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.