Introduction: Left main stem (LMS) disease poses a relevant challenge to interventionalists. For its assessment, a strategy relying on intracoronary imaging is suggested. Quantitative flow ratio (QFR), a novel angiography-based physiological flow index, may offer a novel method to assess relevance of LMS stenoses without the use of a guidewire only based on angiographic projections. We investigated the relationship between QFR on the one hand and intraluminal stenosis area as determined by intravascular imaging on the other hand.

Methods: In 53 patients with LMS disease, we compared functional assessment as determined by QFR with anatomic assessment through OCT (n=28) or IVUS (n=25) measuring the minimal lumen area (MLA) of the stenosis. LMS-QFR was measured using a dedicated approach, averaging QFR over left anterior descending (LAD) and circumflex (LCX) and manually limiting segment of interest to LMS.

Results: QFR showed a consistent correlation with LMS-MLA, as measured by intravascular imaging (R=0.61, p<0.001). QFR could predict with excellent diagnostic accuracy (AUC 0.92) a LMS-MLA<6 mm² and with very good accuracy (AUC 0.80) a LMS-MLA<4.5 mm². Similar results were obtained for other stenosis parameters.

Conclusion: QFR might be a valuable tool to assess the severity LMS disease. Further studies focusing on patient outcomes are needed to validate the effectiveness of this approach.