Following Institutional Review Board (IRB) approval, infants requiring cardiac surgery at the University of Rochester (UR)-Golisano Children’s Hospital between October 5th, 2020 to May 19, 2021 were approached sequentially for study participation. Inclusion criteria included: 1) Age < 1 year at the time of operation; 2) Expected use of CPB during the repair or palliation; and 3) Urgent or elective operation as defined by the Society of Thoracic Surgeons – European Association for Cardiothoracic Surgery Congenital Heart Surgery Mortality (STAT) Categories . Exclusion criteria included: 1) Pre-operative extra-corporeal membrane oxygenation (ECMO); 2) Non-English speaking parents, or 3) Prior inclusion in the current study.
Following the induction of anesthesia, a median sternotomy was performed. CPB was achieved using either direct aortic cannulation or cannulation of a 3.5 mm graft sewn to the innominate artery and bicaval venous drainage. In all cases the CPB circuit was connected to a Terumo System 1 Heart Lung Machine (Terumo Corporation, Tokyo, Japan) with an Fx05 oxygenator and hardshell reservoir with integrated arterial line filter (Terumo Corporation, Tokyo, Japan). CPB prime volume (200mL) was similar for all subjects and included: mannitol, bicarbonate, 25% albumin, heparin, tranexamic acid, Plasma-Lyte 148 and allogeneic red blood cells (RBC) when necessary to achieve a hematocrit >24%. Dilutional ultrafiltration was performed at the conclusion of surgery for most subjects, with the exception of those who required aortic arch reconstruction and regional perfusion, in which case zero-balance ultrafiltration was utilized.
When necessary, myocardial arrest was performed using either one or two doses of a modified blood cardioplegia. Regional cerebral perfusion was used in all subjects requiring aortic arch reconstruction. Deep hypothermic circulatory arrest was not utilized during the study period.
Both cerebral and somatic Near Infra-Red Spectroscopy (NIRS) were used and the nadir daily values recorded. Post-operative electrolyte, fluid, and blood product transfusion guidelines were utilized for every subject per local standard of care . Post-operatively patients were given two-thirds maintenance intravenous fluids (IVF) until the morning of post-operative day (POD) 1 or until extubation from invasive mechanical ventilation (whichever is longer). Maintenance IVF consisted of 10% dextrose and one quarter normal saline (D10 + 0.22% NaCl) in infants < 6 months of age, and 5% dextrose with one half normal saline (D5 + 0.45% NaCl) in infants > 6 months of age. Maintenance IVF rate was calculated using the Holliday-Segar method . Patients were transitioned to a full maintenance IVF rate following extubation and transitioned off IV fluids once tolerating enteral feeds. As per local standard of care, diuretic therapy is not administered prior to POD 2, and only if there is adequate intravascular volume and stable hemodynamics.
Strict measurements of all intake and output were tracked per intensive care unit (ICU) protocol with the aid of an indwelling urinary catheter. Assessment for "fluid overload” was determined at ICU admission, and on POD 0, 1, 2, and 3 by dividing each subject’s net fluid balance (milliliters) by their weight (grams), and multiplying by 100 (i.e., net balance(mL)/weight(gm) x 100). Subject characteristics and details regarding cardiac morphology, surgical procedure and intra-and post-operative data were collected from the electronic medical record.
Pre-operative and post-operative factors potentially impacting renal function were assessed and included: 1) Nephrotoxic agent exposure within 7 days prior to surgery of 3 medications as outlined by Goldstein, et al, along with all non-steroidal anti-inflammatory drugs and intravenous contrast agents ; 2) Chronic kidney injury defined as any evidence of structural nephro-ureteral abnormality or a prior AKI episode without normalization to baseline SCr in those 3 months of age, or a SCr > 0.4 mg/dL for the past 3 months; 3) Renal angina index was defined using a calculation based on inotropic support, fluid overload and creatinine change as outlined by Menon, et al ; 4) Shock defined as arterial lactate > 4 mmol/L on two sequential blood gases after the initial post-operative peak and nadir, or 2 or more signs of end-organ injury; 5) “urgent surgery” defined as non-elective cases that required urgent admission or continued inpatient stay to undergo surgery before discharge home; 6) Severe bleeding was defined as a) bleeding that leads to 1 or more organ dysfunction, or b) bleeding that leads to hemodynamic instability (>20% increase in HR or >20% decrease in BP), or c) requirement for surgical exploration for cardiac tamponade or bleeding in the first 24 hours; 7) Necrotizing enterocolitis (NEC) was defined as stage IIA NEC or greater as outlined by the modified Bell staging criteria ; 8) The highest vasoactive inotropic score (VIS) on POD 0-3 was calculated as outlined by Gaies, et al. .
