Currently available non-hyperaemic pressure ratios (NHPRs)
| NHPR | Calculation | Period of the cardiac cycle | Manufacturer | Threshold | Summary of evidence |
| Instantaneous wave-free ratio (iFR) | Pd/Pa calculated during the WFP within diastole. | WFP in diastole | Philips Healthcare | ≤0.89 | Validated 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 cycle | Abbott | ≤0.89 | VALIDATE 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. | Diastole | Boston Scientific | ≤0.89 | DFR is diagnostically equivalent to iFR in multiple validation studies.8 22 DFR correlates as highly with FFR as do other NHPR’s. |
| Resting Pd/Pa | Resting Pd/Pa averaged over the entire cardiac cycle. | Whole cycle | Not proprietary technology | ≤0.91 | Resting 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.