Doppler ultrasound can be used to assess arterial and venous blood flow velocity within the inter-lobar vessels of the kidney. Since intra-renal arteries and veins are located adjacent to each other, a single scan can yield a Doppler waveform that includes both venous and arterial flow. This post will focus on the Arterial IRD *See video at the end to learn how to perform this assessment
Arterial Intra-Renal Waveform
Usually the arterial flow will be directed towards the probe thus, the arterial waveform will be located above the baseline. While absolute velocity depends heavily on the ultrasound probe angle, relative systolic and diastolic velocities can provide information on renal hemodynamics.
Renal Resistive Index (RRI): Calculated as RRI = [(Vmax – Vmin ) / Vmax], the RRI depends heavily on systemic pulse pressure and arterial compliance. However, changes in intra-renal (afferent/efferent arteriolar) resistance can also influence the RRI. Increased RRI can be seen in cases of altered systemic hemodynamics (shock, venous congestion) or in parenchymal renal disease (acute tubular necrosis, thrombotic microangiopathy, acute rejection in renal transplants).
Usefulness in AKI: Although a high RRI (>0.70) has been associated with persistent AKI, given the myriad of factors affecting the RRI, it is not surprising that the RRI is not very specific. However, monitoring the change in RRI after a therapeutic intervention could be used as an indicator of success of said intervention. Titrating NE to higher MABP, passive leg raise, or fluid administration have been shown to lower the RRI in some patients; however, NE can also increase RRI in other patients. The availability of immediate feedback after therapeutic intervention might help individualize resuscitation efforts.
Key: Monitoring changes in the RRI in response to interventions may be used to guide the optimization of renal perfusion. This could include vasopressor dose titration, vasopressor agent selection, inotropic addition, or fluid challenge.
Clinical Vignette: A patient with septic shock and AKI on NE with a good MABP shows persistent oliguria and a high RRI (0.80). The addition of vasopressin (and lowering of NE) decreased RRI to 0.66. This was followed by increased urine output and SCr improvement
Doppler waveform of flow of the inter-lobar vessels of the kidney can be used to calculate de Renal Resistive Index (RRI)
Monitoring the change in RRI could provide real-time (immediate) feedback to determine the effectiveness of resuscitation efforts
Increased RRI has been linked with AKI and RRI can be modified with resuscitation strategies like vasopressors or fluids
Although mainly determined by pulse pressure and compliance, intrinsic resistance vascular can also alter RRI
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