466 Views Posted on 14-Mar-2011 under Features

By Dr. Kow Entsua-Mensah
Surgeon, National Cardiothoracic Centre

We continue our discussion on common congenital heart diseases with a look at another common lesion known as atrial septal defect.

As the name implies, atrial septal defect (ASD) is a congenital heart disease in which there is a defect in the septum separating the two atria, namely the left and right atria. The atria are the chambers which collect and hold blood before discharging their contents into the ventricles or pumping chambers. The interatrial septum is the tissue wall that divides the right and left atria and prevents mixture of the two blood streams. Without this septum, or if there is a defect in this septum, it is possible for blood to travel from the left side of the heart to the right side of the heart, or vice versa. This blood flow is abnormal and results in the mixing of oxygenated and deoxygenated blood. The direction of flow is from the higher pressure left atrium (containing oxygenated blood), to the lower pressure right atrium (containing deoxygenated blood).

Types of atrial septal defects
There are many types of atrial septal defects, categorized mainly by their location along the interatrial septum or its relation to other structures. The types are:

1. Patent foramen ovale
2. Ostium primum atrial septal defect
3. Ostium secundum atrial septal defect
4. Sinus venosus atrial septal defect (upper and lower)
5. Unroofed coronary sinus


Patent foramen ovale (PFO)
A brief look at the development of the heart and changes occurring soon after birth will enhance our understanding of this condition.

During development of the foetus, (baby in the womb), the interatrial septum develops to separate the left and right atria. However, there is a foramen ovale (foramen: hole, ovale: oval) which allows blood from the right atrium to flow to the left atrium. The non-functional foetal lung is bypassed. Soon after birth, when the lungs come into full use, this foramen ovale closes. This adapts the newborn baby’s heart to life outside of the womb. In approximately 25% of people, the foramen ovale does not entirely seal even up to adulthood. Since the foramen ovale remains open or patent, it is medically termed patent foramen ovale (PFO).

Ostium primum atrial septal defect
A defect in the lower part of the interatrial septum is called ostium primum atrial septal defect. Developmentally it is the first communication between the two atria, (ostium: opening, primum: primary or first). This ostium normally closes in the course of development. Abnormal persistence usually also involves the lower two chambers of the heart, the ventricles, hence it is more commonly classified as an atrioventricular septal defect. Presence of this defect is also associated with a cleft or slit in one of the leaflets of the mitral valve, (anterior mitral valve leaflet).

Ostium secundum atrial septal defect
The ostium secundum atrial septal defect is the most common type of atrial septal defect, making up about 80% of all atrial septal defects corrected at the National Cardiothoracic Centre and comprises 6-10% of all congenital heart diseases.

As we have discussed earlier, the foramen ovale needs the septum secundum, (septum: wall, partition, secundum: second) to help close it and adapt the heart to life outside of the womb. Hence, the secundum atrial septal defect usually arises from an enlarged foramen ovale, inadequate growth of the septum secundum, or excessive absorption of the septum primum. Ten to twenty percent of individuals with ostium secundum ASDs also have mitral valve prolapse, a condition where the valve leaflets balloon out during heart contractions.

Sinus venosus atrial septal defect
A sinus venosus ASD is a type of atrial septal defect in which the defect in the septum involves the venous inflow into the right atrium; either the superior vena cava (upper great vein) or the inferior vena cava (lower great vein).

A sinus venosus ASD that involves the superior vena cava makes up 2 to 3% of all interatrial communications. It is located at the junction of the superior vena cava and the right atrium. It is frequently associated with anomalous drainage of the right-sided pulmonary veins (containing oxygenated blood) into the right atrium (containing deoxygenated blood) instead of the normal drainage of the pulmonary veins into the left atrium.
Unroofed coronary sinus
This occurs when there is a defect in the roof of the vein which drains venous blood from the heart into the right atrium. Its deoxygenated blood finds its way as a result into the left atrium where it mixes up the rich oxygenated blood. There is therefore a step down in the oxygen concentration of blood reaching the rest of the body.

