The Fontan Principle
What is a Fontan-type circulation ?
Let me take some time to explain this, as it is an integral part of the entire operation for conditions like tricuspid atresia, pulmonary atresia, hypoplastic left heart syndrome ( HLHS ) and other single ventricle pathology.
We - you, me, cardiologists, surgeons, and everyone else - have been accustomed to thinking of the heart as having four chambers - two atria and two ventricles. These four chambers acting in unison maintain the circulation of blood.
To understand the Fontan circulation, you must make a "leap of imagination". In your mind, eliminate the right ventricle from the heart ! Tough isn't it ? And how can the heart possibly work without a right ventricle ?
Illogical as it may seem, this however was exactly what Dr.Fontan proved with his operation. In his original repair, he connected the right atrium directly to the pulmonary artery, and closed the ASD. Blood entering the right atrium from the veins passed across this surgical connection into the pulmonary artery and to the lungs. It completely bypassed the right ventricle.
Wait a minute ! There must be a flaw in this somewhere. How can the blood enter the lungs if it is not PUMPED IN by the right ventricle ? Well, that really is what makes this procedure unique. Normally the right ventricle will do the pumping. But in tricuspid atresia - and many other conditions in which a Fontan operation is performed - there is NO right ventricle. So blood flows PASSIVELY into the lungs - without being propelled into them by a right ventricle.
Why is lung blood flow so important ?
Because it is the only place in the body that blood can be purified by the addition of oxygen. So when lung blood flow is very low, oxygen supply is reduced to the entire body. This has many harmful effects, since no organ can perform its work normally without oxygen for energy.
So where does the energy for blood flow to the lungs come from ?
First, you must understand that any fluid flowing in a tube will continue to move, becoming slower and slower, until the resistance offered by the tube makes it stop. In a Fontan type circulation, the left ventricle pumps blood into the aorta and arteries. This blood flows at first rapidly into the different organs. The very same force pushes the blood across capillaries, and through the veins, but with lesser force. Slowly, blood enters the right atrium, and then passes across the surgical connection into the lungs - all the while unaided by a right ventricle.
But by its very nature, this flow depends on many factors. For instance, if the blood vessels in the lung are thick walled and narrow before surgery, they will offer very high resistance to passive blood flow. In such a state, the Fontan operation cannot be performed, or will have a high risk of failure, since the extra energy needed to maintain lung blood flow is not available.
Even normally a small amount of resistance will exist across the lung blood vessels. After a Fontan operation, the pressure in the veins will therfore be higher than normal, in order to overcome this resistance and maintain lung blood flow. The elevated pressure in the veins has a few ill effects.
What are the criteria for selecting a patient for the Fontan operation ? Read the next article to learn more about this.