Are diagonal branches, obtuse marginal (om) branches, and septal branches considered epicardial arteries?

Diagonal Branches, Obtuse Marginal Branches, and Septal Branches Are Epicardial Coronary Arteries

Yes, diagonal branches, obtuse marginal (OM) branches, and septal branches are considered epicardial coronary arteries, with diagonal and obtuse marginal branches being true epicardial vessels while septal branches quickly become intramyocardial vessels after their origin.

Anatomical Classification of Coronary Arteries

The coronary arterial system can be classified into:

1. Main Epicardial Coronary Arteries:

   • Left main coronary artery
   • Left anterior descending artery (LAD)
   • Left circumflex artery (LCx)
   • Right coronary artery (RCA)

2. Major Branches of Epicardial Arteries:

   • Diagonal branches: Originate from the LAD and supply the anterolateral wall of the left ventricle 1
   • Obtuse marginal branches: Originate from the left circumflex artery and supply the lateral wall of the left ventricle 1
   • Septal branches: Originate primarily from the LAD (anterior descending septal perforators) or from the posterior descending artery (posterior descending septal perforators) 1

Characteristics of These Branches

Diagonal Branches

• Typically classified as first, second, and third diagonal branches
• Run along the epicardial surface of the heart
• Supply the anterolateral wall of the left ventricle
• True epicardial vessels that can be visualized on coronary angiography 1

Obtuse Marginal Branches

• Classified as first, second, and third obtuse marginal branches
• Run along the epicardial surface of the heart
• Supply the lateral wall of the left ventricle
• True epicardial vessels that can be visualized on coronary angiography 1

Septal Branches

• Originate as epicardial vessels but quickly penetrate into the myocardium (intramyocardial)
• Anterior septal perforators arise from the LAD
• Posterior septal perforators arise from the posterior descending artery (PDA)
• Supply the interventricular septum 1

Clinical Significance

The classification of these vessels as epicardial has important clinical implications:

1. Visualization during Angiography:

   • Epicardial vessels are visible on coronary angiography and CT angiography
   • Reporting should include all coronary arteries of 1.5-2 mm in diameter, including diagonal and obtuse marginal branches 1

2. Revascularization Considerations:

   • Epicardial vessels are accessible for percutaneous coronary intervention and surgical bypass
   • Intramyocardial vessels (like septal branches after they penetrate the myocardium) may not be easily bypassed surgically 2

3. Ischemia Patterns:

   • Occlusion of diagonal branches affects the anterolateral wall
   • Occlusion of obtuse marginal branches affects the lateral wall
   • Occlusion of septal branches affects the interventricular septum 1

4. Collateral Circulation:

   • Septal branches can serve as important collateral channels in chronic total occlusions 1
   • Epicardial collaterals (including diagonal and obtuse marginal branches) may provide alternative routes for blood flow in coronary occlusion 1

Anatomical Variations

It's worth noting that there can be variations in the origin and course of these branches:

• Septal branches may occasionally originate from diagonal branches, obtuse marginal branches, or directly from the left main coronary artery 3, 4
• Obtuse marginal branches may rarely originate from the LAD or diagonal branches 2, 5

Conclusion

Diagonal branches and obtuse marginal branches are true epicardial coronary arteries that run along the surface of the heart. Septal branches begin as epicardial vessels at their origin but quickly become intramyocardial as they penetrate into the interventricular septum to supply it with blood.