## 1 Introduction  
Complex heart malformations with single ventricular physiology are severe congenital diseases, and in order to survive, patients with these conditions require a staged reconstruction, depending on the type of existing defects. The first operative step is performed immediately after birth and is represented by systemic-pulmonary shunt, pulmonary artery banding or Noorwood surgery. The second stage is the Glenn operation which connects the superior vena cava to the pulmonary artery and is performed in the first year of life. The final step is the Fontan operation which connects the inferior vena cava to the pulmonary artery [1]. Glenn and Fontan palliative surgeries have increased the life expectancy and quality of life of patients with complex congenital heart defects [2]. They have been used in patients with tricuspid valve atresia, pulmonary valve atresia, double-inlet right ventricle, total atrioventricular septal defects, right ventricle with double outlet with transposition of the great vessels and left ventricular hypoplasia with aortic stenosis [1]. Despite the undeniable beneficial role of Fontan surgery, survivors of the intervention develop many short- and long-term postoperative complications: arrhythmias, kidney failure, plastic bronchitis and liver fibrosis [3; 4]. Liver damage secondary to Fontan surgery ranges from mild fibrosis to cirrhosis [5], but isolated cases of hepatocellular carcinoma have also been reported in adolescents and young adults [6]. Liver disease in patients with Fontan surgery occurs secondary to increased central venous pressure due to hemodynamic changes, decreased cardiac output and preoperative injuries (medication, hypoxia, perioperative injuries during systemic-pulmonary shunt or Glenn surgery) [7; 8]. Liver biopsy is the gold standard for diagnosing fibrosis in many pathologies. However, the procedure can have life-threatening complications, especially in children, thus it is difficult to use as a follow-up tool. Given these limitations, several indices that incorporate parameters associated with inflammation or fibrogenesis have been proposed. These non-invasive methods for estimating liver fibrosis have been extensively studied in various pathologies in the adult population and much less in children, especially those addressing congestive liver disease [9]. Elastography is an imagistic technique used to noninvasively evaluate the stiffness of the liver, measuring the changes in the tissue when applying an outer mechanical force or an internal acoustic radiation. Elastography techniques may therefore be grouped into three categories: transient elastography (TE), which uses an external mechanical pulsation; acoustic radiation force impulse (ARFI) elastography, which uses an internal acoustic pulse; and strain elastography, which interprets the tissue deformation caused by external pressure or secondary to physiologic internal phenomena. The ARFI technique is divided into point-shear wave and 2D-shearwave, respectively [10]. TE and SWE have been used and validated in various studies for a wide-spectrum of liver diseases in adults, but cutoff values in children have just recently been published [11; 12]. These methods are relatively inexpensive and easy to use for monitoring the evolution of the disease. Another great advantage over liver biopsy is that they allow extensive hepatic parenchyma assessment [13]. TE is performed using a special device which incorporates a vibrator positioned on the chest wall that generates an elastic wave. This device sends a signal of low amplitude to the liver tissue, inducing an elastic shear wave that propagates through the tissue. The speed of these waves is directly proportional to the stiffness of the tissue. The main strong point of TE is that it is widely available, whereas its weaknesses are the lack of imagistic guidance and the fact that it cannot be used in the case of ascites [10]. The 2D-SWE technique generates shear waves in the liver parenchyma by using the radiation force from a focalized ultrasound beam.