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Single ventricle anomalies and Fontan circulation

Monday, February 19th, 2007

What is a single ventricle cardiac anomaly?

The term “single ventricle anomaly” is purposely non-specific. It is used to describe a group of cardiac defects that may differ quite dramatically from each other but share the common feature that only one of the two ventricles is of adequate functional size.

Because of this feature, the ultimate plan for reconstruction is actually quite similar for most of these anomalies. All will generally undergo staged reconstructive procedures ultimately resulting in a “Fontan circulation.”

Some of the anomalies described as single ventricle defects include:

Tricuspid atresia

Hypoplastic left heart syndrome

Double inlet left ventricle

Many of the heterotaxy defects

Some variations of double outlet right ventricle

Diagnosing and evaluating single ventricle anomalies

In most cases an echocardiogram will be able to define the details of the cardiac anatomy for the initial diagnosis in the newborn period. Echocardiography will also be important in following the anatomy and function of the various components of the cardiovascular system, such as valve or ventricular function, as the child grows.

Cardiac catheterization is only occasionally necessary in a newborn with a single ventricle anomaly, if there are details of the anatomy that cannot be determined by echocardiogram. However, patients with single ventricle anomalies will have a cardiac catheterization prior to the second surgery (Glenn shunt) and again before the third surgery (Fontan procedure). These cardiac catheterizations are done to look at the anatomy, particularly of the pulmonary arteries, and to obtain pressure measurements in the heart. These pressure measurements are important in determining if a patient with a single ventricle anomaly is a good candidate for surgery.

Catheter interventions such as dilation or stenting of pulmonary arteries or coil occlusion of abnormal collateral (extra) vessels may be performed at the time of these cardiac catheterizations.

Fontan Circulation

In the normal heart each ventricle does a separate job. The right ventricle pumps blood to the lungs, the left ventricle pumps blood to the body.

In a single ventricle heart, there is only one ventricle large enough to do the normal job of pumping blood. Thus, we need to configure the circulation to maximize the efficiency of this single ventricle.

This ultimately requires committing the single ventricle to doing the harder work of the heart, pumping blood to the body. The job of getting blood to the lungs must be done without a pump.

Whenever there is only one ventricle large enough to do a normal job of pumping blood, we need to configure the circulation to maximize the efficiency of this single ventricle without overworking it.

The “Fontan circulation” refers to this configuration where the single ventricle pumps blood returning from the lungs to the body, and the blood returning from the body travels to the lungs via direct blood vessel connections without a pumping chamber. In any individual child there may be different procedures needed to achieve this goal.

Requirements for a child to be a good risk candidate for a Fontan circulation

For a heart with a Fontan reconstruction to work well, there are a few crucial features that must be maintained. These key factors must be kept in mind when planning the medical or surgical management of children with single ventricle defects from the first day of life onward.

The single ventricle must not be overworked for a long period of time, in terms of either having to pump too much blood or pump at too high a pressure.

The pulmonary arteries must grow well without stenosis (narrowing) and must remain low resistance (or be very relaxed). If the pulmonary arteries are narrow or if the resistance in these vessels is high, blood will not be able to flow into them without a pump, so the Fontan will not be successful.

Lastly, leaky or tight valves may adversely affect the function of the ventricle or the flow of blood to the lungs.

Operative stages for achieving a Fontan circulation

The type of operation needed in the newborn period is quite varied depending on the specific type of single ventricle cardiac defect. In some babies there is not enough blood flow into the lungs, resulting in cyanosis. In these babies, a tube graft is placed from an artery (usually the left subclavian or left innominate artery) to the pulmonary artery. This is called a systemic to pulmonary artery shunt or Blalock-Taussig (BT) shunt.

In other babies, the flow of blood into the lungs may be excessive, placing an extra burden on the ventricle and exposing the pulmonary arteries to dangerously high pressure. In these babies, a procedure will be performed to restrict blood flow to the lungs. This is done by placing a piece of material or a “band” around the pulmonary artery.

Other newborns have more complex heart disease and require more complex operations, such as the Norwood procedure for patients with hypoplastic left heart syndrome.

