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 node disease can lead to delays in rhythm, referred to as asystole, and classified as pauses and arrests.