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The presence of a concomitant CTO may influence the FFR measurement in donor vessel. We sought to investigate the immediate physiological impact of CTO recanalisation on donor vessel pressure-derived indices.34 out of 40 consecutive patients underwent successful PCI to RCA CTOs. Resting Pd/Pa,iFR and FFR were measured pre and post-successful CTO PCI in donor vessels and collateral FFR in the CTO vessel.The angiographic details are as outlined in table 1. The mean resting Pd/Pa, iFR and FFR pre and post CTO PCI in major donor vessel were (0.891,0.858, 0.759) and (0.903, 0.882, 0.746) (p=0.109, p=0.012, p=0.388) respectively.iFR in the major donor vessel increased from 0.858 to 0.882 (difference, 0.02412; p=0.012). The mean resting Pd/Pa, iFR and FFR pre and post CTO PCI in minor donor vessel were (0.982, 0.969, 0.894) and (0.985, 0.979, 0.885), (p=0.534, p=0.152, p=0.183) respectively. The mean collateral FFR was 0.310. The mean total ischaemic burden on baseline cardiac MRI in RCA territory was 12.6%.Successful recanalisation of a RCA CTO results in a significant increase in the iFR of the major donor vessel but no significant difference was seen in resting Pd/Pa and FFR. Complete collateral regression was not observed in all patients immediately post RCA CTO PCI and this may account for the non-significant change in FFR values.