The hospital laboratory was utilized for all standard laboratory analyses, including hemoglobin, lactate, blood urea nitrogen (BUN), and SCr per local standard of care. Baseline measurements were routinely taken at either the morning of, or 1-3 days prior to surgery. Post-operative measurements were obtained at ICU admission and in the morning during each post-operative day.
Baseline measurements for serum CysC were obtained at time of intra-operative central line placement prior to surgical stimulation, and at 12, 24, 48, and 72 hours following CPB initiation. 1.3 mL of blood was collected in a plasma separator tube, centrifuged at 1100 x g for 10 minutes at room temperature (20°C) and serum supernatant aliquoted into a 2 mL conical microcentrifuge tube within 1 hour of collection and stored at -80°C. The International Federation for Clinical Chemistry and Laboratory Medicine (IFCC) calibration was used for Cystatin C concentration, which was reported in mg/L, as previously outlined by Schwartz, et al . Samples were thawed and analyzed using a commercially available kit for human cystatin C (e.g. N latex CysC Kit standardized to ERM-DA471/IFCC reference material; Siemens Healthineers, Erlangen, Germany) and the Siemens BN II nephelometric analyzer (Siemens Healthcare Diagnostics, Newark, DE) via a latex-enhanced immunoassay using a 1:100 dilution in phosphate buffered saline performed with a six-point calibration generated from multiple dilutions of a human cystatin C calibrator . The lower limit of detection for CysC was 0.27 mg/L. The intra-assay and inter-assay coefficients of variation were below 5%.
Subjects were divided in 2 groups for primary analysis (AKI and non-AKI within POD 0-3) based on meeting the criteria for AKI as per KDIGO criteria:
1) Stage 1 AKI: SCr ≥ 1.5-1.9 times baseline, or SCr increase ≥ 0.3 mg/dl within 48 hours, or a urine output < 0.5mL/kg/hr for 6-12 hours;
2) Stage 2 AKI: SCr ≥ 2 – 2.9 times baseline or urine output < 0.5 mL/kg/hr for ≥ 12 hours;
3) Stage 3 AKI: SCr 3 times baseline, or increase in SCr ≥ 4.0 mg/dL or initiation of renal replacement therapy, or a urine output < 0.3 mg/kg/hr for ≥ 24 hours, or anuria for ≥ 12 hours [8,27].
Data are presented as mean ± standard deviation, frequency and percentage, or median with inter-quartile range (IQR). The percent change in post-operative CysC, SCr, and BUN from baseline (%CysC, %SCr, %BUN respectively) were calculated (e.g. %CysC12hr = (CysC12hr - CysCbaseline)/CysCbaseline x 100) for each post-operative time point as previously described . Continuous variables were evaluated using the Shapiro-Wilk test for normality. Significance was determined using either a 2-tailed Student’s t-test or Mann-Whitney when comparing two groups. To examine differences of SCr, BUN, CysC, and urine output over sequential time points, a repeated measures test was performed. Comparison of categorical variables were evaluated with Fisher’s exact test. Linear regression was used to identify the relationship between CysC and other markers for AKI, as well as change in fluid status. Receiver Operating Characteristic (ROC) curves were constructed to identify variables that could predict AKI during POD 0 and the area under the curve (AUC) quantified. The two variables with the highest predictive ability were combined using a binary logistic regression to identify if together they increased the predictive ability for AKI. A backward stepwise logistical regression method identified variables associated with AKI, such that variables were removed until all p values in the model were significant. All statistics were completed using GraphPad Prism version 5.0b (GraphPad Software, San Diego CA) and SPSS 28 where a P value of < 0.05 was considered statistically significant.