A 66-year-old female presented with atypical chest pain 15 years after coronary bypass grafting with a left internal mammary artery (LIMA) graft to the left anterior descending artery (LAD). Coronary angiography demonstrated a small calibre but undiseased LIMA graft with poor antegrade flow into the distal LAD (Panel A). The conus branch of the right coronary artery (RCA) supplied collaterals to the distal LAD (Panel B), and there was no ischaemia evident on a subsequent myocardial perfusion

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... n. Multidetector computed tomography (CT) coronary angiography confirmed collateral supply from the conus branch to the distal LAD (Panels C and D). On reviewing the angiogram, in light of the CT findings, the conus branch preferentially filled the distal LAD and there was a small amount of back filling of the proximal LAD and circumflex. This was felt to be the reason for the poorly developed LIMA graft. Alternatively, collaterals may have developed due to poor LIMA function. The conus branch of the RCA has special anatomical and functional significance in the development of collaterals between the right and left coronary arteries. In this case, collaterals from the conus artery were sufficiently well developed to preferentially fill the LAD, which may explain the relative failure of the LIMA graft. We describe a case of single, well-developed epicardial collateral vessel providing blood supply to the distal LAD preferentially over an LIMA bypass graft. This case also highlights the difficulties in imaging this 'watershed' area of the LAD beyond graft insertion and the strengths of using different imaging modalities to visualize the area, both anatomically and functionally. (Panel A) Coronary angiogram showing patent LIMA graft and poor filling of LAD and diagonal branch (D1). (Panel B) Collaterals from the conus branch of RCA supplying to the LAD. (Panel C) Multiplanar reconstruction of multidetector CT showing proximally occluded left coronary artery and a small calibre but patent LIMA graft. (Panel D) Volume-rendered 3D reconstruction shows the course of conus artery. Published on behalf of the