## 4 Discussion
### Liver Elastography
In our study, we used SWE to compare the values of liver stiffness in Fontan and Glenn postoperative patients with those obtained in a group of healthy controls. Many reports documented that measurements of liver stiffness using this method can accurately estimate the degree of fibrosis in liver disease of different etiologies [12; 34]. \(\mathrm{E_{median}}\) liver stiffness values determined by SWE were statistically significantly higher in patients with Fontan surgery compared to the control group (4.85 kPa vs. 3.91 kPa, \(p\) = 0.0049) but also compared to the group of postoperative Glenn patients (4.85 kPa vs. 3.99 kPa, \(p\) = 0.01). Similar results were obtained for \(\mathrm{V_{median}}\): 1.25 m/s in Fontan patients vs. 1.12 m/sec in controls (\(p\) = 0.01) and 1.16 m/s in Glenn patients, respectively (\(p\) = 0.08). Contrariwise to our study, [PERSON] and co. included older Fontan patients [28], thus with longer exposure to the Fontan hemodynamics, and used a different system for SWE evaluation (SuperSonic Imaging Aiexplorer). These methodological differences could explain, at least partially, the impact on the values of hepatic stiffness. In addition, we obtained slightly higher values of hepatic stiffness in patients with Glenn surgery compared to the control group, without exceeding the threshold of statistical significance. Transient elastography is another valuable noninvasive tool used in multiple chronic hepatic pathologies [35], and the liver stiffness values were compared to biopsy-proven fibrosis in Fontan patients. Liver stiffness values overestimated the level of fibrosis by one stage in 7 out of 10 patients, and in 9 out of 10 subjects overestimated the fibrosis level by two stages [36]. We performed transient elastography in a segment of patients who underwent Fontan and Glenn surgeries. The results were compared to the age-specific cut-offs published recently by our research team [11]. In post-Fontan patients, liver stiffness was significantly increased in 6 out of 7 patients suggesting advanced fibrosis, compared with post-Glenn patients where slightly increased liver stiffness was observed in 6 out of 14 patients (Mean \(\mathrm{E_{median}}\) 11.31 kPa in the Fontan group vs. 4.14 kPa in the Glenn group, \(p\) = 0.003; mean \(\mathrm{V_{median}}\) 1.86 m/s in Fontan vs. 1.25 m/s in Glenn, \(p\) = 0.05). For Fontan patients, the liver stiffness values determined by TE were significantly higher than those obtained by SWE, but this difference was not obtained for the Glenn group. It is possible that liver stiffness results determined by TE are falsely elevated by congestion secondary to the increase in inferior vena cava pressures. We found only one study in Fontan patients including both elastographic methods which did not document this finding, but a different machine was used for SWE evaluation [37]. Further studies comparing these two methods and validation with biopsy are needed. We found no significant correlations between E\({}_{\text{median}}\) and V\({}_{\text{median}}\) results obtained by SWE and TE, respectively. An explanation for the lack of correlation of values between the two types of measurements could be given by the relatively small number of measurements by TE, with the statistical test not having sufficient power. In addition, although the reproducibility of intra- and inter-observer elastographic measurements is considered to be high for both SWE and TE (intra-class correlation coefficient >0.9% [38]), a certain difference between measurements may be present even if they are performed by an experienced user. In the case of increased liver stiffness, a greater variability between the measurements obtained by different elastography systems is described [38]. Biochemical markers of fibrosis do not show such variability, so their determination in parallel with elastography can determine a more accurate diagnosis of liver fibrosis. For patients with Fontan surgery, the values of hepatic stiffness determined by Fibroscan were much higher, possibly due to significant congestion.