sinus rhythm

Sinus rhythm is the heart beating the way it is meant to — each beat started by the sinoatrial node, the natural pacemaker, and then handed off in proper order through the conduction system. It is the default, healthy rhythm, and on an ECG it looks neat and predictable.

Clinicians recognize it by a simple pattern: every beat begins with a P wave (the atria depolarizing under the node's command), each P wave is followed by a QRS complex (the ventricles depolarizing), and the rhythm is regular. When the resting rate falls between about 60 and 100 beats per minute, this is normal sinus rhythm. The same node-driven rhythm can simply be fast (sinus tachycardia, as during exercise or fever) or slow (sinus bradycardia, as in athletes or during sleep) — still sinus, just at a different tempo.

Sinus rhythm is the reference point against which arrhythmias are judged. In atrial fibrillation, for instance, the orderly P waves vanish and the rhythm becomes irregular because the sinoatrial node is no longer in charge. A common goal of treatments — from medications to cardioversion to ablation — is to restore or maintain sinus rhythm, returning the heart to its natural, coordinated pattern.