What causes PSVT?

To understand PSVT, you first need to know how your heart beats normally.

Your heart is a set of muscles that form four chambers: the left atrium and right atrium on top, the left ventricle and right ventricle on bottom. A heartbeat occurs when the muscles contract to circulate blood through your body.

Normally, the signal for your heart's muscles to contract begins in the right atrium--in a group of cells called the sinus node--which sends out a tiny electrical impulse through both atria. The signal reaches another group of cells at the middle of the heart called the atrioventricular (AV) node. The AV node then passes the signal on to the ventricles. As a result, your heart contracts in two stages--first the atria contract (pushing blood into the ventricles), then the ventricles contract (pumping blood through your body).

In the most common form of PSVT, the signal leaving the AV node is somehow sent right back to the atria. (Physicians call this "reentry.") This causes the AV node to send out another signal before it is supposed to. Something like a "continuous loop" of electrical signals is created, making the heart beat unusually fast. There are other, slightly different ways that PSVT can occur. But the result is the same: a very rapid heartbeat.