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Sinoatrial node (SA node)

The sinoatrial node (SA node), often referred to as the natural pacemaker of the heart, is a specialized cluster of cardiomyocytes located in the superior posterolateral wall of the right atrium, near the junction of the superior vena cava (SVC) and the right atrial appendage. It is typically crescent-shaped and measures about 10–20 mm in length and 2–3 mm in thickness.

The SA node generates spontaneous electrical impulses through slow diastolic depolarization, setting the pace of the heartbeat. These impulses spread through the atria, leading to atrial contraction, and then travel to the atrioventricular (AV) node and subsequently the His–Purkinje system, coordinating ventricular contraction.

The SA node is supplied mainly by the SA nodal artery, which arises from the right coronary artery (RCA) in ~60% of individuals and from the circumflex artery (LCx) in ~40%.

Clinically, the SA node is vital in conditions such as sick sinus syndrome, arrhythmias, ischemia, and ablation therapy planning.

Synonyms

  • SA node

  • Pacemaker of the heart

  • Keith–Flack node

Function

  • Initiates the normal cardiac rhythm by spontaneous depolarization

  • Generates impulses at 60–100 beats per minute under normal physiological conditions

  • Coordinates atrial contraction and ventricular filling

  • Responds to autonomic nervous system input (sympathetic ↑ rate, parasympathetic ↓ rate)

  • Acts as the primary pacemaker of the heart

MRI Appearance

T1-weighted images:

  • SA node itself is too small to be distinctly seen; visualized indirectly as part of right atrial wall

  • Appears as low signal tissue within atrial myocardium near the SVC-RA junction

T2 Cine (Cardiac-gated):

  • Cine imaging provides indirect assessment by showing atrial contraction timing relative to electrical activity

  • Dysfunction may appear as abnormal atrial contraction patterns or atrial standstill

  • Used more for functional correlation rather than direct visualization

T1 Post-Contrast (Gadolinium-enhanced MRI):

  • Contrast highlights surrounding atrial myocardium and venous inflow tract

  • In late gadolinium enhancement (LGE): fibrosis or scarring in SA nodal region can be seen in arrhythmic patients

  • Useful in mapping sinus node dysfunction due to ischemia or fibrosis

MRA (Magnetic Resonance Angiography):

  • Used to visualize the SA nodal artery origin and course

  • Helps evaluate for vascular compromise of the SA node

CT Appearance

CT Coronary Angiography (CCTA):

  • SA node itself too small to visualize, but SA nodal artery can be identified in high-resolution scans

  • Critical in preoperative and interventional planning (e.g., atrial ablation) to avoid injury to the artery

  • CT may also reveal fibrosis, calcification, or adjacent atrial abnormalities that impact nodal function

CT Delayed Enhancement (CTDE):

  • May demonstrate fibrotic changes in the right atrium near the SA node region

  • Provides alternative to MRI in patients unsuitable for CMR

CT image

Sinoatrial node (SA node) anatomy CT axial image -img-00000-00000

MRI image

Sinoatrial node (SA node) MRI axial image