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Single Ventricle Physiology

Single Ventricle Physiology

The normal heart has four chambers, with most of the pumping done by the two lower chambers (the ventricles).  It is often easier to think of the heart as having  two sides. The left side of the heart pumps blood to the body, and the right side of the heart pumps blood to the lungs. The blood pumped by the left ventricle should be full of oxygen.

When the left ventricle pumps, a large blood vessel (the aorta) takes blood to all of the tissues of the body. The body uses up the oxygen, and then the unoxygenated blood returns to the right ventricle. The right ventricle pumps blood to the lungs so it can pick up more oxygen. The blood then returns to the left ventricle, so it can be once more pumped to the body. This cycle is continually repeating itself.

Single ventricle physiology is a medical phrase meant to include a variety of cardiac defects which have only one functioning ventricle. Sometimes the ventricle is a single left ventricle, sometimes it is a single right ventricle, and sometimes it is difficult to tell. The lesions are grouped together because they are often treated the same.

Newborn stage

All children with single ventricle physiology have a similar pattern of blood flow as newborns - the blood is pumped by their only ventricle. After the blood is pumped, some of the blood will go to the lungs, and some will go to the body. How this is arranged is different from baby to baby. Sometimes, there are two large arteries coming from the heart, one to the lungs and one to the body. Other times, there is only one vessel coming from the heart going to the body. No matter what the particular arrangement, all children have a "Y" shaped circulation where the blood flow comes from the heart to both the lungs and the body.

Problems with newborn circulation: There are three problems with the newborn circulation for children with single ventricles:

1.  The children are blue: Ordinarily, the blood which the body receives is fully oxygenated. The blood pumped by the single ventricle is made up of blood which comes back from the body (unoxygenated) and lungs (oxygenated). Thus, when the heart pumps is only partially oxygenated blood, the body does not receive the normal amount of oxygen. (Blood which isn't fully oxygenated has a bluish tinge to it, which is why children who have this condition are referred to as being "blue").

2.  Balance of blood between the body and the lungs: The balance of blood between the body and the lungs is critical. However, there may be too much blood going to the body and not enough to the lungs or vice versa.

3.  The Single Ventricle has to pump too much blood: The ventricle is designed to pump as much blood as the body or lungs need for any given minute, hour, or day. However, the ventricle is not designed to pump enough blood for both the body and lungs. The ventricle can adapt to that for a period of time but if a child is left in this circulation indefinitely, the ventricle is at risk for failing.

Short-term solution

The short-term solution is to balance the amount of blood flow between the body and the lungs. If there is too much blood going to the lungs, blood flow must then be reduced by constricting the artery to the lungs (Pulmonary Artery Banding). If the amount of blood going to the lungs is insufficient, an artificial blood vessel must be inserted to increase the blood flow to the lungs (Blalock-Taussig Shunt). 

In the most fortunate situation, there is nothing that needs to be done for the newborn with a single ventricle, as the blood flow between the body and lungs is already balanced. In the most complicated situation, this means re-doing all of the blood vessels as it leaves the heart to provide unobstructed blood flow between the ventricle and the body (this involves complicated open heart surgery, such as a Norwood procedure for Hypoplastic Left Heart Syndrome). For all newborns, however, there must be unobstructed blood flow to the body with a proper amount of blood flow going to the lungs. The issue of how to balance the blood flow between the body and lungs can be discussed with you by your doctor.

Long-term solution

The long-term solution is an operation called the Fontan procedure [link to Fontan procedure page]. The Fontan procedure is where the blood which returns from the body is directly routed to the lungs without passing through the heart. Thus, there is no ventricle pumping blood through the lungs as there normally is. However, it does address all three problems described above: 

1.  The children are pink: All of the blood which goes to the body came from the lungs, so it is full of oxygen.

2.  No issues about balance between the lungs and the body: All of the blood which leaves the heart goes to the body, and then to the lungs and back to the heart. There is no "Y" shape in the circulation for an imbalance to occur.

3.  No extra work on the heart: The ventricle once again only pumps what the body needs; the ventricle's work has decreased from the newborn stage.

Although the Fontan procedure is the best that can be offered for single ventricle patients at this time, it is far from perfect. With a Fontan, there is no ventricle to pump blood through the lungs. Only the "pressure" within the veins drives blood through the lungs. If there is any significant resistance to flow through the lungs, a Fontan may not work. Fontan procedures are usually performed at approximately 3-4 years of age.

Bidirectional Glenn Procedure

Among doctors and nurses, a Fontan refers to the procedure where the two large veins draining unoxygenated blood back from the body (one draining the upper body and one draining the lower body) are connected directly to the lungs. In current medical practice, these are done at separate operations.

 The upper vein is connected first (usually at about 6 months of age), and the lower vein is connected second (2-5 years of age). The name "Fontan procedure" refers to the operation where the lower vein is connected, and the repair is complete. The first operation where the upper vein is connected is called a bidirectional Glenn procedure.

In a bidirectional Glenn procedure, the large vein which brings unoxygenated blood back from the upper body is connected directly to the lungs. This blood is oxygenated by the lungs and then returned to the heart. The blood which goes to the lower body goes directly back to the heart itself, and then gets re-pumped to the body without going to the lungs. Since unoxygenated blood from the lower body gets mixed in the heart with the oxygenated blood from the upper body and then is pumped to the body, the blood which goes to the body is not fully oxygenated. The usual oxygen saturation for a child with a bidirectional Glenn is around 80%. 

There are a number of reasons why the Fontan is staged, with the upper vein connected to the lungs at one operation and the lower vein connected at a different operation. The best explanation is that recovering from a Fontan can be very hard, and the heart has to be in the best shape possible in order for the Fontan to be successful. In the newborn stage, there is volume overload because the heart has to pump to both the heart and the lungs. Thus, hearts are generally dilated and not in perfect shape. A bidirectional Glenn does not require additional work by the heart and is not as hard to recover from as the complete Fontan; thus, a child whose heart is not in perfect shape will have a safer surgery with a bidirectional Glenn then with a Fontan. When the Fontan surgery is performed, the heart is entering the operation in better shape after a bi-directional Glenn than if the Fontan was performed with the newborn circulation.

The risk and recovery of these operations varies from patient to patient, and will be discussed with you by your doctor.

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