Normal sinus rhythm (NSR) is the normal functioning rhythm of the heart. It is not an arrhythmia in the true sense of the word, because it is the normal state of functioning of the heart and not a pathologic state. While the heart is in NSR, it cycles through automaticity, depolarization, and repolarization to produce the coordinated contraction and relaxation of the muscle needed to propel blood through the circulatory system.
Most textbooks spend just a few paragraphs, at most, to present this topic; however, we are going to present it more thoroughly because it is the foundation of arrhythmia recognition. Knowing how and why things occur will greatly facilitate your understanding of the more complex arrhythmias.