Just like in medical school, it’s important to understand what happens normally before you start tackling abnormal cases. Your heartbeat begins when a group of cells in the upper right chamber or atrium, called the sinus node, fire up electrical signals. These signals spread through the cells of your heart like a wildfire until they reach the bottom chambers (ventricles) and cause the heart muscle to contract, which pumps blood to your lungs and the rest of your body. Normally (unless you are as athletic as Lance Armstrong or Usain Bolt), your heart cranks up a fresh set of electrical signals to produce a new heartbeat 60 to 100 times a minute.
However, during atrial fibrillation, the electrical signals that produce heartbeats don’t start in the sinus node. Instead, they come from another part of the heart or from neighboring pulmonary veins. And instead of traveling in an orderly way from top to bottom, the electrical impulses rev up and scatter like insects when you lift up a log. As a result, instead of the usual, healthy pitter-patter, your heart starts sputtering rapidly and may beat up to 175 times a minute. That often results in less blood flow being pumped from your heart to the rest of your body.