node.
Types of AVNRT There are two types of AVNRT, called the common form and the uncommon form. Common AVNRT In common AVNRT, the anterograde conduction is via the slow pathway and the retrograde conduction is via the fast pathway ("slow-fast" AVNRT). Because the retrograde conduction is via the fast pathway, stimulation of the atria (which produces the inverted P wave) will occur at the same time as stimulation of the ventricles (which causes the QRS complex). As a result, the inverted P waves may not be seen on the surface ECG since they are buried with the QRS complexes. Uncommon AVNRT In uncommon AVNRT, the anterograde conduction is via the fast pathway and the retrograde conduction is via the slow pathway ("fast-slow" AVNRT). Multiple slow pathways can exist so that both anterograde and retrograde conduction are over slow pathways. ("slow-slow" AVNRT). Because the retrograde conduction is via the slow pathway, stimulation of the atria will be delayed by the slow conduction tissue and will typically produce an inverted P wave that falls after the QRS complex on the surface ECG. Fast and slow pathways vs. accessory pathways The fast and slow pathways should not be confused with the accessory pathways that give rise to Wolff-Parkinson-White syndrome (WPW) syndrome or atrioventricular re-entrant tachycardia (AVRT). In AVNRT, the fast and slow pathways are located within the right atrium in close proximity to the AV node and exhibit electrophysiologic properties similar to AV nodal tissue. Accessory pathways that give rise to WPW syndrome and AVRT are located in the atrioventricular valvular rings, they provide a direct connection between the atria and ventricles, and have electrophysiologic properties similar to ventricular myocardium. Treatment An episode of supraventricular tachycardia (SVT) due to AVNRT can be terminated by any action that transiently blocks the AV node. This is because the AV node is an essential portion of the reentrant circuit in AVNRT. Medical therapy can be initiated with AV nodal slowing drugs such as beta blockers or calcium channel blockers. After being diagnosed with AVNRT, patients can also undergo an electrophysiology (EP) study to confirm the diagnosis and subsequent catheter ablation of the slow pathway which effectively cures the patient of AVNRT. See also | ||||||||
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