2020 Nov 21. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan–.
The sinoatrial node (SA) is the default pacemaker and therefore a crucial component of the heart's conduction system. It is located subepicardial and is crescent in shape. In an average adult, a sinoatrial node is 13.5 millimeters in length and is innervated by the vagus and sympathetic nerves. The sinoatrial nodal artery supplies blood to the sinoatrial node, it branches off the right coronary artery in 60% of cases, whereas in 40% of cases it comes off the left circumflex coronary artery. Sinus bradycardia is a cardiac rhythm with appropriate cardiac muscular depolarization initiating from the sinus node generating less than 60 beats per minute (bpm). The diagnosis of sinus bradycardia requires visualization of an electrocardiogram showing a normal sinus rhythm at a rate lower than 60 bpm. Where a normal sinus rhythm has the following criteria:
Regular rhythm, with a P wave before every QRS.
The p wave is upright in leads 1 and 2, P wave is biphasic in V1.
The maximum height of a P wave is less than or equal to 2.5 mm in leads 2 and 3.
The rate of the rhythm is between 60 bpm and 100 bpm.