Common or single atrium
Common (or single) atrium is a failure of development of the embryologic components that contribute to the atrial septal complex discussed earlier. It is frequently associated with heterotaxy syndrome, a disorder that results in certain organs forming on the opposite side of the body.

Occurrence
As a group, atrial septal defects are detected in 1 child per 1500 live births. PFO are quite common (appearing in 10 - 20% of adults) but asymptomatic and therefore undiagnosed. ASDs make up 30 to 40% of all congenital heart disease that is seen in adults.

The ostium secundum atrial septal defect accounts for 7% of all congenital heart lesions. This lesion shows a female preponderance, with a male : female ratio of 1: 2.

The untreated atrial septal defect
Most individuals with small to moderate sized uncorrected secundum ASD do not have significant symptoms up to early adulthood. About 70% develop symptoms by the time they are in their 40s. Symptoms are typically decreased exercise tolerance, easy fatigueability, palpitations (consciousness of heart beats) and dizziness.

Complications of an uncorrected secundum ASD include pulmonary hypertension, (abnormally high pressures in the blood vessels of the lungs), right-sided heart failure, atrial fibrillation or flutter, (rapid uncoordinated beating of the atria), stroke, and Eisenmenger's syndrome; a state where the usual left atrium-to-right atrium direction of blood flow is reversed. At this stage, the ASD is generally considered to be inoperable .

While pulmonary hypertension is unusual before 20 years of age, it is seen in 50% of individuals above the age of 40. Progression to Eisenmenger's syndrome occurs in 5 to 10% of individuals late in the disease process. I must emphasise that when the ASD is large, then the progression to the complicated state is rapid.



Echocardiogram of an atrial septal defect.


Causation of disease
In unaffected individuals, the chambers of the left side of the heart are under higher pressure than the chambers of the right side of the heart. This is because the left ventricle has to produce enough pressure to pump blood throughout the entire body, while the right ventricle only has to produce enough pressure to pump blood to the lungs.

In the case of a large ASD (>9mm), which may result in a clinically remarkable left-to-right shunt, blood will shunt from the left atrium to the right atrium. This extra blood from the left atrium may cause a volume overload of both the right atrium and the right ventricle. If untreated, this condition can result in enlargement of the right side of the heart and ultimately heart failure.
Any process that increases the pressure in the left ventricle can cause worsening of the left-to-right shunt. This includes hypertension and coronary artery disease.
The right ventricle will have to pump out more blood than the left ventricle due to the left-to-right shunt. This constant overload of the right side of the heart will cause an overload of the entire pulmonary vasculature. Eventually pulmonary hypertension may develop.
The pulmonary hypertension will cause the right ventricle to face increased afterload (from high pressures in the lungs) in addition to the increased preload that the shunted blood from the left atrium to the right atrium caused. The right ventricle will be forced to generate higher pressures to try to overcome the pulmonary hypertension. This may lead to right ventricular failure (dilatation and decreased contractile function of the right ventricle) or elevations of the right sided pressures relative to left sided pressures.
When the pressure in the right atrium rises to the level in the left atrium, there will no longer be a pressure gradient between these heart chambers, and the left-to-right shunt will diminish or cease.
If left uncorrected still, the pressure in the right side of the heart will be greater than the left side of the heart. This will cause the pressure in the right atrium to be higher than the pressure in the left atrium. This will reverse the pressure gradient across the ASD, and the shunt will reverse; a right-to-left shunt will exist. This phenomenon is known as Eisenmenger's syndrome.
Once right-to-left shunting occurs, a portion of the oxygen-poor blood will get shunted to the left side of the heart and ejected to the peripheral vascular system. This will cause signs of cyanosis, generally seen as bluish discoloration of the lips, fingers and toes.

Diagnosis
In children Most individuals in with a significant ASD are diagnosed in the mother’s womb using ultrasound. In Ghana, at the Cardio Centre, most ASDs are diagnosed in early childhood with the use of echocardiograpghy.

