What we call normal rhythm is stimulated by a group of cells in the upper chamber of our heart called the atrium. This group of cells is in an area of the atrium called the sinus node and functions as a "natural pacemaker" stimulating the atria and then the ventricles of our heart to contract.

In normal function (what doctors call normal sinus rhythm, generally with rates from 50 to 90 beats per minute in the resting individual), these cells can increase the rate due to activity (running,...