Unmasking Myocardial Bridge-Related Ischemia by Quantitative Flow Ratio Functional Evaluation.

Introduction: A myocardial bridge (MB) is a condition where a segment of an epicardial coronary artery passes through the myocardial muscle. While traditionally regarded as benign, MBs have been associated with various cardiovascular conditions. Therefore, assessing their hemodynamic impact is crucial for informed treatment decisions. Intracoronary functional assessments, such as fractional flow reserve (FFR) and instantaneous wave-free ratio (iFR), have proven useful, especially under inotropic stimulation. However, their invasive nature limits their widespread clinical application. The Quantitative Flow Ratio (QFR) has emerged as a minimally invasive alternative for functional evaluation of MBs, though data on its use are still limited. This study aims to compare the diagnostic efficacy of FFR, iFR, and QFR for evaluating MBs both at rest and under stress conditions.

Methods: Patients with confirmed MB on the LAD and typical angina (or abnormal non-invasive tests indicating myocardial ischemia) were included. According to a prespecified protocol, all patients underwent functional intracoronary evaluation with FFR and iFR at rest and after dobutamine and atropine intravenous infusion. QFR was also calculated for all cases both at rest and during dobutamine infusion. FFR values ≤0.80, iFR values ≤0.89 and QFR values ≤0.84 were considered indicative of significant myocardial ischemia.

Results: A total of 21 patients were included. Median FFR remained unchanged from rest (0.85) to stress (0.85), with only one patient showing a positive stress-FFR. In contrast, median iFR significantly decreased from 0.91 to 0.79 (p < 0.001), with stress-iFR ≤ 0.89 in 18 patients. Resting QFR did not indicate significant hemodynamic impact of the MB (median 0.90), but under inotropic stimulation, ischemia was detected in 18 patients (median 0.79, p < 0.001). QFR and iFR were concordant during stress in 19 patients, showing a significant positive correlation (Spearman's ρ = 0.702, p = 0.037) and comparable sensitivity (0.86).

Conclusions: QFR, computed during inotropic infusion, shows high sensitivity for detecting MB-related ischemia, comparable to stress-iFR and superior to stress-FFR. The correlation between stress-induced iFR and QFR suggests QFR as a reliable, minimally invasive alternative for functional lesion-specific evaluation in MB patients. Larger studies are necessary to confirm these preliminary findings and standardize QFR use in dynamic coronary stenosis assessments.