The ramus (intermediate) artery is a branch of which coronary artery?

Ramus Intermedius Origin

The ramus intermedius (RI) artery is a branch of the left main coronary artery (LMCA), arising when the LMCA trifurcates instead of the typical bifurcation pattern. 1, 2

Anatomical Pattern

Normally, the LMCA bifurcates into two vessels:

• Left anterior descending artery (LAD)
• Left circumflex artery (LCX)

However, in some individuals, the LMCA demonstrates a trifurcation pattern, giving rise to three vessels:

• LAD
• LCX
• Ramus intermedius (RI) - the third branch arising between the LAD and LCX 1, 2

Clinical Significance

The presence of an RI has important implications:

Anatomical impact: When an RI is present, the proximal diameters of both the LAD and LCX are significantly smaller compared to patients without an RI 3. The RI essentially "steals" caliber from the other two branches.

Distribution patterns: The RI can course closer to the LAD, closer to the LCX, or take a middle position between them, which correlates with the relative sizes of the LAD and LCX 3.

Diagnostic challenge: Angiographically, the RI can be difficult to distinguish from a high-origin diagonal branch or a high-origin obtuse marginal branch. IVUS studies show that only 54% of angiographically-identified RIs are confirmed as true RIs upon intravascular ultrasound examination 4. This has critical implications for revascularization planning in left main bifurcation lesions.

Collateral potential: In cases of multivessel coronary occlusion, a patent RI can serve as a critical collateral source of myocardial blood supply 2.