Sinoatrial Node
The sinoatrial node is the primary pacemaker of the heart, a small cluster of specialized autorhythmic cells located in the posterior wall of the right atrium near the superior vena cava. It spontaneously depolarizes at the highest intrinsic rate of any cardiac tissue, setting the heart rate under normal conditions.
Also known as: SA Node, Sinus Node, Keith-Flack Node, Pacemaker of the Heart
Anatomy & Physiology
| Location | Posterior wall of the right atrium at the junction of the superior vena cava and the right atrium (sulcus terminalis). It is a crescent-shaped structure approximately 10–20 mm long and 2–3 mm wide in humans. |
| Function | The SA node acts as the dominant cardiac pacemaker by generating spontaneous action potentials at an intrinsic rate of 60–100 bpm in humans (60–140 bpm in dogs, 140–240 bpm in cats). Depolarization spreads outward through both atria via gap junctions and specialized internodal pathways, culminating in atrial contraction and triggering conduction through the AV node. |
| Conduction Velocity | 0.05 m/s (within the node); atrial myocardium conducts at ~1.0 m/s |
| Blood Supply | SA nodal artery, which arises from the right coronary artery (RCA) in approximately 60% of humans and from the left circumflex artery (LCx) in the remaining 40%. Occlusion of this artery can cause sinus node dysfunction or sick sinus syndrome. |
Clinical Relevance
The SA node is the origin of all normal cardiac rhythms. Dysfunction produces sick sinus syndrome, sinus bradycardia, sinus pauses, and sinus arrest. Increased sympathetic tone (exercise, pain, fever) accelerates SA node firing; increased vagal tone (vagal maneuvers, high athletic fitness) slows it. In dogs, respiratory sinus arrhythmia — cyclic rate variation tied to breathing — originates at the SA node and is considered a normal finding. SA node disease is treated with artificial cardiac pacing.
Associated Pathologies
- Sick sinus syndrome
- Sinus bradycardia
- Sinus tachycardia
- Sinus pause / sinus arrest
- Sinoatrial exit block
- Respiratory sinus arrhythmia
- Tachy-brady syndrome
- Inappropriate sinus tachycardia
Frequently Asked Questions
Where is the SA node located in the heart?
The SA node is located in the posterior wall of the right atrium at the junction with the superior vena cava, within a region called the sulcus terminalis. It sits just beneath the epicardium and is approximately 10–20 mm long in humans.
What is the intrinsic firing rate of the SA node?
The SA node fires at an intrinsic rate of 60–100 beats per minute in humans. In dogs the range is approximately 60–140 bpm, and in cats it is 140–240 bpm. Under autonomic influence, sympathetic stimulation increases this rate and parasympathetic (vagal) stimulation decreases it.
What happens when the SA node fails?
When the SA node fails to generate an impulse or the impulse cannot exit the node, subsidiary pacemakers take over. The AV node assumes pacing at 40–60 bpm (junctional rhythm) and, if that fails, the ventricular Purkinje system paces at 20–40 bpm (idioventricular rhythm). Clinically, SA node failure manifests as sick sinus syndrome, sinus pauses, or complete sinus arrest, and may require permanent pacemaker implantation.
Why is sinus arrhythmia normal in dogs?
Respiratory sinus arrhythmia occurs because vagal tone in dogs is normally high and modulated by breathing — heart rate increases during inspiration and decreases during expiration. This is a physiologically normal finding reflecting healthy autonomic tone and should not be mistaken for a pathological arrhythmia.
Which coronary artery supplies the SA node?
In approximately 60% of people, the SA node is supplied by the SA nodal artery, which is a branch of the right coronary artery. In the remaining 40%, it arises from the left circumflex artery. This is why right coronary artery occlusion (inferior MI) can cause sinus node dysfunction.
See It in Action
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