This is a continuation of Dr. Cripes comments concerning adult congenital heart disease:
Fontan physiology represents a unique set of circumstances in cardiac performance. This circulation is often associated with CHD, but may be seen in other cardiac pathology. The primary situation occurring, regardless if it is a result of a congenital malformation of the heart or massive RV dysfunction, is the loss of a traditional right sided pumping mechanism. The return of systemic venous blood is directed passively into the pulmonary circulation. There is no RV to pump blood. In Fontan physiology pulmonary blood flow and cardiac output are dependent on a transpulmonary gradient (TPG). The TPG may be used as a guide to gage the forward flow of blood in the pulmonary circulation. The TPG is estimated by taking the CVP minus the LVEDP (PCWP if using a Swan-Ganz catheter). An ideal TPG is approximately 5-8 mmHg.
Goals in caring for patients with Fontan physiology may be broken down into pre-pulmonary/cardiac, pulmonary and cardiac. The pre-pulmonary/cardiac goals in Fontan physiology include the presence of an unobstructed venous return to pulmonary circulation. The importance of an adequate preload cannot be overemphasized. These patients are exquisitely preload sensitive. The patient needs widely patent anastomotic connections in the case of a surgically corrected CHD.
The pulmonary component may be thought of in terms of decreasing or increasing resistance to blood flow across the pulmonary vascular bed. Patients need a low pulmonary vascular resistance, as well as unobstructed pulmonary arteries. Normal lung parenchyma with no pulmonary vascular disease is typically a low resistance system (e.g. 1 Woods unit). Events that may raise PVR such as acidosis, hypercarbia and hypoxia will be harmful. Ventilator settings should include low (<15-20 mm Hg) mean airway pressures and normal alveolar ventilation. Overventilation with high airway pressures (e.g. Valsalva breaths) and the resultant decrease in venous return and increased PVR may be more harmful than hypoventilation, atelectasis and the resulting hypercarbia/acidosis. High levels of PEEP will increase pulmonary vascular resistance and cause a decrease in pulmonary blood flow.
In terms of cardiac effects on Fontan physiology, the maintenance of NSR with normal systolic and diastolic function is important. Ideally there will be no outflow obstruction and a competent valve system.
In summary the most important objective when caring for patients with Fontan physiology include maintenance of adequate intravascular volume or preload. An appropriate ventilation strategy which minimizes high mean airway pressures, while also preventing atelectasis, hypoxia and hypercarbia is important. Cardiac function should be maintained as NSR with optimal performance.
Dr. Cripe has recently completed his training in Anesthesiology at the University of Pennsylvania