Atrioventricular nodal reentrant tachycardia (AVNRT)

The atria are the heart’s upper chambers; the ventricles are the lower chambers. Reentrant tachycardias are fast heart rates caused by electrical signals that loop back on themselves.

Normally, an electrical signal starts at the sinoatrial or SA node in the right atrium and propagates out through both atria, including bachmann’s bundle in the left atrium, and then contracts both atria. It’s then delayed just a little bit as it goes through the atrioventricular, or AV node, before it passes through the Bundle of His and on to the Purkinje fibers of the left and right ventricles, causing them to contract as well.

Usually, the only place where a signal can go from the atria to the ventricles is at the AV node, and once that signal gets to the purkinje fibers, it stops and the heart tissue waits for another signal from the SA node. With an atrioventricular reentrant tachycardia, or AVRT, the electrical signal actually uses a separate accessory pathway to get back up from the ventricles to the atria, which causes the atria to contract before the SA node sends out another signal. The signal then moves back down the AV node to the ventricles and purkinje fibers, contracts the ventricles, and goes back up that accessory pathway. This cycle repeats, which is why AVRT can result in rates as high as 200-300 bpm. This type of tachycardia is known as a supraventricular tachycardia because the signal causing the fast rate originates above the ventricles. The most common type of AVRT is Wolff-Parkinson-White syndrome, where the accessory pathway is called the Bundle of Kent. This type of reentry is known as an anatomical reentrant circuit because the accessory pathway is a fixed, anatomically-defined pathway.

Another type of reentrant circuit is atrioventricular nodal reentrant tachycardia, or AVNRT. AVNRT, just like AVRT, is a type of supraventricular tachycardia; however, with AVNRT it’s in or near the AV node, which similarly contracts the ventricle and the atria every time it goes around.

Specifically, there are two separate electrical pathways that make up this loop. One of these pathways has heart tissue that has slow electrical conduction; it’s called the alpha pathway. The other has fast conduction; it’s called the beta pathway. Additionally, the alpha pathway has a short refractory period, which is the time it takes to conduct another signal. The beta pathway, on the other hand, has a long refractory period. Once you have all these things, you’ve got yourself a recipe for AVNRT.

So, let’s say a signal comes down from the SA node in the right atrium, the signal goes down the fast pathway, and reaches the other end before the slow pathway. Next, it splits to travel down to the ventricles, as well as up the alpha pathway, where it meets the slow signal and they both cancel each other out.

At this point, both enter into their refractory period. Because the alpha pathway is shorter, it comes out of refractory sooner and is ready for another signal, while the beta pathway’s still in refractory. So, if another signal comes by, it’ll start down the slow pathway, but since the fast is still in refractory, it’ll be blocked.

At some point while the signal’s going down the alpha side, that beta side will come out of refractory, and then it’ll be ready to go. So, now as the signal exits the alpha pathway to the ventricles and enters the refractory period, it also travels up the beta pathway, and by the time it reaches the alpha pathway again, that pathway’s out of refractory!