Atrioventricular Septal Defect – definition, diagnosis and treatment
Atrioventricular septal defect (AVSD) is a relatively uncommon congenital cardiac condition. There appears to be an association between the condition and Down’s syndrome (although the presence of one does not indicate with certainty that the other is also there).
An AVSD is a large hole in the wall dividing the right and left sides of the heart. It is located centrally, and therefore both the collecting chambers (the atria) and the pumping chambers (the ventricles) are involved: the hole lies between all four.
It is possible to have a partial AVSD (a defect involving the atria but with no hole at ventricular level): this is a less severe variant of this lesion; it will not be discussed in detail here.
In a normal heart, the left atrium (collecting chamber) receives oxygenated blood from the lungs, which is then pumped by the left ventricle around the body. The right atrium receives de-oxygenated blood from the veins, which is then pumped into the lungs by the right ventricle. Thus, the right and left sides of the heart deal with de-oxygenated and oxygenated blood respectively: the arrangement ensures a complete division between de-oxygenated and oxygenated blood. If there is a large hole between all four chambers, then that division necessarily breaks down. Both the right and left sides of the heart will receive a mixture of oxygenated and de-oxygenated blood. Thus, blood that is already partially oxygenated will be pumped back into the lungs, and the systemic circulation will receive blood that is not adequately oxygenated.
There is a further problem arising from the arrangement in AVSD, which arises from the disparity in pressures between the right and left sides of the heart. Because there is a hole, there will be a “shunt” – i.e., the side of the heart which is pumping at a higher pressure will push blood across the hole and into the other side of the heart.
The shunt will initially probably be from left to right, because the left side of the heart operates at a higher pressure its pumping chamber pushes out blood at a higher volume: hence, blood will be forced from the left to the right side of the heart. This is a dynamic process, and over time the direction of the shunt may reverse (e.g. right – left, rather than left – right).
Typically, this means that the pulmonary arteries receive more blood than they can cope with, and over time this increases pulmonary vascular resistance. If this is left uncorrected, it will lead to permanent damaging changes to the lung fields. Conversely, the left side of the heart will not be able to pump a sufficient supply of blood around the body as each time it pumps, it is losing some of its volume to the lungs. Thus, the body receives a blood supply that is inadequately oxygenated and quantitatively insufficient.
The symptoms and long-term effects of AVSD
Children suffering from this condition will characteristically exhibit marked symptoms very early in life. Typically, you will find the following complications:
- Cyanosis (a blue tinge to the skin, usually most pronounced around the fingertips and lips), and dyspnoea (problems with breathing).
- Heart failure may be present at or shortly after birth. If present, then a greyish appearance to the skin accompanied by sweating, and/or signs of respiratory distress (wheezing or grunting) and/or fatigue may all be present. The child’s heart may be irregular.
- When measured, oxygen saturation levels are likely to be significantly lower than usual.
- There are likely to be problems with feeding.
Longer term, if left untreated, AVSD will cause death – usually as a consequence of cardiac failure – in the sufferer’s late twenties or early thirties. Historically, clinicians were reluctant to operate upon patients with Down’s syndrome and AVSD. This was because the lifespan of patients with Down’s was then in the range late twenties – early thirties in any event, and consequently it was thought unethical to put this group through a procedure which then carried a 25% mortality risk for no real benefit. The position is now very different, and survival rates are far higher.
Diagnosing and treating AVSD
AVSD can sometimes be seen on ultrasound antenatally; if so, it is a marker for Down’s syndrome (although it is not diagnostic of the condition).
Assuming it is not made antenatally, diagnosis is usually made soon after birth by diagnostic imaging. Echocardiography (a form of ultrasound) will usually confirm the diagnosis.
It may be necessary to clarify the diagnosis by further investigation – possibly including a cardiac catheterisation. During this procedure, a camera is threaded up the blood vessels towards the heart and imaging of the affected areas is then performed.
Once the diagnosis has been made and clarified, surgical treatment is likely to be indicated. Unless the baby’s condition is so bad that immediate corrective surgery is required, it is likely that monitoring will be instituted. After the child has grown for some months, definitive surgery will usually be offered.
Surgery is conducted on cardiopulmonary bypass. A median sternotomy (a large cut down the centreline of the chest and through the breastbone) is made. The ribs are spread. The atrial and ventricular holes are closed. The left atrioventricular valve will usually require repair.
As with any cardiac procedure which involves either time spent on cardiopulmonary bypass and either aortic cross-clamps or possibly total circulatory arrest, there is a limited amount of time within which to effect the repair, as the likelihood of damage to the heart and other organs in the body mounts with the passage of time.
AVSD does carry one significant risk factor – namely, clotting. If a patient with this condition suffers an embolus (clot) in a vein, then because there is a large communication between the two sides of the heart, it is possible for fragments of that clot to pass from the venous into the arterial circulation. If that happens, then those fragments can lodge in the blood vessels in the brain. If that happens, then a stroke will occur.