Physiology of single ventricle circulation: basic hydraulics explains basic complications

Congenital heart diseases (CHD) result from heart and/or major blood vessels defects arising in early pregnancy and are the most common congenital malformations, with an overall incidence of about 1 out of 100 live births. In particular, CHD with hypoplastic left or right ventricles (i.e. with single ventricle-SV-physiology) affect about 2 out of 1000 live birth. In these CDH with SV, the only treatment option is a palliative surgical procedure, which is usually staged, with the aim of restoring the normal physiology without anatomical repair of the CHD (i.e.a separation between oxygenated and deoxigenated circulation as complete as possible). The final stage is a single circuit (Fontan circulation), with the SV pumping blood through the systemic and pulmonary trees connected in series via the total cavo-pulmonary connection (TCPC). Despite most of these so called Fontan patients have a good quality of life and reach adulthood, nevertheless a large number of complications may arise, leading to late Fontan failure. When this is caused by ventricular failure, heart transplantation is the only available option for survival. There is large consensus among clinicians in considering abnormally high venous blood pressure in TCPC as the leading cause of complications, but the basic haemodynamic factors driving that occurrence are not clear enough yet (Gewillig and Brown, 2016). The present contribution is aimed to highlight the fundamental physiological elements at the basis of venous hypertension by comparing behavior of pressure grade lines in double (dvc) and single (svc) ventricle circulation, respectively. The cardiovascular system is described by the simplified circuit depicted in Figure 1, according to the lumped parameter modeling methodologyhere adopted(Ursino, 1998). Only main functional elements are considered, with compliance and resistance effects assigned to large vessels (Ao, Cve, PuA and PuVe) and to systemic (SR) and pulmonary (PR) microvasculature, respectively. Cardiac atria are modelled as passive chambers in which pressure variations occur according to volume changes within the chamber, while in ventricles also the activation of myofibers is considered; finally, a resistance effect is assigned to heart valves during their opening phase. Boundary conditions are assigned in the systemic ventricle, with prescribed physiological pressure wave. Initial pressure is also given in large vessels. Model calibrationand validation have been performed according to literature data. Instantaneous pressure grade line along the dvc and svc circuit has been extracted from simulation results, in order to have a clear and immediate perspective on the effect of any specific functional compartment on the instantaneous pressure behavior. Steady flow simulations revealed thatin the absence of pulmonary heart (right ventricle) pumping action, physiological pressure can be maintained in TCPC only for unattainable low pressure in the systemic heart. Pulsatile flow simulation confirmed that result, showing that the leading factor of abnormally high pressures in TCPC is the limited capability of the single ventricle to express a sufficiently strong suction effect. This finding might open new possibilities in designing surgical or technical solutions able to mitigate the early and long term complications of Fontan circulation.