Doppler Renal Resistance Index for the Prediction of Response to Passive Leg-Raising Following Cardiac Surgery

Purpose: Doppler-based renal resistance index (RI) can be measured at the bedside of critically ill patients. This study was designed to assess if the RI predicted an increase in cardiac output (CO) following passive leg-raising (PLR) in patients admitted to the intensive care unit after cardiac surgery.

Methods: During this single center prospective study, Doppler assessment of RI and measurements of CO using the thermodilution method were performed, after surgery, in the intensive care unit before and after PLR. A positive response to PLR was defined as a ≥10% increase in CO.

Results: We included 30 patients. The mean RI was higher before (0.694 ±0.069) than after PLR (0.679 ± 0.069) (P = .02) with a median change of -0.012 (IQR: -0.042;0.000). Following PLR, 9 patients (30%) had a >10% increase in CO. In patients with a positive PLR response, the decrease in the RI during PLR was more pronounced than in patients who did not respond to PLR (PLR ± 0.042 (IQR: -0.051; -0.040) vs PLR ± -0.008 (IQR: -0.032; 0.015) (P = .004). There was a significant negative association between RI change in response to PLR and a 10% increase in CO following PLR (OR: 1.63 (CI:1.07-2.47) (P = .02) per -0.01 change).

Conclusion: An increase in CO following PLR was associated with a significant decrease in RI. Variations of RI in response to PLR should be further studied as a tool to predict fluid responsiveness. However, their clinical utility could be limited by the small magnitude of the variations.