Rarely, a baby with a single ventricle anomaly will have “just right” flow into the lungs so that an equal amount of blood flows to the body and the lungs. These babies do not require intervention in the newborn period.

Whatever is needed in the newborn period, the aim is typically to balance the blood flow between the lungs and the body, achieving stable oxygen levels and adequate heart function.

The second stage for most children with single ventricle anomolies is undertaken around three to six months of age. The operation is called a “bi-directional Glenn” or sometimes a “hemi-Fontan.”

During the Glenn operation the large vessel that drains blood from the head and upper body back to the heart (the superior vena cava) is taken off the heart and sewn directly to the pulmonary artery. If a prior BT shunt was present, it is removed. If a pulmonary artery was previously placed, it may be removed but can also be left in place in some situations.

The Glenn operation has two major advantages in most children. First, because the connection is a direct one between two blood vessels, rather than made of artificial matter, it has the ability to grow with the child.

Second, it removes some of the work of pumping blood to the lungs from the single ventricle so that the ventricle will no longer have to pump all of the blood to the lungs in addition to all of the blood to the body, which places it at risk for early heart failure. In most cases this stage is tolerated the best of all the stages with a survival rate of 95 percent or better.

After the Glenn operation most children will have oxygen saturation levels of 75 percent to 85 percent.

The third and final stage in the reconstruction of a single ventricle heart defect is the Fontan completion operation. This operation is usually performed at 2 or 3 years of age, based on the child’s size and clinical status.

During the Fontan operation, blood returning to the heart from the lower half of the body (via the inferior vena cava) is connected directly the blood from the pulmonary arteries. Up until now this blood has bypassed the lungs resulting in oxygen levels lower than normal.

After a Fontan operation, oxygen levels will be nearly normal (90s). The two most common methods of performing the Fontan completion today are the “lateral tunnel” and the “extra-cardiac” techniques.

In the lateral tunnel method, a tunnel-like patch is placed inside the atrium so that blood returning from the inferior vena cava is directed through this tunnel. A connection is then made between the end of the tunnel / top of the right atrium and the underside of the pulmonary artery.

In the extra-cardiac method, the inferior vena cava is taken off of the heart and a synthetic tube, usually Gore-texTM, is sewn directly to the top of the inferior vena cava and to the underside of the pulmonary artery, routing the blood flow outside of the heart.

In either method, a hole or “fenestration” is often made between the Fontan circuit and the right atrium so that if pressures become very high in the Fontan circuit, there is a “pop-off” into the heart. Patients with fenestrations may have a more stable post-operative course with smaller and less prolonged plerual effusions (a common complication after Fontan surgery). Many fenestrations close spontaneously many months after surgery, but can also be closed during a cardiac catheterization procedure if deemed necessary.

Currently, when patients have been well prepared for Fontan completion, the success rates are 90 percent and higher.

Will a child who has had a Fontan operation be able to function as well as a child with a normal, two-ventricle heart?

After a successful Fontan surgery, the reconstructed single ventricle heart has achieved its maximal efficiency in terms of ventricular work and near normal oxygen levels, but its capacity for work will usually not match that of a normal heart when examined using sophisticated testing.

The limitations children experience due to their heart defect, though, can vary greatly. At one end of the spectrum there are children with Fontan circulations who have participated in competitive sports such as swimming and gymnastics. Most children are on a blood thinner called coumadin after their Fontan to prevent clots from forming in the Fontan circuit. When a child is on a blood thinner, you must take extra careful to avoid falls or head trauma as they are at increased risk for internal bleeding.

Other children may have significant limitation in their capacity for exercise. Most children fall somewhere in between the extremes.

How long a heart with a single ventricle reconstruction can function is not known. It is now just over 30 years since the first successful Fontan operation was performed and many improvements in surgical technique and medical management have occurred over this time period.

Late complications including irregular rhythms and heart failure may be occur. Some speculate that most single ventricle hearts will not function efficiently beyond 30 to 40 years, but improvements in surgical technique and medical care may increase this age significantly. In some cases, if the ventricular function deteriorates significantly, heart transplantation may be considered.

Because of the possibility of late complications, continued regular follow-up with a cardiologist for the life of a patient with a single ventricle anomaly is essential.