AVNRT

Although this rhythm is commonly referred to as SVT, SVT refers to any supraventricular tachycardia (ie atrial fibrillation with RVR). AV nodal reentrant tachycardia is the more accurate name for SVT. It is a reentrant tachycardia involving the AV node, where the depolarization continuously circles within the AV node, propagating to the atria and ventricles. A closely related arrhythmia, AVRT (AV reentrant tachycardia), is the same process, except the circuit/loop involves the AV node and an accessory pathway.

AVNRT is the most common cause of palpitations in patients with structurally normal hearts. It is very uncommon for patients to be unstable with true AVNRT. AVNRT occurs due to the 2 pathways in the AV node. The slow pathway has a short refractory period and the fast pathway has a long refractory period. Usually, the fast pathway conducts impulses from the atria to the ventricles; the conduction down the slow pathway meets the conduction from the fast pathway that has started to come back up the slow pathway and is, therefore, terminated. There are multiple reasons why AVNRT may develop, with the most common being from a PAC. A PAC can arrive while the fast pathway is still refractory with the impulse going down the slow pathway; this then circles back to the fast pathway as it is no longer refractory when the impulse reaches the end of the slow pathway.

The rate ranges between 140-280bpm and is regular. The heart rate may be helpful to differentiate from other causes of narrow complex tachycardias, as conduction through the AV node from the atria usually is not greater than ~160bpm in adults (technically 220-age). Therefore, in adults of middle age or more, most heart rates are not >170/180 bpm that are from the atria (unless there is an accessory conduction pathway). It is usually narrow complex, though can be wide complex if there is aberrancy present (more likely to be wide with faster rates). P waves may or may not be visible; if they are visible, they are noted to be retrograde P waves as the atria are activated from the AV node. This means the P waves are usually hidden in the QRS complex, but can be found after it or have inverted morphology with very short PR interval just before the QRS complex. Because retrograde P waves are initiated from the AV node, their morphology will be opposite those of sinus P waves (they will be inverted in the inferior leads and upright in V1). Sometimes, this is called negative polarity. Patients commonly have ST depression due to the tachycardia. However, this commonly resolves after resolution of SVT. Rarely, these rhythms will also display QRS alternans. This is an alternating QRS amplitude; it is differentiated from electrical alternans in that the QRS amplitudes are normal, unlike with electrical alternans.

Slow-Fast AVNRT

Slow-fast AVNRT occurs due to the PAC mechanism listed above and accounts for the vast majority of AVNRT. This usually produces the 'typical' SVT findings on an EKG. The retrograde P wave is obscured in the QRS complex or occurs at the end of the QRS complex as pseudo r' or S waves. The retrograde P wave is commonly hidden as the ventricles and atria receive electrical impulses from the AV node almost simultaneously. This is because the ventricles are initiated at the end of the slow pathway and the atria are initiated at the end of the fast pathway (in other words, retrograde conduction through the fast pathway is what separates the timing of the ventricles and atria receiving the electrical impulses).

Fast-Slow AVNRT

This is an uncommon (10%) cause of SVT, where the conduction goes down the fast pathway and then through the slow pathway for retrograde conduction. Because the ventricles get activated much quicker than the atria with this pathway (as retrograde conduction through the slow pathway is what separates impulse propagation to the ventricles and impulse propagation to the atria), the retrograde P waves are more likely to be visible after the QRS complex.

Slow-Slow AVNRT

This is the most rare form of SVT. The impulse travels down the slow AV nodal pathway and then up (retrograde) slow left atrial fibers. This causes the P wave to appear before the QRS complex as the slow left atrial fibers are extremely slow to conduct. This can be confusing as the P waves effectively appear before the QRS complex. However, they will have the morphology of retrograde P waves, allowing differentiation from a sinus tachycardia.

Examples:

Further Reading:

https://litfl.com/supraventricular-tachycardia-svt-ecg-library/

http://hqmeded-ecg.blogspot.com/search/label/AVNRT%20AV%20nodal%20re-entrant%20tachycardia?updated-max=2020-03-06T12:16:00-06:00&max-results=20&start=1&by-date=false