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Supraventricular tachycardia (SVT) is caused by a problem with electrical impulses in the heart.
Heartbeats are normally initiated by a small group of cells at the top of the heart called the sinoatrial node, which acts as the heart's natural pacemaker.
The sinoatrial node produces electrical signals that pass through the muscles of the upper heart chambers (atria), causing them to contract and pump blood into the lower heart chambers (ventricles).
The signal then passes into another group of cells in the middle of the heart called the atrioventricular (AV) node. From here, the signals travel into the ventricle muscles, causing them to contract and pump blood out of the heart.
Episodes of SVT occur when a problem develops in this system. This causes faster signals to be sent around the heart, increasing the speed at which the heart beats.
In most cases, the problem is temporary and lasts for a few seconds, minutes, or in some cases hours.
There are several different types of SVT, which are classified by the specific problem in the heart that disrupts the electrical system.
A type of SVT called Wolff-Parkinson-White syndrome occurs as a result of an abnormal electrical connection between the atria and ventricles, which creates a short circuit in the heart's electrical system. This happens because people with Wolff-Parkinson-White syndrome are born with an extra strand of muscle tissue between these chambers.
In other cases of SVT, a short circuit occurs, even though the heart is otherwise normal.
SVT can also be caused by an electrical signal from another part of the heart overriding the signal from the sinoatrial node.
SVT is usually triggered by extra heartbeats (ectopic beats), which everyone has. It may also be triggered by:
However, in the majority of cases, there's no identifiable trigger for SVT.