Key Points

• The resistive index (RI) quantifies downstream vascular resistance via Doppler ultrasound.
• Angle independence and reproducibility enhance clinical reliability.
• Organ-specific RI thresholds inform diagnosis, prognostication, and management.
• Real-time calculators streamline interpretation and decision-making.

Fundamental Principles

Definition and Formula
Resistive index is calculated as RI = (PSV – EDV) / PSV, yielding a dimensionless value that reflects microvascular resistance.

Technical Advantages
Reproducible across insonation angles, RI remains less operator-dependent than absolute velocity measurements.

Renal Applications

Normal and Pediatric Values
Adult renal arteries typically exhibit RI between 0.50 and 0.70. Pediatric values are higher in early childhood and normalize by six years of age.

Intrinsic Renal Disease
An RI above 0.70 suggests conditions such as acute tubular necrosis, glomerulonephritis, or obstructive uropathy. Elevated RI correlates with interstitial edema and increased tubular pressure.

Renal Transplant Assessment
Post-transplant kidneys demonstrate dynamic RI changes. An RI exceeding 0.80 beyond the immediate postoperative period signals possible rejection, vascular thrombosis, or ureteric obstruction. Early transient RI elevation (< 72 hours) is common and typically without clinical consequence. Persistent high RI predicts poorer graft survival and may prompt biopsy or immunosuppression adjustment.

Prognostic Significance
Higher RI values correlate with accelerated decline in renal function and increased mortality in chronic kidney disease. In critical care, RI predicts acute kidney injury by reflecting ventilator pressures, hypoxemia, and systemic inflammation.

Hepatic Applications

Normal and Transplant Ranges
Hepatic arterial RI of 0.55–0.80 is considered normal in both native and transplanted livers.

Transplant Monitoring
Low hepatic RI (< 0.55) early after transplant suggests arterial anastomotic stenosis or arteriovenous shunting. High RI (> 0.80) beyond one week may indicate chronic rejection, venous congestion, or microvascular compression.

NAFLD Fibrosis Evaluation
In non-alcoholic fatty liver disease, RI values exceeding 0.90 correlate with advanced fibrosis and may guide decisions on liver biopsy for staging.

Testicular Applications

Normal Values and Infertility Screening
Intratesticular RI averages 0.54 in fertile men. An RI above 0.60 associates with oligoasthenospermia. Capsular artery RI demonstrates approximately 90% sensitivity and 77% specificity for identifying subfertile patients.

Carotid Applications

Stenosis Detection
A side-to-side RI difference greater than 0.10 indicates severe internal carotid artery stenosis or occlusion, aiding in non-invasive vascular assessment.

Hypertension Evaluation
Hypertensive individuals exhibit mean carotid RI of 0.78 versus 0.56 in normotensive controls. Elevated RI correlates with increased arterial stiffness and cardiovascular risk.

Obstetric Applications

Umbilical Artery Surveillance
Umbilical artery RI provides essential insights into placental vascular resistance. During a normal pregnancy, RI values gradually decrease or remain stable from mid–third trimester, reflecting improved placental perfusion.

Early Detection of Placental Insufficiency
A rising RI—especially above the 95th percentile for gestational age—indicates increasing placental resistance. This change often precedes clinical signs of fetal compromise by days to weeks, enabling timely intervention.

Prediction of Fetal Growth Restriction (FGR)
Persistent elevation of RI correlates strongly with FGR. Doppler studies show that an RI > 0.75 at 28–32 weeks gestation predicts small-for-gestational-age infants with high specificity.

Absent and Reversed End-Diastolic Flow
Absent end-diastolic flow (AEDF) carries a > 60% risk of adverse perinatal outcome within one week. Reversed end-diastolic flow (REDF) is associated with > 80% perinatal mortality if delivery is not expedited.

Middle Cerebral Artery (MCA) and Cerebroplacental Ratio
Combining umbilical RI with MCA RI to calculate the cerebroplacental ratio (CPR) enhances predictive accuracy for fetal acidemia. A CPR < 1.0 indicates brain-sparing physiology and warrants increased surveillance or early delivery.

Technical Considerations and Limitations

Heart rate, arrhythmias, and systemic hemodynamics influence RI measurements. Proper angle correction and sample volume placement are critical for accuracy. Operator training and standardized protocols enhance reproducibility across institutions.

References

Viazzi F, Leoncini G, Derchi LE, Pontremoli R. Ultrasound Doppler renal resistive index: a useful tool for the management of the hypertensive patient. J Hypertens. 2014;32(1):149-53.

Surya H, Kumari N, Singh AK, et al. Renal resistive index measurements by ultrasound in hospitalized cirrhosis patients with acute kidney injury. World J Hepatol. 2024;16(6):865-75.

Cuttone G, Casarin A, Scremin F, et al. Exploring the Utility of Renal Resistive Index in Critical Care: A Narrative Review. J Clin Med. 2025;14(4):1234.

García-Criado A, Gilabert R, Nicolau C, et al. Doppler ultrasound findings in the hepatic artery shortly after liver transplantation. AJR Am J Roentgenol. 2009;192(1):128-35.

Tana C, Dietrich CF, Di Gregorio M, et al. Hepatic artery resistive index (HARI) and non-alcoholic fatty liver disease (NAFLD): pathophysiological and clinical implications. Eur Rev Med Pharmacol Sci. 2016;20(12):2622-33.

Pinggera GM, Mitterberger M, Steiner E, et al. Assessment of the intratesticular resistive index by colour Doppler ultrasonography measurements as a predictor of spermatogenesis. BJU Int. 2008;101(6):722-6.

Zolfaghar-Khani M, Majidi H, Feizzadeh B, Sabaghi M. Diagnostic accuracy of resistive index of capsular and intratesticular branches of testicular arteries in infertile men with oligoasthenospermia: a case-control study. Biomed J. 2020;43(6):18-22.

Shakeri AB, Tubbs RS, Shoja MM, et al. Diagnostic value of the resistivity index of the common carotid artery in patients with severe internal carotid artery stenosis or occlusion. Folia Morphol (Warsz). 2008;67(3):175-8.

Agarwal A, Kumar A, Singh AK, et al. Sonographic and Doppler evaluation of carotid artery in hypertensive and normotensive subjects: a hospital-based cross-sectional study. Cureus. 2024;16(10):e71579.

Rocha AS, Araujo Júnior E, Nardozza LM, Moron AF. Doppler Ultrasound of the Umbilical Artery: Clinical Application. Rev Bras Ginecol Obstet. 2022;44(4):411-8.

Srikumar S, Debnath J, Ravikumar R, et al. Doppler indices of the umbilical and fetal middle cerebral artery at 18-41 weeks of normal gestation: a pilot study. Med J Armed Forces India. 2017;73(3):232-41.

 

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