- Acute Kidney Injury (AKI) is commonly seen in patients with COVID-19, especially in those requiring intubation and mechanical ventilation.
- Point-of-care ultrasound (POCUS) is widely used for the early recognition of and management of critically ill patients.
- The use of point-of-care renal ultrasound in the ICU setting is currently limited, but certain measurements have shown potential in aiding in early recognition of AKI especially in septic and post-cardiac surgery patients.
- Renal Resistive Index (RRI) is a noninvasive ultrasound test that may help in early recognition and management of AKI.
What is the Renal Resistive Index?
- RRI is the sonographic Doppler measurement of flow velocities in the intraparenchymal renal arteries. It has been shown to correlate with renal vascular impedance.
- Generally, values above 0.7 are considered abnormal. Renal transplant patients with values above 0.8 are associated with higher mortality.
- Critically Ill COVID-19 patients with AKI have been shown to have a higher RRI than those without AKI.
How is RRI calculated?
- Start with getting a long axis view of the kidney.
- Apply color doppler to identify pulsatile flow in the kidney.
- Place pulse wave doppler gate over the interlobar pulsatile vessels.
- Peak systolic velocity (PSV) and End diastolic velocity (EDV) are measured.
RRI = PSV-EDV/PSV
- Values above 0.7 are considered abnormal.
- Many factors can influence RRI. These include age, mean arterial blood pressure, and serum lactate levels.
- Research is ongoing looking at RRI as a predictor for AKI in patients with sepsis.
- RRI is a bedside ultrasound Doppler test that may help predict AKI in critically ill ICU patients.
- Some studies have shown that RRI can help predict AKI in patients post-cardiac surgery and in septic shock. Larger studies are ongoing.
- More research is needed, but RRI may help predict and prognosticate AKI in patients with COVID 19.
- Renberg, M., Jonmarker, O., Kilhamn, N. et al. Renal resistive index is associated with acute kidney injury in COVID-19 patients treated in the intensive care unit. Ultrasound J13, 3 (2021). https://doi.org/10.1186/s13089-021-00203-z
- Watchorn J, Huang DY, Joslin J, Bramham K, Hutchings SD (2020) Critically ILL COVID-19 patients with acute kidney injury have reduced renal blood flow and perfusion despite preserved cardiac function. Shock, A Case-Control Study Using Contrast Enhanced Ultrasound. https://doi.org/10.1097/shk.0000000000001659
- Di Nicolo P, Granata A (2019) Renal intraparenchymal resistive index: the ultrasonographic answer to many clinical questions. J Nephrol 32:527–538. https://doi.org/10.1007/s40620-018-00567-x