A postoperative renal resistive index (RRI)>0.70 has the best threshold to early predict acute kidney injury (AKI). The response of RRI to a postoperative fluid challenge (FC) is unknown. The aim of our study was to assess the impact of a FC on RRI in suspected hypovolaemia patients after orthopaedic surgery.

In this single-centre observational study, we prospectively screened 156 patients in the recovery room after having undergone a hip or knee replacement.

Forty-six patients with a RRI>0.70 and requiring FC were included. RRI and cardiac output (CO) were measured before and immediately after a fluid challenge with 500mL of isotonic saline. A decrease in RRI>5% was considered significant (renal responders).

Overall, FC resulted in a consistent decrease in RRI (from 0.74 [0.72–0.79] to 0.70 [0.68–0.73], P<0.01). Thirty-four patients (74%) showed a significant decrease in their RRI (from 0.74 [0.73–0.79] to 0.69 [0.67–0.72], P<0.05, versus non-responders: from 0.73 [0.72–0.75] to 0.72 [0.71–0.79], P=NS). CO increased equally among renal responders and non-responders (P=0.56). No correlation was found between changes in RRI and CO (r²=0.04; P=0.064). AKI was more common in renal non-responders (7/12) than in responders (3/34, P=0.001).

After major orthopaedic surgery, a FC can decrease RRI in suspected hypovolaemia patients at risk of postoperative AKI, but the changes are not correlated to changes in CO. Decreases in RRI were associated with better renal outcome.