Supraventricular tachycardia (SVT) is a fast heart rhythm involving the top part of the heart and/or the normal connection between the top and the bottom of the heart, the atrioventricular node (AV node). SVT is caused by an extra pathway in about 90% of people and an irritable spot in the remaining 10%.
AV node re-entry tachycardia (AVNRT)
This is the most common form of SVT, comprising about 60% of cases. AVNRT involves a short circuit that is actually part of the normal pathway (the AV node) that takes the heartbeat from the top to the bottom of the heart. Instead of having one AV node, patients with AVNRT have two: a fast and a slow AV node pathway. The SVT occurs when an early heartbeat blocks in one pathway and then goes down the other slowly enough so that when it reaches the far end of the first pathway, it can turn around and come up it back to the top again and then back down again – and so on like a dog chasing its tail. Usually, the slow pathway conducts electrical impulses to the bottom of the heart and the fast pathway conducts it back to the top. But in rare cases, the circuit consists of the fast pathway to the bottom and the slow pathway to the top or even two different slow pathways. Usually these different pathways are very near the main part of the AV node, but they may occur at some distance either on the right or left side of the heart. These pathways never are hereditary as far as we know.
AV re-entry tachycardia (AVRT)
This is the second-most common form of SVT, comprising about 30% of cases. AVRT involves a short circuit usually somewhat removed from the normal pathway, the AV node. These accessory pathways are present since birth and result from incomplete separation of the top from the bottom part of the heart.
Only about 1% of these pathways are hereditary. They can allow conduction from the top to the bottom of the heart only, from the bottom to the top only or both directions. When the accessory pathway allows conduction from top to bottom or both ways then the Wolff-Parkinson-White syndrome (WPW) is said to exist. The accessory pathway can cause SVT as long as it conducts from the bottom to the top. This happens when an early beat goes down the normal pathway with enough slowing so that when it reaches the bottom end of the accessory pathway, it can turn around and come up it to the top and then back down the AV node and so on, like a dog chasing its tail. More rarely, the early beat can go down the accessory pathway and then come back up the AV node. Most of these accessory pathways are just under the inner lining of the heart, but occasionally can run near the outer lining, usually within a vein on the back of the heart called the coronary sinus. Usually, only one accessory pathway is present, but 10-15% of people may have more than one.
This least common type of SVT involves one or more irritable spots that have formed after birth in one of the atria (the top chambers). These spots usually form around the valves that separate the top from the bottom of the heart (the tricuspid valve on the right side and the mitral valve on the left side of the heart). They can also form within the pulmonary veins on the left side or along the crista terminalis (a ridge running around inside of the right atrium). They are virtually never hereditary. They may occur by themselves or with other arrhythmias such as atrial flutter or atrial fibrillation. Patients with atrial tachycardia are more likely to have high blood pressure or something else wrong with their heart than patients with other types of SVT.