What is the heart defect called Single Ventricle ?
While the normal heart has two ventricles, in some birth defects, one of these ventricles may be absent or poorly developed. This condition is called Single Ventricle or Univentricular heart.
The ventricular structure may resemble the normal left ventricle or the normal right ventricle. Sometimes, it resembles neither, and this is called Indeterminate ventricle morphology.
In a single ventricle heart, there are two normal atria – right and left. These open into the ventricle through an atrio-ventricular (AV) valve. There might be
two AV valves, both opening into the ventricle – a condition called Double Inlet Ventricle – or one AV valve only, with the other one being absent (Atresia)
The single ventricle connects with the aorta and pulmonary artery. These two great arteries may be normal or interchanged in position. Some patients have obstruction of the great arteries – pulmonary stenosis and sub-aortic stenosis.
What happens in Single Ventricle ?
The main functional disturbance in single ventricle is the mixing of blood inside the ventricular chamber.
Impure blood from the veins enters the right atrium and from there reaches the single ventricle. Pure blood returning from the lungs flows into the left atrium and into the single ventricle. Inside the ventricle, both mix together. From here, the mixed blood flows into the aorta and pulmonary artery.
The clinical picture depends on the relative amount of the total blood flow that reaches the lungs.
Three scenarios are possible:
1. The pulmonary artery may be normal, with no obstruction. Because the lung blood vessels are thin walled, they offer lower resistance to the flow of blood than the aorta and its branches. So a larger proportion of blood from the single ventricle will flow into the pulmonary artery and into the lungs. In many ways, this situation is similar to a very large VSD. This increased volume of blood returns back to the ventricle and is once again pumped out, resulting in a high workload on the single ventricle. Soon, unable to tolerate this stress, the ventricle fails.
2. The pulmonary artery may be obstructed (stenosis). This increases the resistance to blood flow into the lungs. A larger proportion of blood from the single ventricle will flow into the aorta and its branches. The amount of blood flowing into the lungs for purification and addition of oxygen is thus reduced. So blood returning to the single ventricle from the veins is more impure and contains less oxygen. This causes oxygen content in blood leaving the ventricle into the aorta also to be reduced, producing a bluish discoloration called cyanosis.
3. The pulmonary artery may be mildly narrowed. The resistance in both the pulmonary artery and aorta are nearly the same in this condition. As a result, blood from the single ventricle will flow in almost equal proportion into both the great arteries. This situation is called a “balanced circulation“, and may not produce any clinical symptoms for a long time.
Should Single Ventricle be treated ?
The natural history of single ventricle conditions is highly variable depending on the functional condition as described earlier. In a group of patients with balanced circulations, survival at 10 years may be over 90%.
Overall, as a group, two thirds of single ventricle patients will survive one year after birth, and only one third will live to their tenth birthday. In view of this poor course of events, single ventricle almost always needs early surgery, sometimes soon after diagnosis is made.
What are the surgical options for single ventricle ?
Operation for single ventricle depends on many different factors including the functional type, age, symptom status, condition of the lung blood vessels and pulmonary resistance and the presence of associated defects.
As an initial procedure, many patients will need one of the palliative operations to tide over crises and to prepare the pulmonary arteries for definitive repair. After palliative procedures, patients are followed up at periodic intervals and plans made for repair at the earliest optimal time.
1. Modified B-T Shunt
This is the same operation that is used in patients with Tetralogy of Fallot. A tube graft is used to connect the subclavian branch of the aorta to the pulmonary artery. It is necessary in patients with narrowing of the pulmonary artery causing low oxygen saturation and cyanosis. The BT shunt will increase lung blood flow, improve oxygen saturation and relieve cyanosis.
2. Pulmonary Artery Banding
This operation is performed in the setting of increased lung blood flow due to a normal pulmonary artery. It is a closed heart operation (that is, it does not require connecting the patient to the heart lung machine) that may be carried out through an incision on the left side of the chest, or through a midline incision that cuts open the breastbone.
