Fractional flow reserve and non-hyperemic indices: Essential tools for percutaneous coronary interventions

Table 5 Non-hyperemic pressure ratio indices

Acronym	Full name	Manufacturer	Method	Threshold	Evidence
iFR	Instantaneous wave-free ratio	Philips Heathcare	Mean five Pd/Pa during the WFP	0.89	DEFINE FLAIR[13], SWEDEHAEART TRIAL[12]
RFR	Resting full-cycle ratio	Abbott	Lowest value of Pd/Pa across the whole cardiac cycle	0.89	VALIDATE RFR[68], RE-VALIDATE RFR[69]
DFR	Diastolic hyperemia-free ratio	Boston Scientific	Average 5 consecutive cardiac cycles Pd/Pa ratio over the approximateda diastolic period	0.89	Comparison of different diastolic resting indexes to iFR[15]
dPR	Diastolic pressure ratio	ACIST/Opsens Medical	Average 5 beats Pd/Pa ratio at the pressure peak-to-peak midpoint	0.89	Diastolic pressure ratio: New approach and validation vs the iFR[69]
Resting Pd/Pa		Not proprietary technology	Pd/Pa averaged over the entire cardiac cycle	0.91

^aDiastole approximated as negatively sloped segment of tracing where Pa falls below mean Pa. Pd: Distal coronary artery pressure; Pa: Aortic pressure; WFP: Wave Free Period. This table represents the multiple non-hyperemic pressure ratios available at this day. Each manufacturer has led its trial to support the use of its non-hyperemic pressure ratios (NHPR) indices. These studies compared the new calculations methods to the instantaneous wave free ratio (iFR) and showed good correlation between NHPR. These indices have the same cut-off value of 0.89 below which a coronary artery stenosis is considered functionally significant.