Fontan single ventricle phys
Advanced, Clinical Subspecialties, Pediatrics
To best understand the physiology of the Fontan single ventricle, an understanding of the anatomy is essential. Staged procedures (i.e. B-T shunt, Glenn) are performed prior to ultimately reaching Fontan physiology, with the end result illustrated below (note that some variations exist). Venous return via the SVC and IVC now flow directly into the pulmonary circulation with the IVC connection via the right atrium (pictured) or via an extra-cardiac conduit. Pulmonary flow returns to the left atrium, then single ventricle (hypoplastic left heart pictured below) and out via the aorta.
The goal of the fontan circulation is to balance Qp:Qs and is accomplished by creating a system where venous return flows directly into the pulmonary vasculature, then into the left atrium, single ventricle, and is ejected systemically. Since venous return is directly into the pulmonary circulation, changes in PVR and venous filling pressure can greatly affect flow and cardiac output. This is why the fenestration between the IVC and right atrium is created. In situations that increase PVR (hypoxemia, acidosis, increased intrathoracic pressure, etc) the fenestration serves as a pressure relief valve to maintain ventricular filling and cardiac output despite decreased pulmonary flow. This also illustrates the importance of maintaining venous filling pressures as ventricular filling, via pulmonary flow or fenestration shunting, is very dependent on venous pressure. Given that some shunting will likely occur along with blood returning from the coronary sinus and mixing with oxygenated blood, oxygen saturations in these patients will likely be low-normal at baseline.