The surgeon will place a “band” (a narrow tape of fabric) around the pulmonary artery and tighten it just enough to
- allow adequate blood flow into the lungs to avoid cyanosis
- reduce the excessive flow, so that pulmonary artery pressure is one half of the aortic pressure
Pulmonary Artery Banding has some disadvantages. The band may be too tight or too loose. Rarely, the band may migrate to one of the pulmonary artery branches and narrow it. In single ventricle conditions, one of the important drawbacks is the later development of sub-aortic obstruction after a banding operation.
3. Damus-Stansel-Kaye Operation
This is an another operation that can be done in patients who have sub-aortic narrowing. The pulmonary artery is divided just below the level where it branches into the right and left pulmonary arteries. The pulmonary artery is then sutured to an opening in the side of the aorta.
Blood from the single ventricle can now flow into the pulmonary artery and then into the aorta, bypassing the area of sub-aortic narrowing. Blood flow into the lungs is achieved by creating a modified BT shunt to one of the branch pulmonary arteries.
4. Norwood Operation and Arterial Switch Operation
These rather more complex operations are necessary in cases where the narrowing in the sub-aortic region is combined with a more generalized under-development of the entire aortic arch. The Norwood operation is explained in the section on Hypoplastic Left Heart Syndrome and the Arterial Switch operation in the article on Transposition of the Great Arteries.
Sometimes surgery may be necessary in between palliative and definitive procedures to treat complications that arise in the interim.
1. VSD enlargement
In some cases of single ventricle, especially after PA banding, obstruction may develop in the sub-aortic region. Usually this is due to narrowing of a pre-existing VSD. Surgery may be required to enlarge this VSD be excising its margins.
2. Bidirectional Glenn Shunt
In cases where patients are not yet suitable for Fontan operation (due to presence of risk factors like sub-aortic stenosis, high lung blood vessel resistance, distortion of pulmonary artery branches) but symptom status worsens, a Glenn shunt may be done on one or both sides, followed by a completion Fontan operation at a later date.
Definitive Repair for Single Ventricle
There are two kinds of repair procedures for Single Ventricle – Ventricular Septation and Fontan procedure.
1. Ventricular Septation
In this operation, an attempt is made to re-create an artificial partition in the large ventricular chamber so that blood from the left and right atria will be separated inside the ventricle.
The operation requires many pre-conditions, and is not suitable for a large percentage of patients.
What are the conditions for ventricular septation to be possible ?
- Double Inlet ventricle (not atresia)
- Two normal AV valves, without leak (regurgitation)
- Suitable Chordal Anatomy – this means that the chordae tendineae of the
- AV valves should permit placement of a partition between the two sides of the ventricle
- No associated intra-cardiac defects
- Minimal or no pulmonary outflow narrowing
- Left ventricle or Indeterminate ventricle morphology
How is the ventricular septation operation carried out ?
If all the above conditions are met, a septation operation can be performed. This is an open heart operation performed with the patient hooked up to the heart-lung machine.
With the heart beat stopped, the surgeon opens the right atrium and evaluates the ventricle through the tricuspid valve. After choosing the level of partition, multiple sutures are passed into the ventricle wall. These sutures are then threaded into a suitably shaped patch made of Dacron fabric backed with pericardium to make it less porous and reduce the risk of clot formation on the patch.
The sutures are tied down, securing the patch in place.
Now pure blood from the left atrium will cross the mitral valve to the left side of the ventricle and leave through the aortic valve. Impure venous blood from the right atrium will pass across the tricuspid valve into the right side of the ventricle and into the pulmonary valve.
What are the risks of septation ?
Ventricular septation is a major operation with a rather high mortality rate of 30% to 40%. One major complication of this operation is damage to the conduction system of the heart causing heart block.
As a routine, pacemaker leads are inserted at the time of surgery, and many patients require insertion of a permanent pacemaker later.
How do patients do after septation ?
The limited number of patients followed up late after septation makes this a difficult question to answer reliably. Of those patients evaluated subjectively, exercise tolerance and life style were near normal.
Objective studies however demonstrated ventricular function to be inferior compared to patients who had a Fontan operation. Late re-operation for AV valve regurgitation has been required for some patients.
The Fontan operation has been discussed in great detail in the section on Tricuspid Atresia.
The role of paediatric heart transplantation is still being evaluated, and the same arguments as in cases of HLHS apply here.
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