Normal Sinus Rhythm

To pump blood through the body, the heart "runs" on electrical impulses that stimulate its chambers (atria and ventricles) to rhythmically contract and relax with each heartbeat. Most healthy hearts have normal sinus rhythm. "Normal" refers to a heart (pulse) rate between 60 and 100. "Sinus" refers to the sinoatrial or SA node -- The origin of the electrical impulse that "starts" each beat.

We trace the flow of electricity through the heart with an ECG (electrocardiogram) as shown in the animation. The "P wave" represents the electricity flowing through the atria or upper chambers of the heart. The "QRS complex" represents the flow through the ventricles, the bottom chambers, as they contract to push the blood out from the heart. The "T wave" represents repolarization, the electrical "resetting" of the heart for the next beat. In the normal sinus rhythm the ECG waves are usually regular, meaning their peaks are evenly spaced.

Atrial Fibrillation

The most common abnormal heart rhythm is atrial fibrillation, which causes an "irregularly irregular" heartbeat, with no repeat pattern. Instead of a single electrical impulse from the SA node, about 400 per minute "fire" in the atria - but only a few are transmitted through the AV node. On the ECG, the P wave is replaced by a bumpy line that doctors refer to as "fibrillatory waves," and the QRS complex is unevenly and irregularly spaced. In atrial fibrillation the atria are not contracting effectively, so the blood swirls, pools, and may form clots that could travel to the brain, causing a stroke.