Cardiac surgery-associated acute kidney injury (CSA-AKI) is a frequent and important complication often attributed to decreased kidney blood flow. Ultrasound measurement of renal vein blood flow has been associated with adverse cardiac outcomes but is understudied in cardiac surgery. The renal vein flow index (RVFI) was defined using transesophageal echocardiography (TEE) during cardiac surgery to assess its precision, variability throughout surgery, and relationship to systemic perfusion defined by the cardiac index (CI).

Prospective.

University hospital.

Patients undergoing cardiac surgery.

None.

RVFI was repeatedly measured using TEE every 30 minutes throughout the entire surgery. RVFI precision was assessed using measurement error estimated from RVFI residuals and intraclass correlation coefficient. To quantify the variability of RVFI over the course of surgery, box plots with medians and interquartile ranges were assessed. The relationship between CI and RVFI was analyzed using linear regression with bootstrapping. Data from 10 participants included 324 RVFI measurements from 108 ultrasound images. The estimated measurement error was 0.030 units, which was well below the clinically meaningful threshold of 0.1 units. The intraclass correlation coefficient was 0.99 for RVSI measurements obtained within three consecutive cardiac cycles. RVFI varied across surgery (range 0-1.0 units), and each CI increase of 0.5 L/min/m² was associated with a 0.058 (95% confidence interval 0.04-0.08, p < 0.001) decrease in RVFI.

Intraoperative measurement of RVFI by TEE has excellent precision. TEE can detect a clinically meaningful change in RVFI. RVFI varied over the course of surgery and was associated with CI, consistent with expectations for a physiological measure of renal blood flow.