Second most common tachycardia in children
Below you find the Holter-ECG tracing of a 16 y/o female:
As you can see in the top row, there is regular sinus rhythm of 75 bpm. The sixth beat falls early and shows a different P morphology, i.e. a premature atrial contraction (PAC). The subsequent beat (8th) comes in even earlier with the P before the T peak. Also of note, there is sudden prolongation of the PR interval. That is called a "Jump", as the conduction towards the AV node (AVN) "jumps" from the fast to the slow conducting pathway. This proves presence of dual AVN physiology, and also in this case initiates supra ventricular tachycardia (SVT).
Looking closer at the tachycardia, it is a very regular narrow complex tachycardia with P waves hidden in the S waves. RP interval is short, somewhat about 50-60ms. Combining our findings, this is very suspicious of AV nodal reentry tachycardia (AVNRT).
AVNRT is the second most common tachycardia in pediatric patients. It is, however, the most common form of SVT in adult patients. Presence of dual AVN physiology (present in about 30% of all people; see picture below) sets the stage for AVNRT.
Dual AVN physiology
Typically, tachycardia is initiated by premature contractions, often appearing when patients are physically or emotionally stressed (sports, excitement, fear). Sometimes tachycardia might even be induced by spicy foods.
The "jump", which actually is a sudden prolongation of the atrium to his bundle conduction time (AH-jump), results from the fast pathway being rendered refractory while the slow pathway continues conduction. Sometimes you may spot this "jump" on the ECG. That is why it is always advisable to take a look at the beginning of a tachycardia.
The tachycardia itself is quite regular, as is typical for reentry based arrhythmias.
Due to the fact that the reentry circuit is set within the AVN, retrograde atrial conduction via the fast pathway and integrate ventricular conduction via the bundle of His happen almost synchronously. Thus, the P waves are either hidden within the QRS or my appear at its´ end, usually not later than 90ms after R peak.
In typical AVNRT the electrical impulse uses the slow pathway to enter the AVN, and the fast PW to retrogradely activate the atria ("Slow-Fast-AVNRT"). However, depending on conduction properties of the pathways, timing of the impulse and refractory periods, it is possible for the circuit running the other way around (i.e. "Fast-Slow-AVNRT" or atypical AVNRT).
Since the AVN is involved in the reentry circuit, vagal maneuvers or adenosine may terminate tachycardia by slowing/blocking AVN conduction.
According to current guidelines, catheter ablation is the mainstay of therapy. Success rates for AVNRT ablation are about 96%, with low overall risk. Following successful ablation therapy there is no need for further medical treatment, as the arrhythmia is cured.