Table 1

All the functional parameters tested in patients with acute coronary syndrome

ParameterDefinitionFormulaAbnormal valueOutcome
FFRRatio of mean distal coronary pressure to mean aortic pressure during maximal hyperaemiaPdistal/Paorta during hyperaemia≤0.80Coronary stenosis functional severity
iFRMeasurement of intracoronary pressure during the diastolic ‘wave-free’ periodPdistal/Paorta during ‘wave free period’<0.90Coronary stenosis functional severity
CFRRatio of mean distal coronary peak flow velocity during maximal hyperaemia to mean peak flow velocity at restVelocity during hyperaemia/velocity during basal conditions<2*Coronary resistance
Of note, a reduced CFR can be caused by a significant epicardial stenosis, by coronary microvascular dysfunction or by both
HSRRatio between the pressure drop across the stenosis and distal peak flow velocity during maximal hyperaemia(Paorta-Pdistal)/velocity during hyperaemia>0.8 mmCoronary stenosis resistance
IMRProduct of distal coronary pressure by the mean transit time during maximal hyperaemiaDistal pression* mean transit time during hyperaemia<32–35 U Microvascular resistance
HMRRatio of pressure gradient and flow velocity measured during maximal hyperaemiaDistal pressure/mean flow velocity during hyperaemia>3.25 mm Hg/cm·s*Microvascular resistance

  • *As research on IMR and HMR is scarce, no general cut-off values have yet been proposed. In clinical studies, different cut-off values for IMR between 32 and 35 U are used and for HMR a cut-off value of 3.25 mm Hg/cm·s depending on the median value in the respective study group and/or ROC curves is used. For CFR a cut-off value of 2.0 is generally accepted.

  • CFR, coronary flow reserve; FFR, fractional flow reserve; HMR, hyperaemic microvascular resistance; HSR, hyperaemic stenosis resistance; iFR, instantaneous wave-free ratio; IMR, index of microvascular resistance; Paorta, proximal perfusion pressure; Pdistal, distal coronary pressure; ROC, receiver operating characteristic.