Sinoatrial (SA) node !
The sinoatrial node is the heart's natural pacemaker, in the wall of the right atrium; it starts each heartbeat, 60 to 100 times a minute at rest.
Official name: Sinuatrial node
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What it is
A small cluster of special cells in the wall of the right atrium, near where the superior vena cava enters.
What it does
The heart's natural pacemaker: fires 60-100 times a minute at rest, starting each heartbeat (sinus rhythm).
Key facts
- Where
- Right atrial wall at SVC junction, upper end of crista terminalis; SA nodal artery from RCA (~60%) or circumflex (~40%)
- Function
- Pacemaker: starts each beat at 60–100/min at rest; sympathetic speeds it, vagus slows it
Relations
- Above
- Opening of the superior vena cava into the right atrium
- In front
- Base of the right auricle (right atrial appendage)
- Deep
- Muscle of the crista terminalis, the ridge that separates the rough pectinate wall of the right atrium from the smooth sinus venarum
- Superficial
- Epicardium and the sulcus terminalis, where incisions or cannulas near the caval junction can reach it
How to find it
It cannot be seen with the naked eye. In a dissection it is sought in the sulcus terminalis at the junction of the superior vena cava and right atrium, beside the base of the right auricle; the SA nodal artery, which often loops around the base of the superior vena cava, leads to it.
On imaging
Not seen on routine imaging; its activity shows on the ECG as normal P waves. Its site is inferred on cardiac CT or MRI at the junction of the superior vena cava and right atrium, along the crista terminalis. Coronary CT angiography can trace the SA nodal artery, usually from the right coronary. Echocardiography assesses the atria, not the node.
Common problems
- Sick sinus syndrome: the node fires too slowly or pauses, causing tiredness, dizziness or fainting, mostly in older adults.
How it is diagnosed
ECG and heart-rhythm monitoring (Holter).
Treatment
- A pacemaker when slow rhythms cause symptoms.
Clinical notes
- Sinus node dysfunction often takes a tachy-brady form, with bursts of atrial fibrillation alternating with slow rhythms or pauses
- Occlusion of the right coronary artery before its SA nodal branch can cause sinus bradycardia or sinus arrest
- Surgery at the junction of the superior vena cava and right atrium (venous cannulation, atrial or congenital repairs) can injure the node or its artery and cause sinus dysfunction afterwards
- Focal atrial tachycardias often arise along the crista terminalis, the same ridge that carries the node, and are mapped and ablated there
Anatomical variants
- The SA nodal artery comes from the right coronary artery in about 60% of hearts and from the circumflex in about 40%; rarely both supply it
Development
Forms in the wall of the right horn of the sinus venosus, which becomes part of the right atrium; this explains its position at the caval junction.
Related conditions
- Cardiac arrest (VF, pulseless VT, asystole, PEA)
- Sinus node dysfunction (sick sinus syndrome) and symptomatic bradycardia
- Syncope (vasovagal, orthostatic hypotension, cardiac)
- Premature atrial and ventricular contractions
- Postural orthostatic tachycardia syndrome (POTS) and inappropriate sinus tachycardia
- Beta-blocker and calcium-channel-blocker overdose
- Atrial fibrillation
Test yourself
Think of the answer, then tap the question to see it.
Where is the sinoatrial (SA) node located?
Under the epicardium where the superior vena cava meets the right atrium, at the upper end of the crista terminalis (sulcus terminalis outside).
Why does the SA node set the heart rhythm?
It has the fastest built-in rate of the conduction system; if it fails, slower pacemakers in the AV junction or the ventricles take over.
Which artery supplies the SA node?
The SA nodal artery, from the right coronary artery in about 60% and the circumflex in about 40%.
What defines sinus rhythm on the ECG?
A P wave before every QRS complex, upright in lead II and inverted in aVR.
Educational information, not medical advice. For any symptom, see a health professional.
Its names
- Official Latin (TA2)
- nodus sinuatrialis TA2 3953
- Official Spanish (DeCS)
- Nodo sinoatrial DeCS D012849
- English (TA2)
- Sinuatrial node
- Also called
- sinoatrial node · SA node · Flack's node · node of Keith-Flack · Koch's node
Related
Sources: Terminologia Anatomica (TA2); DeCS; Anatomy, Thorax, Sinoatrial Nodal Artery; Anatomy, Thorax, Heart.
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Representative model, for educational purposes only.
