Acute kidney injury (AKI) is an important complication following cardiac surgery with cardiopulmonary bypass (CPB). Early identification of patients at risk of AKI may facilitate timely clinical management. This prospective observational study investigated the association of perioperative renal regional oxygen saturation (rSO₂), measured using near-infrared spectroscopy (NIRS), and neutrophil gelatinase-associated lipocalin (NGAL) levels with postoperative AKI. A total of 85 adult patients undergoing elective cardiac surgery with CPB were enrolled. Bilateral renal rSO₂ was measured during four predefined perioperative phases. NGAL levels were evaluated preoperatively and at 24 and 48 hours postoperatively. The primary objective was to assess the discriminatory performance of renal rSO₂ and NGAL, individually and in combination, for identifying postoperative AKI. AKI was defined and staged according to the Kidney Disease: Improving Global Outcomes (KDIGO) criteria during the first seven postoperative days. Secondary objectives included determining the incidence and severity of AKI and investigating associated patient-related and perioperative factors.
This prospective, single-center observational study was conducted at Haseki Training and Research Hospital, Istanbul, Türkiye, between March 20, 2025, and February 20, 2026. The study received ethics committee approval on March 19, 2025 (approval number: 34-2025), and written informed consent was obtained from all participants. Among 95 adult patients assessed for eligibility, 85 patients undergoing elective cardiac surgery with cardiopulmonary bypass (CPB) were enrolled. Renal regional oxygen saturation (rSO₂) was monitored using bilateral near-infrared spectroscopy (NIRS) probes placed over the renal regions. Perioperative measurements were collected during four predefined phases: A0: Before induction of anesthesia. A1: From induction of anesthesia to initiation of CPB. A2: From initiation of CPB to separation from CPB. A3: From separation from CPB to the end of surgery. During A1, A2, and A3, right and left renal rSO₂ values and other perioperative parameters were recorded every 15 minutes. Mean values were calculated separately for each phase. Bilateral mean renal rSO₂ was calculated as the arithmetic mean of the corresponding right- and left-sided values and was used as the primary renal oxygenation parameter. Urine output was recorded for each perioperative phase and expressed as mL/kg/h. NGAL levels were measured preoperatively (T0) and at 24 (T24) and 48 (T48) hours postoperatively. Postoperative AKI was assessed during the first seven postoperative days according to KDIGO criteria using serum creatinine changes and/or urine output. Participants were classified into AKI and non-AKI groups according to postoperative AKI development. The diagnostic performance of renal rSO₂ and NGAL was assessed using receiver operating characteristic (ROC) curve analysis. The primary combined biomarker model incorporated bilateral mean renal rSO₂ during CPB (A2) and NGAL measured at 24 hours postoperatively (T24). Additional analyses investigated temporal changes in biomarkers and potential patient-related and perioperative factors associated with postoperative AKI.
Study Type
OBSERVATIONAL
Enrollment
85
Bilateral renal regional oxygen saturation (rSO₂) was monitored using near-infrared spectroscopy (NIRS). Measurements were obtained before anesthesia induction (A0) and recorded at 15-minute intervals during three perioperative phases (A1, A2, and A3). The mean rSO₂ values were calculated separately for each kidney, and the bilateral mean was used as the primary renal oxygenation parameter.
Blood samples were collected to measure neutrophil gelatinase-associated lipocalin (NGAL) levels at three predefined time points: preoperatively (T0), and at 24 (T24) and 48 (T48) hours postoperatively. NGAL levels were evaluated for their association with postoperative acute kidney injury.
Sultangazi Haseki Training and Research Hospital
Istanbul, Istanbul, Turkey (Türkiye)
Discriminatory Performance of Perioperative Renal Oxygen Saturation for Postoperative AKI
The discriminatory performance of bilateral mean renal rSO₂ for identifying postoperative AKI will be evaluated separately for each perioperative phase (A0, A1, A2, and A3) using ROC curve analysis. The area under the curve (AUC) and its 95% confidence interval will be calculated.
Time frame: Perioperatively (A0-A3), with postoperative AKI assessed through day 7.
Discriminatory Performance of NGAL for Postoperative AKI
The discriminatory performance of NGAL levels measured preoperatively (T0), at 24 hours (T24), and at 48 hours (T48) postoperatively will be evaluated separately using ROC curve analysis. AUC values and corresponding 95% confidence intervals will be calculated.
Time frame: Preoperatively and at 24 and 48 hours postoperatively, with AKI assessed through postoperative day 7.
Combined Discriminatory Performance of Renal Oxygen Saturation and NGAL for Postoperative AKI
The primary combined biomarker model will incorporate bilateral mean renal rSO₂ during CPB (A2) and NGAL measured at 24 hours postoperatively (T24). Binary logistic regression will be used to derive predicted probabilities, and ROC analysis will be performed to calculate the AUC and its 95% confidence interval. Incremental discrimination will be assessed by comparing AUCs with those of the individual biomarker models.
Time frame: During CPB and at 24 hours postoperatively, with AKI assessed through postoperative day 7.
Incidence of Postoperative Acute Kidney Injury
The proportion of patients developing postoperative AKI, defined according to KDIGO criteria using serum creatinine changes and/or urine output.
Time frame: Within 7 days after surgery.
Severity of Postoperative Acute Kidney Injury
Postoperative AKI severity will be classified according to KDIGO stages 1, 2, and 3, based on serum creatinine changes and/or urine output.
Time frame: Within 7 days after surgery.
Patient-Related and Perioperative Factors Associated With Postoperative AKI
Associations of demographic characteristics, comorbidities, preoperative renal function, CPB duration, aortic cross-clamp duration, and relevant intraoperative clinical and hemodynamic variables with postoperative AKI will be evaluated using logistic regression. Odds ratios and 95% confidence intervals will be reported.
Time frame: From the preoperative period through postoperative day 7.
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