The Sinus Node
The onset of depolarisation in the healthy heart begins within a focus of pacemaker cells found at the upper right border of the heart. These cells which give the initial activation are collectively known as the sinus (or sinoatrial) node.
Healthy rhythms are classified by heart rate as follows:
- sinus rhythm — a heart rate between 50 and 120 bpm.
- sinus tachycardia — a heart rate over 120 bpm.
- sinus bradycardia — a heart rate below 50 bpm.
Sinus node disease can lead to delays in rhythm, referred to as asystole, and classified as pauses and arrests.