We conducted a prospective observational study between 2021 and 2022, enrolling infants and children who had undergone cardiac surgery for congenital heart disease within the first 48 h after the operation, provided that specific conditions were met. These conditions included invasive monitoring of central venous and arterial pressures through catheterization of the internal jugular vein and femoral artery, absence of obstruction between the inferior vena cava (IVC) and the right atrium, exclusion of Glenn bidirectional shunt or Fontan operation as surgical procedures, extubated with spontaneous breathing, presence of an adequate acoustic window for echocardiographic examination, and maintenance of stable hemodynamic status in the patient. During echocardiography, all patients were awake and in a calm state.

The central venous pressure was measured using Arrow pediatric three-lumen central venous catheter (Arrow International LLC, Morrisville, NC 27560 USA) inserted into the internal jugular vein and Bioptimal disposable pressure monitoring kit (Biosensors International, Shanghai International Holding Corp. 20,537 Hamburg, Germany). The positioning accuracy of the central venous catheter was assessed by examining the chest X-ray of every patient [4].

The measurements were standardized.

The echocardiographic examinations were performed in the intensive care unit using the Philips Affinity 70 C echocardiography machine (Philips Healthcare, USA), equipped with probes operating at 5 and 8 megahertz (MHz) frequencies.

All echocardiographic examinations were conducted by a senior and well-trained fellow of pediatric cardiology, with the patient positioned in a supine posture. Standard echocardiographic images were acquired from various windows, including subcostal, apical, parasternal, and suprasternal views.

In the right ventricular focused four-chamber view, measurements of Doppler variables related to the tricuspid valve and dimensions of the right atrium were obtained in accordance with established guidelines [7,8,9]. Doppler evaluation of the hepatic vein and inferior vena cava, as well as the measurement of the maximum and minimum diameter of the inferior vena cava at a proximal point to the junction of the hepatic vein, and descending aorta at the same level, was conducted in the subcostal window, following the previously described methodology. The superior vena cava was examined from the suprasternal window [10,11,12] (Fig. 1). The measurements were standardized.

A The heart and the connections of the right atrium to the surrounding vessels. B The Doppler profile of the inferior vena cava, superior vena cava, and hepatic vein. C The measurement of maximal and minimal diameters of the inferior vena cava. D The pulse-wave Doppler of the tricuspid valve flow. E The tissue Doppler of the lateral annulus of the tricuspid valve (IVC inferior vena cava, SVC superior vena cava, HV hepatic vein, TV tricuspid valve, PW pulse Doppler, TD tissue Doppler)

The diameters of the inferior vena cava and right atrial volume were indexed by the body surface area. The ratio between the minimum and maximum diameters of the inferior vena cava (IVC) to the minimum and maximum diameters of the descending aorta (DAO) at the same level and inferior vena cava collapsibility and distensibility indices is calculated using the following formulas [13]:

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\(\frac}_}}} }}}_}}} }} = \frac}}}}\)

\(} = \frac} - }}}}}\)

\(} = \frac} - }}}}}\)

The data distribution was evaluated using the Shapiro–Wilk test to assess its normality. Descriptive statistics, including the mean, standard deviation, median, interquartile range, minimum, and maximum, were provided for continuous variables. The presentation of categorical variables included the absolute counts and corresponding percentages.

The correlation between invasively measured central venous pressure and numerical echocardiographic variables was evaluated using linear regression, and Pearson r correlation coefficient and R square values were reported. The Chi-square or Fisher’s exact test assessed the association between categorical variables. The statistical analysis was conducted using IBM SPSS Statistics for Windows, version 27 (IBM Corp., Armonk, NY, USA). A significance level of P < 0.05 was used to determine statistical significance.

Informed parents or guardians’ consent was obtained. This study received approval from the Institutional Research Ethics Committees. The study was conducted in accordance with the ethical guidelines outlined in the 2013 Declaration of Helsinki [14].