One of the most popular values of the vascular duplex ultrasound is the resistivity index (RI) developed by Leandre Pourcelot, this value reflects the resistance to blood flow caused by microvascular bed distal to the site of measurement. It is used in arteries that have no reverse flow, the maximum achievable value is equal to one.
The formula of this tool is:
The RI is based on the fact that in a territory the high resistance of distal vessels produces a low diastolic flow in the artery responsible for bring irrigation to this area, increasing the difference between peak systolic velocity and end diastolic velocity. By contrast, a low resistance of distal vascular bed is characterized by increased diastolic flow which decreases the value of the numerator of our formula and is expressed numerically as a RI lower.
Here are two examples, the first we will calculate the RI of the external carotid artery that supply arterial flow to the facial muscles, this is higher than the RI of the internal carotid artery supplying the brain, showed in the second figure and calculated automatically by the ultrasound equipment.
Calculate the RI of the external carotid artery = (115 cm / sec - 18 cm / sec) / 115 cm / sec = 0.84
RI of the internal carotid artery, is less than the RI of the external carotid artery
The points that have become popular to the RI beyond the simplicity from the standpoint of management, are closely related to the clinical utility such as in the renal transplant evaluation, study of cancer or hypertension intracranial.
Read about the pulsatility index (spanish)
Derechos de autor Dra. Onelia Greatty