A normal adult heart beats between 60 and 100 times a minute. A heart rate over 100 beats a minute is called tachycardia. Some tachycardias are relatively harmless and need no treatment, but others can be life-threatening. Your heart is a muscular pump that circulates blood all around your body. There are four hollow chambers in your heart — the two upper chambers are the atria, and the lower, more muscular chambers are the ventricles. Each heartbeat begins in the right atrium. There, the heart’s natural pacemaker, called the sinus node, sends an electrical signal that causes the atria to contract, filling the ventricles with blood. A split second later, the electrical impulse travels across the atrioventricular (AV) node into the ventricles. This makes the ventricles contract, sending blood throughout the body. In people with tachycardias, this normal rhythm is disrupted somewhere along the electrical path, causing the heart to beat too quickly.