Coronary collateral circulation is a network of interconnecting vessels that provides alternative blood flow to the myocardium when the primary coronary arteries are occluded or severely stenosed.
Definition and Overview
Coronary collaterals are pre-existing anastomotic connections between coronary artery territories that can enlarge and remodel in response to ischemia. They function as natural bypass channels that can limit infarct size and preserve myocardial function after coronary occlusion.
Classification of Collaterals
By Vessel Type
Arterioluminal:
- Connect small coronary arteries directly to the cardiac chambers
- Represent Thebesian veins connecting to the arterial system
- Minimal functional significance
Arteriosinusoidal:
- Connect arterioles to myocardial sinusoids
- May provide limited collateral flow
Arterioarterial:
- Direct connections between coronary arteries
- Most clinically significant collateral type
- Can enlarge to supply significant blood flow
By Size (Rentrop Classification)
Used during coronary angiography to grade collateral filling:
| Grade | Description |
|---|---|
| 0 | No visible collateral filling |
| 1 | Very faint collateral filling of side branches (not the epicardial vessel) |
| 2 | Partial filling of the epicardial vessel |
| 3 | Complete filling of the epicardial vessel |
By Source
Homocollaterals: Connect branches of the same coronary artery (e.g., septal-to-septal within the LAD)
Heterocollaterals: Connect different coronary arteries (e.g., RCA to LAD via septal collaterals)
Major Collateral Pathways
Septal Collaterals (Most Important)
The septal perforating arteries form the most extensive collateral network.
Connections:
- LAD septal perforators to PDA septal perforators
- LAD septal perforators to posterior descending septal perforators
Advantages:
- Protected by the septum from external compression
- Short, direct connections
- High capacity for flow
Epicardial Collaterals
Vieussens Ring: Anastomotic connection between the conus branch of the RCA and the LAD at the pulmonary infundibulum.
Other epicardial pathways:
- LAD to diagonal to obtuse marginal (LCx territory)
- Posterior LV branches of RCA to LCx marginal branches
- Acute marginal (RCA) to distal LAD
Apical Collaterals
Connections at the cardiac apex between:
- LAD and PDA (wrap-around LAD to PDA)
- LAD and posterior LV branches
These are common but provide less flow than septal collaterals.
Atrial Collaterals
Connections between atrial branches:
- Sinoatrial nodal artery to other atrial branches
- Atrial branches to the AV nodal artery
Development of Collaterals
Preexisting Collaterals
Collateral vessels exist in the normal heart but are small (< 100 microns) and carry minimal flow:
- Present from birth
- Formed during embryonic development
- Normally carry < 1% of total coronary blood flow
Arteriogenesis (Collateral Remodeling)
When a coronary artery becomes severely stenosed (> 70%), the pressure gradient across the collateral network drives flow.
Stages:
- Shear stress increase: Flow through preexisting collaterals increases
- Endothelial activation: Nitric oxide, adhesion molecule expression
- Monocyte recruitment: Macrophages clear debris, secrete growth factors
- Smooth muscle proliferation: Vascular wall remodeling
- Maturation: Collateral diameter increases up to 20-fold
Timeline:
- Pressure gradient: Immediate
- Increased flow: Hours to days
- Visible collaterals on angiogram: 2-4 weeks
- Mature collaterals: 1-3 months
Angiogenesis (New Vessel Formation)
In chronic ischemia, new capillary networks form through:
- Hypoxia-inducible factor (HIF) activation
- Vascular endothelial growth factor (VEGF) signaling
- Endothelial cell proliferation and migration
Factors Affecting Collateral Development
Positive Factors
| Factor | Effect |
|---|---|
| Chronic ischemia | Strongest stimulus for collateral development |
| Gradual coronary occlusion | Allows time for collateral maturation |
| Exercise training | Enhances collateral flow |
| Younger age | Better collateral remodeling |
| Normal endothelial function | Required for arteriogenesis |
| Nitric oxide | Mediates flow-dependent remodeling |
Negative Factors
| Factor | Effect |
|---|---|
| Diabetes mellitus | Impaired collateral formation |
| Advanced age | Reduced collateral response |
| Smoking | Endothelial dysfunction impairs remodeling |
| Hyperlipidemia | Reduces collateral flow |
| Hypertension | Associated with poorer collaterals |
| Obesity | Reduced collateral development |
| Female sex | Conflicting evidence |
Clinical Significance
Myocardial Protection
Infarct size reduction:
- Well-developed collaterals can reduce infarct size by 30-50%
- Preserve left ventricular function after coronary occlusion
- Reduce mortality in acute myocardial infarction
Ischemic preconditioning:
- Small ischemic episodes protect against subsequent larger ischemia
- Collateral recruitment is one mechanism
- Can be induced by angina before infarction
Angina Pectoris
Stable angina:
- Patients with good collaterals may have less angina
- Collaterals protect against exercise-induced ischemia
- May allow longer survival in chronic total occlusion
Angina-to-MI ratio:
- Patients with pre-infarction angina have better collaterals
- Pre-infarction angina is associated with smaller infarcts
Chronic Total Occlusion (CTO)
Collaterals maintain myocardial viability in CTO:
- 60-80% of CTO segments receive collateral flow
- Myocardium remains viable for months to years
- Successful recanalization improves regional function
Types of CTO Collaterals:
| Collateral type | Rentrop Grade | Clinical Significance |
|---|---|---|
| Septal | 2-3 | Best for retrograde CTO crossing |
| Epicardial | 2-3 | Higher risk of perforation |
| Apical | 1-2 | Limited for retrograde access |
Collaterals in Acute MI
Protective effects:
- Smaller CK-MB peak
- Better preserved LV function
- Lower incidence of cardiogenic shock
- Improved survival at 30 days and 1 year
Determinants of collateral protection:
- Duration of occlusion (sudden vs. gradual)
- Pre-existing collaterals (prior angina)
- Territory at risk (proximal vs. distal occlusion)
Therapeutic Enhancement of Collaterals
Approaches under investigation:
- Growth factor therapy: VEGF, FGF (limited clinical success)
- Gene therapy: HIF-1 alpha, VEGF genes (experimental)
- Cell therapy: Endothelial progenitor cells (investigational)
- External counterpulsation: Enhanced external counterpulsation (EECP) improves collateral flow
- Exercise training: Best-established non-pharmacologic approach
Imaging Collaterals
| Modality | Advantages | Limitations |
|---|---|---|
| Coronary angiography | Gold standard, interventional guidance | Invasive, radiation |
| CT coronary angiography | Noninvasive, anatomic detail | Radiation, contrast |
| MRI | Myocardial perfusion, viability | Lower resolution for small collaterals |
| PET | Quantitative flow assessment | Radiation, limited availability |
| Doppler guidewire | Physiologic assessment of collateral flow | Invasive, limited to catheterization |
Coronary Steal
Coronary steal occurs when pharmacologic vasodilation diverts flow away from collateral-dependent myocardium:
Mechanism:
- Vasodilators (adenosine, dipyridamole) dilate resistance vessels
- Normal arteries dilate more than collateral-dependent territories
- Flow is redirected from the collateral-dependent region to the normal region
- Can cause ischemia in collateral-dependent myocardium
Clinical relevance:
- Important during pharmacologic stress testing
- Contraindication to dipyridamole in patients with significant coronary disease
- Adenosine stress perfusion imaging relies on this phenomenon