The arterial<unk> ty<unk>  (RI) is a measure of pulsatile blood flow that reflects the resistance to blood flow caused by microvascular bed distal to the site of measurement. A<unk>  of 0 corresponds to continuous flow; a<unk>  of 1 corresponds to systolic but no diastolic flow; and a<unk>  greater than 1 corresponds to reversed diastolic flow.
Formula = (Peak systolic velocity - End diastolic velocity) / Peak systolic velocity
The RI is altered not by vascular resistance alone but by the combination of vascular resistance and vascular compliance.
Resistive index
Extraction ratio
E/a ratio
Oxygen Enhancement Ratio
Volumetric flow rate
Anion gap
Polarization density
Ventilation/perfusion ratio