Resting Full-Cycle Ratio (RFR) in the Assessment of Left Main Coronary Disease: Caution Required

Hyperaemia-free indices have been gaining traction in recent times due to the practical advantages they offer over the fractional flow reserve (FFR) in the evaluation of angiographically intermediate coronary lesions. More recently, a new hyperaemia-free index, the resting full-cycle ratio (RFR), was described and found to correlate closely with the instantaneous wave-free ratio (iFR). The comparison between FFR and these hyperaemia-free indices, however, is nuanced and remains an ongoing area of debate and investigation. Herein, we highlight one of the important differences between the RFR and FFR, specifically in relation to the assessment of left main coronary lesion. We contend that the interchangeability of these indices cannot always be assumed and clinicians need to be aware of these limitations in their clinical practice.