SVT is the most common type of arrhythmia in children, affecting about one in every 1,000 children. An extra electrical pathway in the heart causes the fast heart rate. The heart has four chambers: two upper chambers (called atria) and two lower chambers (called ventricles). Blood returning from the body first enters the right atrium, which squeezes blood into the right ventricle. Then it pauses to allow the ventricle to pump blood to the lungs. Oxygen-rich blood returns to the left atrium, gets pushed into the left ventricle, and then pumped back to the body. The sinus node is a specialized structure in the right atrium that controls the normal heart rate by releasing electrical pulses. These pulses travel across the heart and control when the muscle squeezes or pumps. Electrical signals usually travel along one path from the upper to the lower chambers. In SVT, the child has an extra pathway for the signal to follow. The signal travels down the normal pathway and then back up the extra one allowing the electricity to get stuck going in a loop. The signal then travels around and around, rather than one-way, and causes the heart to beat faster. Infants with SVT have heart rates of more than 180 beats per minute — and as high as 300 beats per minute. A normal heart rate for children younger than a year is between 100 and 180 beats per minute when calm.