RI — resistive index

Interactive tool

Calculate RI and PI

Enter the peak systolic velocity and the end-diastolic velocity obtained from the same spectral trace. If the time-averaged velocity is available, the calculator also returns Gosling's pulsatility index.

How to use it

Instructions

Formulas used

• RI (Pourcelot) = (PSV − EDV) / PSV — ranges from 0 to 1; it equals 1 when diastolic flow is absent.
• PI (Gosling) = (PSV − EDV) / TAMAX, where TAMAX is the time-averaged maximum velocity (the spectral envelope trace, not the mean velocity TAMEAN used for volume flow) — it has no upper limit and is more sensitive than RI when diastolic flow is reversed.
• Both are ratios: they are not affected by angle-correction error, which instead distorts PSV and EDV taken individually. This is why they remain reliable in intraparenchymal arteries, where the angle cannot be determined.

Obtaining a valid trace

1. Small sample volume centred in the lumen, gain set to give a sharp spectral outline without background noise.
2. Wall filter at its minimum: a high filter cuts off low diastolic velocities and overestimates RI.
3. At least 3 consecutive, regular cardiac cycles; in atrial fibrillation, average over 5–10 cycles.
4. For abdominal arteries, measure during a brief inspiratory breath-hold; avoid compressing with the probe, which artificially increases resistance.
5. Record heart rate and blood pressure: bradycardia, tachycardia, aortic regurgitation and a wide pulse pressure change RI independently of the vascular bed.

Choosing the vascular territory

The calculator adapts the interpretation to the territory selected, because normal values differ radically: an RI of 0.75 is abnormal in a renal interlobar artery, physiological in a femoral artery at rest, and clearly abnormal in an internal carotid artery, where it indicates high distal resistance.

Clinical references

Guidelines and interpretation

Renal artery and parenchyma

In renal artery stenosis the typical downstream pattern is tardus-parvus: reduced RI, acceleration time > 70–100 ms, systolic acceleration < 3 m/s², loss of the early systolic peak. Intraparenchymal RI also predicts the response to revascularisation: values ≥ 0.80 indicate advanced parenchymal damage and a reduced likelihood of functional improvement after stenting.

Carotid territory

• Internal carotid artery: normal RI 0.55–0.70 (low-resistance cerebral bed, with abundant diastolic flow).
• External carotid artery: higher RI, typically > 0.75, with little diastolic flow. The difference in RI is one of the criteria for distinguishing the two vessels, together with the temporal tap test.
• A symmetrically raised internal carotid RI reflects increased distal cerebral resistance (chronic hypertension, small-vessel disease, age); if asymmetrical, it points to an intracranial stenosis or a distal obstruction.
• Downstream of a tight stenosis RI falls and the systolic upstroke is blunted: here too the sign is tardus-parvus.

Limb arteries

• At rest flow is triphasic and high-resistance: RI close to or above 1.00 because of the reversed diastolic component. In this situation PI is more informative than RI, because RI saturates at 1.
• After exercise or during hyperaemia the bed dilates, forward diastolic flow appears and RI falls physiologically.
• A monophasic waveform with low RI downstream of a stenosis indicates collateral compensation and an upstream pressure drop.

References

Sources

• Pourcelot L. (1974) — Applications cliniques de l'examen Doppler transcutané — definition of the resistive index.
• Gosling RG., King DH. (1974) — Arterial assessment by Doppler-shift ultrasound — pulsatility index.
• Radermacher J. et al., N Engl J Med 2001 — Use of Doppler ultrasonography to predict the outcome of therapy for renal-artery stenosis.
• AIUM / SIUMB — Practice parameters for Doppler examination of the renal vessels and supra-aortic trunks.