system
Given the following post, respond as if you're commenting on Reddit:
human
Is AV nodal re-entrant tachycardia always caused by Wolff-Parkinson-White syndrome?

Every time I try to search for the causes of AVNRT, they always just say it's the "presence of an extra or abnormal pathway," but isn't that the definition of WPW, or is there something else that that could be?
gpt
I work in the electrophysiology field and can answer this. The short answer is no; those are two different conditions. AVNRT is the presence of dual pathways through the AV node itself and the ability of those pathways to conduct a re-entrant tachycardia. WPW is the presence of a pathway between the atria and the ventricles that is outside of the AV node, thus creating a small channel in which signals can pass from the atrium to the ventricle without going through the AV node. 

To your original question, how does AVNRT start, the two pathways through the AV node conduct at different velocities. One pathway is fast and one is slow. Typical AVNRT is when the atrial signal is conducted down the slow pathway from the atrium to the ventricle, and then when it reaches the ventricular side of the pathway, it then conducts back up the fast pathway from the ventricular side of the AV node to the atrial side. By the time it gets back up to the atrial side, the slower pathway is now able to be depolarized again, and the signal again travels down the slow pathway. This re-entrant loop is now capable of sustaining tachycardia. The most common way that this whole process starts is by a premature atrial beat which reaches the fast pathway when that pathway is refractory, and thus it conducts down the slow pathway instead. From there, it goes back up the fast pathway, which is no longer refractory, and it's off to the races. Let me know if that helps!