Invited Commentary to ‘Hemodynamic performance of 16-20mm extracardiac Goretex conduits in adolescent Fontan patients at rest and during simulated exercise’

Single-ventricle (SV) defects are among the most complex congenital heart defects (CHDs). In the USA, children born with SV heart defects constitute ∼2 per 1000 births. The severity of SV defects can lead to substantial physical, socioeconomic and mental challenges for the affected families, patients and the entire medical system. Long caregiving hours and poor quality of life (QoL) overwhelm families of patients with congenital heart defects, and the medical care of SV patients costs a total of ∼400 million per year. The standard palliative operation for SV patients is the staged Fontan procedure, in which the total cavopulmonary connection (TCPC) connects the systemic and pulmonary circulations directly, bypassing the subpulmonary ventricle. While this surgery offers favourable short-term outcomes, long-term complications are prevalent among this patient population. Previous studies have shown that TCPC haemodynamics are correlated with patients' QoL [1], lung development [2], liver diseases [3] and exercise capacity [4, 5]. The study conducted by Rijnberg et al. focused on exercise performance, a universal issue for all Fontan patients. Their study investigated haemodynamics in the TCPC at resting and during simulated exercise conditions in 51 Fontan patients with 16–20-mm extracardiac conduits. TCPC haemodynamics was obtained using computational fluid dynamics (CFD) simulations that incorporate cutting-edge features, including pulsatile flow, patient-specific anatomy, and a Fontan-specific non-Newtonian rheological model [6, 7]. The primary findings are statistically significant negative correlations between the mean cross-sectional area (CSAmean) of TCPC vessels and TCPC haemodynamics at rest and simulated exercise and between normalized TCPC resistance and peak oxygen uptake. Moreover, they found that larger Fontan conduits result in a more favourable pressure drop through the TCPC (<1 mmHg at rest and exercise) when compared to smaller ones. Rijnberg's study added new, independent evidence to advocate for the necessity of optimal baseline TCPC haemodynamics. It reconfirms the importance of optimizing TCPC haemodynamics through surgical planning and continuously monitoring Fontan geometry in patient management [8]. Moreover, this work highlighted the importance of TCPC resistance, a metric that has not yet been widely utilized in the Fontan assessment. In addition, this metric has recently been linked with QoL and liver fibrosis in Fontan patients [1, 3]. Therefore, it would be intriguing to read a follow-up study from Rijnberg's group about QoL and the liver disease of their patient cohort. Particularly, their cohort has patients with relatively small Fontan conduits size, i.e., 16 and 18 mm, which would denote higher TCPC resistance. Like any study, Rijnberg's work could be strengthened by using patient-specific boundary conditions in their CFD simulations. The clinical relevance of TCPC haemodynamics simulated by CFD relies on patient's characteristics [9]. Rijnberg's simulations were conducted without patient-specific flow rates for hepatic veins. The authors estimated the flow rates using an area-weighted approach based on the flow rate difference between the Fontan conduit and subhepatic inferior vena cava, a procedure that is not optimal and is a significant limitation to their work. In addition, subjects in previous studies have undergone exercise in the MRI scanner [4, 5], and this should be considered by the authors to expand their work. This could provide exercise CFD with patient-specific velocity in time and space [7, 10], thereby enhancing the clinical relevance of their simulations and findings. It would be useful to re-analyze the data from the present study. First, Rijnberg et al. concluded that no correlations were observed between power loss and Fontan exercise capacity; however, they did not index their measurements to flow and body surface area as previous studies have, which actually demonstrated such correlations [4, 5]. In addition, their calculations of power loss factored in the effect of the confluence between hepatic veins and subhepatic inferior vena cava, which occurs in healthy individuals and has been excluded in previous investigations. Power loss analysis excluding this confluence may be useful. Second, the normalization of CSAmean in their study could be misleading. Respiratory cycle-averaged flow rates were used to normalize CSAmean, leading to a unit of mm2/l/min, which is identical to the reciprocal of the units of velocity. In the study, the normalized CSAmean was negatively correlated with pressure gradient and power loss, which have well-known relationships with the flow velocity. Though negative correlations between CSA (typically the minimum CSA) and TCPC haemodynamics have been reported previously [4], similar correlations (with mean CSA) as reported in this study may not be reliable. Nevertheless, Rijnberg et al. have added to our knowledge of Fontan haemodynamics and will be useful in future work designing the optimal TCPC. As play is so important in a child's development and exercise is part of a healthy lifestyle in adults, the current study aids in the ultimate well-being of the SV patient and sheds new light and insights into the current limitations of the Fontan.