How is the Right Coronary Artery (RCA) divided into proximal, mid, and distal segments?

Right Coronary Artery Segmentation: Proximal, Mid, and Distal Divisions

The Right Coronary Artery (RCA) is divided into proximal, middle, and distal segments based on its course through the right atrioventricular groove, with specific anatomical landmarks defining each segment according to the American Heart Association guidelines. 1

Anatomical Definition of RCA Segments

The RCA originates from the right coronary sinus and follows the right atrioventricular groove. The segmentation is as follows:

Proximal RCA (Segment 1)

• Begins at the origin from the right coronary sinus
• Extends through the initial portion of the right atrioventricular groove
• Best visualized using:
  • Precordial short axis at level of aortic valve
  • Precordial long axis (inferior tangential) of left ventricle
  • Subcostal coronal projection of right ventricular outflow tract
  • Subcostal short axis at level of atrioventricular groove 2

Middle RCA (Segment 2)

• Continues from the proximal segment along the right atrioventricular groove
• Follows the acute margin of the heart
• Best visualized using:
  • Precordial long axis of left ventricle (inferior tangential)
  • Apical 4-chamber view
  • Subcostal left ventricular long axis
  • Subcostal short axis at level of atrioventricular groove 2
• The proximal and mid RCA can be observed in the atrioventricular groove from the third intercostal space at the left and right sternal border 2

Distal RCA (Segment 3)

• Extends from the middle segment to the crux of the heart (where the atrioventricular groove meets the posterior interventricular groove)
• In right-dominant circulation, continues beyond the crux to give rise to the posterior descending artery
• Best visualized using:
  • Apical 4-chamber (inferior)
  • Subcostal atrial long axis (inferior) 2

Clinical Importance of RCA Segmentation

Understanding RCA segmentation is crucial for:

1. Accurate diagnosis of coronary lesions: Proximal RCA occlusions tend to have more severe acute presentations with higher incidence of right ventricular infarction (17.8% vs 10.6% in non-proximal occlusions) 3

2. Proper echocardiographic assessment: Different imaging planes are required for optimal visualization of each segment 2

3. Interpretation of ECG findings: Proximal RCA occlusion may cause right ventricular infarction with ST-segment elevation in right precordial leads (V3R and V4R) 1

Technical Considerations for Imaging

When performing echocardiography to visualize the RCA:

• Use the highest-frequency transducer possible for detailed evaluation
• Record studies in dynamic video or digital cine format for future comparison
• Make measurements from inner edge to inner edge
• Exclude points of branching which may have normal focal dilation 2

The feasibility of visualizing different segments varies significantly:

• Proximal RCA can be completely visualized in approximately 40% of patients with antegrade flow 4
• Middle RCA is visible in about 28% of patients 4
• Distal RCA is visible in about 54% of patients 4

Anatomical Variations

The caliber of the RCA proximal segment averages 3.42 ± 0.66 mm, decreasing to 2.9 ± 0.50 mm at the acute angle of the heart 5. These measurements are important for recognizing normal versus pathological findings during imaging studies.

Understanding these anatomical divisions is essential for accurate interpretation of coronary angiography, echocardiography, and other cardiac imaging modalities when evaluating coronary artery disease.