The sinoatrial (SA) node is the heart's natural pacemaker, a small crescent of specialised muscle cells that fire spontaneously faster than any other cardiac tissue. Each impulse it generates spreads across both atria and starts a heartbeat. At rest it usually fires about 60-100 times per minute.
| Location | Wall of the right atrium just beneath the epicardium, at the junction of the superior vena cava with the atrium, at the upper end of the crista terminalis. |
|---|---|
| Parts |
|
| Nerve Supply | Vagus nerve, mainly right (slows firing); Sympathetic cardiac nerves (speed firing) |
| Blood Supply | Sinoatrial nodal artery (from the right coronary or circumflex artery) |
Sets the heart rate by generating rhythmic electrical impulses. Its rate is raised by sympathetic stimulation and adrenaline during exercise or stress, and lowered by vagal (parasympathetic) tone at rest.
Well-trained endurance athletes often have resting rates in the 40s or lower because of high vagal tone, which is normal when it causes no symptoms. Sick sinus syndrome, common in older adults, causes slow or pausing rhythms and sometimes alternating fast rhythms, and often needs a pacemaker. The SA nodal artery comes from the right coronary artery in about 60% of people and from the circumflex in about 40%.
Degeneration of the SA node causing slow rates, pauses or alternating fast and slow rhythms. Dizziness, fatigue and fainting are common and a pacemaker is often needed.
A regular rhythm from the SA node below 60 beats per minute. It is normal in athletes and during sleep but can also reflect medication effects, hypothyroidism or nodal disease.
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