Renal Resistive Index as a Predictor of Acute Renal Impairment in High-risk Patients

Acute kidney injury (AKI) is a common clinical problem encountered in critically ill patients, frequently in the setting of multiple organ failure, and is an independent risk factor for increase hospital stay and mortality risk.

Early-stage acute kidney injury was first assessed based on the risk, injury, failure, loss and end-stage (RIFLE) criteria in 2004, and then by the Acute Kidney Injury Network (AKIN) criteria in 2007. The Kidney Disease: Improving Global Outcomes (KDIGO) classification, based on both the AKIN and RIFLE criteria, was introduced in 2012, offering an assessment based on baseline creatinine and urine output.

The best strategy in clinical practice is to identify AKI as early as possible, reverse its cause, and even improve the sequelae. In the past decades, several serum creatinine (SCr)-based classification systems have been proposed to define AKI.

The limitations of SCr is that the determinants of SCr (rate of production, apparent volume of distribution, and rate of elimination) are variable. Therefore, there is an unmet need for other objective measures to help detect AKI in a timely manner. The role of several biomarkers in the early prediction or risk assessment of AKI has been proposed, including kidney tubular damage markers (e.g., neutrophil gelatinase-associated lipocalin (NGAL), kidney injury molecule-1 (KIM- 1), liver-type fatty acid-binding protein (L-FABP) and cystatin C).

Cystatin C is a protein from the family of cysteine proteinase inhibitors and is of interest as an early marker of decreased renal function. It is a protein that is synthesized at a constant rate by all cells containing nuclei, secreted into biological fluids: plasma, pleural, ascitic, cerebrospinal fluid, freely filtered through the glomerular membrane (due to its low molecular weight), fully metabolized in the kidneys, not secreted by the proximal renal tubules.

Renal resistive index (RRI) is a noninvasive instrument to evaluate kidney hemodynamics, and it is obtained by analysis of intrarenal arterial waves using Doppler ultrasound.