A 67-year-old man arrived at the emergency department complaining of continuous chest discomfort and shortness of breath for the past two days, with a marked worsening of both symptoms during the two hours prior to presentation. 

ECG showed a biphasic T wave in lead III without other significant ST-segment changes. 

Emergent coronary angiography via left distal radial access revealed subtotal occlusion of the distal LCX and total occlusion of the RCA with well-developed collaterals—suggesting a chronic total occlusion (CTO).
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The LCX was presumed the culprit and was treated with a drug-coated balloon (DCB). Despite successful treatment, the patient still has chest discomfort. Echocardiogram showed inferolateral hypokinesis. Re-evaluation led to RCA wiring, which proved unexpectedly easy—raising concerns about subintimal passage or the possibility of an acute thrombotic occlusion rather than CTO. IVUS confirmed wire position within the true lumen and revealing plaque rupture and thrombus, thereby establishing the RCA as the true culprit lesion. Prompt PCI was performed: balloon pre-dilation followed by deployment of a drug-eluting stent. IVUS ensured optimal stent expansion and confirmed absence of dissection or malapposition. Post-procedurally, the patient stabilized, and follow-up ECG showed T-wave inversion in the inferior leads, consistent with evolving infarction in the RCA territory.
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Collateral flow to an acutely thrombosed artery may mimic a CTO, leading to misidentification and mistreatment of the true culprit lesion. Re-evaluation and dynamic decision-making are essential. In this case, IVUS proved critical at two key points: (1) confirming wire position within the true lumen, and (2) revealing thrombus and plaque rupture—distinguishing an acute thrombotic occlusion from a presumed CTO in the RCA.