Table 1

Currently available non-hyperaemic pressure ratios (NHPRs)

NHPRCalculationPeriod of the cardiac cycleManufacturerThresholdSummary of evidence
Instantaneous wave-free ratio (iFR)Pd/Pa calculated during the WFP within diastole.WFP in diastolePhilips Healthcare≤0.89Validated by randomised prospective clinical trials: DEFINE-FLAIR and SWEDEHEART.3 5 iFR non-inferior to FFR for MACE at 1 year.
Resting full-cycle ratio (RFR)The lowest Pd/Pa over the entire cardiac cycle. Mean of 4–5 consecutive cycles.Whole cycleAbbott≤0.89VALIDATE RFR and RE-VALIDATE RFR both show diagnostic equivalence between RFR and iFR.7 33 RFR correlates as highly with FFR as do other NHPR’s.
Diastolic hyperemia-free ratio (DFR)Average Pd/Pa over the approximated* diastolic period averaged over five consecutive cardiac cycles.DiastoleBoston Scientific≤0.89DFR is diagnostically equivalent to iFR in multiple validation studies.8 22 DFR correlates as highly with FFR as do other NHPR’s.
Resting Pd/PaResting Pd/Pa averaged over the entire cardiac cycle.Whole cycleNot proprietary technology≤0.91Resting Pd/Pa provides excellent agreement with iFR and FFR. Resting Pd/Pa has slightly lower sensitivity to stenosis severity than iFR.
  • *Diastole approximated as negatively sloped segment of tracing where instantaneous Pa falls below mean Pa.

  • FFR, fractional flow reserve; Pd/Pa, ratio of mean distal coronary artery pressure to mean aortic pressure in the resting state; WFP, wave-free period.