What is the difference between epicardial (outer layer of the heart) collaterals and septal (relating to the septum, a wall of tissue) collaterals in coronary artery disease?

Epicardial vs. Septal Collaterals in Coronary Artery Disease

Epicardial collaterals run along the surface of the heart and are more tortuous and difficult to navigate, while septal collaterals travel through the interventricular septum and are safer to use during interventional procedures due to their lower risk of perforation and ability to be safely dilated. 1

Anatomical Differences

Epicardial Collaterals

• Located on the outer surface (epicardium) of the heart
• Follow a more tortuous and unpredictable path
• More visible on angiography
• Higher risk of perforation during intervention
• Cannot be safely dilated with balloons
• More susceptible to compression during cardiac contraction

Septal Collaterals

• Course through the interventricular septum
• Generally straighter and less tortuous
• Sometimes invisible on angiography but still crossable using "surfing technique"
• Lower risk of perforation during intervention
• Can be safely dilated with small balloons to facilitate microcatheter passage 1
• Better protected within myocardial tissue

Clinical Importance and Functional Differences

Procedural Considerations

• Septal collaterals are preferred for retrograde approaches during chronic total occlusion (CTO) interventions due to:
  • Safer navigation with soft-tip, polymer-jacketed guidewires
  • Lower risk of cardiac tamponade if perforation occurs
  • Ability to be dilated with small balloons when necessary 1

Assessment Methods

• Both types of collaterals can be evaluated using:

  • Coronary pressure measurements
  • Flow velocity measurements
  • Collateral flow index (CFI) calculations 1

• The Werner classification is commonly used to grade collateral size:

  • CC0: No continuous connection
  • CC1: Threadlike connection
  • CC2: Side branch-like connection 1

Clinical Significance

Impact on Outcomes

• Well-developed collaterals (of either type) can:
  • Prevent myocardial ischemia during coronary occlusion
  • Reduce infarct size
  • Preserve ventricular function
  • Improve survival outcomes

Quantitative Assessment

• Collateral flow sufficient to prevent ischemia during coronary occlusion typically amounts to 20-25% of normal coronary flow 2
• In patients without ECG signs of ischemia during coronary occlusion, collateral flow index (CFI) values average around 46%, compared to only 18% in patients with ischemic ECG changes 1

Practical Applications in Interventional Procedures

Retrograde CTO Intervention

When planning retrograde approaches for CTO interventions:

1. Evaluate collateral channels for:

   • Size (larger is better)
   • Tortuosity (straighter is better)
   • Bifurcations (fewer is better)
   • Entry and exit angles
   • Distance from collateral exit to distal cap 1

2. Consider these key differences:

   • Septal collaterals: Safer, easier to navigate, can be dilated
   • Epicardial collaterals: Higher risk, more tortuous, cannot be safely dilated

3. Important caveat: When collateral circulation originates from the left anterior descending artery supplied by a mammary artery graft, access via this pathway increases the risk of global ischemia and should be avoided whenever possible 1

By understanding these differences between epicardial and septal collaterals, interventional cardiologists can make more informed decisions about procedural approaches, particularly in complex cases like chronic total occlusions, potentially improving both procedural success and patient outcomes.