Patient and study design

This study was carried out retrospectively on paediatric patients aged less than 18 years who had undergone congenital heart surgery with cardiopulmonary bypass between November 1, 2021, and November 1, 2023. Cases involving heart surgery without cardiopulmonary bypass procedures, incomplete medical records, or those experiencing gastrointestinal bleeding before the operation were excluded from the study.

The study data were obtained from the electronic medical records in our hospital’s data system, and the examined variables included demographic characteristics, laboratory test data, and variables related to surgery. The study was conducted in compliance with the Declaration of Helsinki and received approval from the local committee.

Definition and variables

During the study period, a total of 1100 patients were evaluated for cardiac surgical procedures. These procedures encompass any intrathoracic or cardiac intervention. Gastrointestinal bleeding, which is bleeding from the upper or lower gastrointestinal tract, can present in various clinical forms depending on the severity and location of the bleeding. The definition of gastrointestinal bleeding includes a positive occult blood test result in specimens such as faeces or gastric juice. For each patient who underwent a cardiac surgical procedure, the following data were recorded: demographics (age, sex, operative procedure, cardiopulmonary bypass time, temperature of cardiopulmonary bypass, perfusion mode of cardiopulmonary bypass, and postoperative factors (length of hospital stay, length of mechanical ventilation, use of renal replacement therapy, use of extracorporeal membrane oxygenation (ECMO), postoperative ejection fraction, postoperative dialysis, postoperative lactate, postoperative hepatic function [alanine aminotransferase (ALT) and aspartate aminotransferase (AST)], postoperative platelet count, and survival to hospital discharge [%]). Patient age was categorised into three groups: neonates (<28 days), infants (29 days to 1 year), and children (1–18 years). Operative procedure risk stratification was based on The Society of Thoracic Surgeons-European Association for Cardio-Thoracic Surgery score, and The Society of Thoracic Surgeons-European Association for Cardio-Thoracic Surgery mortality categories 1–5 were considered to indicate complicated surgery. ^{Reference O’Brien, Jacobs and Pasquali7}

In our research, we identified hepatic injury as the presence of ALT level ≥100 U/L and/or AST level ≥100 U/L 30 days post-surgery. The modified Ulate low cardiac output syndrome score was employed for the diagnosis of low cardiac output syndrome. ^{Reference Ulate, Yanay, Jeffries, Baden, Di Gennaro and Zimmerman8} The low cardiac output syndrome score was calculated by assigning one point for each of the following criteria: (1) tachycardia (>20% above postinduction heart rate in the operating room), (2) oliguria (<1 mL/kg/hour), (3) capillary filling time >3 seconds, (4) need for volume expansion (beyond maintenance IV fluids) (>30 mL/kg/day), (5) decreased near-infrared spectroscopy (NIRS) measurements (cerebral and renal NIRS <50% and 75% of arterial saturations, respectively), (6) elevated arterial lactate levels (>2 mmol/L or >0.75 mmol/L/hour increase), and (7) need for vasoactive-inotropic infusions over 0.5 μg/kg/minute milrinone. Each parameter was allotted one point, and the low cardiac output syndrome score was calculated hourly. If the total low cardiac output syndrome score was 3 or higher at any time during the first 24 hours, the patient was considered to have low cardiac output syndrome. ^{Reference Öztürk, Öztürk and Dıkmen9}

Blood samples for lactate level measurement were obtained from the arterial cannula inserted during surgery. Lactate levels are routinely measured on cardiac ICU admission and are frequently monitored thereafter (i.e., 6, 12, 24, and 48 h, and more often if clinically indicated) during the postoperative period.

The vasoactive inotropic score was calculated for each patient using the standard formula for the first 24 postoperative hours, and the maximum score was recorded. The vasoactive inotropic score formula is as follows: vasoactive inotropic score = Dopamine dose (µg/kg/min) + dobutamine dose (µg/kg/min) + 100 × epinephrine dose (µg/kg/min) + 10 × milrinone dose (µg/kg/min) + 10,000 × vasopressin dose (Units/kg/min) + 100 × norepinephrine dose (µg/kg/min). ^{Reference Gaies, Gurney and Yen10}

Statistical analysis

Data were analysed using SPSS Statistics for Windows 15.0, a software programme developed by SPSS Inc., located in Chicago, Illinois, USA. To describe continuous data, the median along with the interquartile range was utilised, while absolute count and percentages were used to present categorical data. The Pearson chi-square test and Mann–Whitney U test were applied to compare variables between groups. To evaluate the discrimination of factors predicting mortality and morbidity, logistic regression models were employed, and the area under the receiver operating characteristic curve was assessed using the results. The estimated area under the curve and its 95% confidence interval are presented in the results. A significance level of p < 0.05 was set for statistical significance.