In adults Adults up to the age of 70 years and above have had surgical correction of ASDs at the Cardio Centre. The development of signs and symptoms due to an ASD are related to the size of the intracardiac shunt. Individuals with a larger shunt tend to present with symptoms at a younger age.

Adults with an uncorrected ASD will present with symptoms of shortness of breath with minimal exercise, congestive heart failure, or cerebrovascular accident (stroke). They may be noted on routine testing to have an abnormal chest x-ray or an abnormal ECG.
Physical examination
A thorough physical examination is performed in order to elicit physical signs related directly to the intracardiac shunt and those that are secondary to the right heart failure that may be present in these individuals.

Laboratory investigations
Haemoglobin level, sickling status, liver and kidney function tests and blood group determination are some of the routine laboratory investigations carried out at the National Cardiothoracic Centre.

Chest X-ray
This gives a lot of information about the size of the heart and the state of blood flow to the lungs. This gives valuable indirect information about the size of the ASD and the extent of the left to right shunt.

Electrocardiogram
This is the recording of the electrical activity of the heart. Specific findings raise the suspicion of the presence of an ASD and its sub types.

Echocardiography
In transthoracic echocardiography, (viewed with the probe on the chest) an atrial septal defect may be seen on color flow imaging as a jet of blood from the left atrium to the right atrium.

Because better visualization of the atria is achieved with transesophageal echocardiography, (performed with a swallowed echocardiogram probe) this test may be performed in individuals with a suspected ASD which is not clearly visualized on transthoracic imaging.
Transcranial Doppler (TCD) Bubble study
A less invasive method for detecting a PFO or other ASDs than transesophagal ultrasound is Transcranial Doppler with bubble contrast. This method reveals the impact of the ASD or PFO on the brain.
Right heart catherterisation
If pulmonary hypertension is present, the evaluation may include a right heart catheterization. This involves placing special fine tubes into the right side of heart to determine the pressures and oxygen content. The information it gives advices doctors on the safety or otherwise of an intended operation.

Treatment
Once someone is found to have an atrial septal defect, a determination of whether it should be corrected has to be made.

Meanwhile, the individual is commonly placed on medications that aim to reduce the extra work of the heart and the fluid and pressure load on the lungs. The excess fluid is usually passed as urine.
Methods of closure of an ASD include surgical closure and percutaneous closure.

Catheter or percutaneous closure
This is indicated in suitable ASDs, i.e. the closure of secundum ASDs with a sufficient rim of tissue around the septal defect.

Surgical ASD closure
Surgical closure of an ASD involves opening up at least one atrium and closing the defect with a patch under direct visualization.
Associated conditions
The lungs act as the filters of particulate matter and infectious agents returning to the heart from the extremities. Due to the communication between the atria that occurs in ASDs, these materials end up on the arterial side

Decompression sickness
ASDs, and particularly PFOs, are a predisposing risk factor for decompression sickness in divers because a proportion of venous blood carrying inert gases, such as helium or nitrogen does not pass through the lungs. The only way to release the excess inert gases from the body is to pass the blood carrying the inert gases through the lungs to be exhaled. If some of the inert gas-laden blood passes through the PFO, it avoids the lungs and the inert gas is more likely to form large bubbles in the arterial blood stream causing decompression sickness.

Paradoxical emboli
Venous thrombi (clots in the veins) are quite common. Embolisation, (dislodgement of thrombi) normally go to the lung and cause pulmonary emboli. This can cause any phenomenon that is attributed to acute loss of blood to a portion of the body, including cerebrovascular accident (stroke), infarction of the spleen or intestines, or even a distal extremity i.e. finger or toe.

This is known as a paradoxical embolus because the clot material in a normal heart cannot gain access to the arterial system directly from the venous side.
Viral Meningitis
It is likely that a virus can pass through PFO, therefore not being filtered by the lungs, and sent directly to the brain. This can cause a type of sinusitus which can lead to viral meningitis.