Sinus rhythmis the normal rhythm of the heart and results from proper activation of the entire heart in proper sequence. The sinoatrial (or sinus) node located in the right atrium spontaneously beats periodically to begin each activation, which proceeds to activate both atria before reaching the atrioventricular (AV) node. At the AV node, which under normal circumstances is the only electrical connection between the atria and ventricles, activation proceeds more slowly to allow for optimal ventricular filling. The activation then enters the His-Purkinje system, which allows a rapid spreading throughout the ventricles and induces a strong, synchronized contraction.
Any variation from normal sinus rhythm is termed an arrhythmia.