In a patient undergoing coronary artery bypass grafting (CABG) with competitive flows, which graft is more likely to close, the Left Internal Mammary Artery (LIMA) graft or the Saphenous Vein Graft (SVG)?

LIMA Grafts Are More Likely to Close with Competitive Flow

In the setting of competitive flows during CABG, arterial grafts (including LIMA) are more susceptible to closure than saphenous vein grafts (SVG), particularly when grafting vessels with less severe stenoses. This is a critical technical consideration that directly impacts graft patency and patient outcomes.

Mechanism of Competitive Flow-Related Graft Failure

Arterial Graft Vulnerability

• Arterial grafts, including LIMA, are more prone to closure from competitive flow because they are muscular conduits that respond to low-flow states with vasospasm and eventual atrophy 1
• The radial artery (another arterial conduit) demonstrates this vulnerability most clearly: it is "susceptible to spasm and atrophy when used to graft a coronary artery that is not severely narrowed" 1
• Arterial grafts should not be used to bypass vessels with less than critical stenosis (<90% for right coronary artery) specifically because of competitive flow-related failure 1

SVG Relative Resistance

• SVGs, while having inferior long-term patency overall (50-60% at 10 years), are less susceptible to acute competitive flow-related closure because they lack the muscular reactivity of arterial conduits 1
• SVG failure mechanisms are primarily related to endothelial damage during harvesting, intimal hyperplasia, and accelerated atherosclerosis rather than competitive flow 1, 2
• Early SVG failure (10-25% within first year) is predominantly due to thrombosis from endothelial injury, not competitive flow 1, 2

Clinical Evidence and Guidelines

Stenosis Severity Requirements

• The 2021 ACC/AHA/SCAI guidelines explicitly state that radial artery grafts should target vessels with "subocclusive stenoses" 1
• Arterial grafting of the right coronary artery is only reasonable when critical (>90%) stenosis is present (Class IIb recommendation) 1
• Using an arterial graft to bypass the right coronary artery with less than critical stenosis (<90%) is classified as Class III: HARM 1
• Radial artery grafts should be used for left-sided coronary arteries with severe stenoses (>70%) and right-sided arteries with critical stenoses (>90%) 1

LIMA-Specific Considerations

• While LIMA to LAD has >90% patency at 10 years when used appropriately, this superior performance assumes grafting to a significantly stenosed vessel 1, 3
• The LIMA's resistance to atherosclerosis (only 4% develop atherosclerosis, 1% develop hemodynamically significant stenoses) does not protect it from competitive flow-related closure 3
• The LIMA's protective mechanisms—continuous internal elastic lamina and endothelial release of prostacyclin and nitric oxide—prevent atherosclerosis but do not prevent flow-mediated atrophy 3

Critical Pitfalls to Avoid

Target Vessel Selection

• Never use arterial grafts (including LIMA) to bypass vessels without adequate stenosis severity, as competitive flow will promote graft failure 1, 2
• Ensure target vessels have appropriate stenosis: LAD and left-sided vessels should have ≥70% stenosis; right coronary artery should have ≥90% stenosis for arterial grafting 1
• Composite radial artery grafts have been found to be "more vulnerable to the effect of chronic native competitive flow" in observational studies 1

Intraoperative Assessment

• Use transit-time flow measurement (TTFM) intraoperatively to assess graft flow parameters 4
• Arterial grafts demonstrate lower pulse index (PI) values when competitive flow is present 4
• Consider alternative conduit strategies if target vessel stenosis is inadequate 1, 2

Practical Algorithm for Conduit Selection

When Competitive Flow Is Anticipated:

1. Reassess target vessel stenosis severity before selecting arterial conduit 1
2. If stenosis is <70% for left-sided vessels or <90% for right coronary artery, strongly consider SVG over arterial graft 1
3. If arterial grafting is still desired, ensure target vessel has adequate distal runoff and consider intraoperative flow assessment 1, 2
4. For patients with limited life expectancy where long-term patency is less critical, SVG is appropriate even for more severe stenoses 2

Optimal Scenarios for LIMA Use:

• LIMA to LAD remains Class I recommendation when LAD has significant stenosis 1, 5
• LIMA demonstrates 85-88% patency at 10 years when used appropriately 6
• Vessel size >2.0 mm diameter significantly improves graft patency (88% vs 55% for vessels ≤2.0 mm) 6