Atrial Septal Defect (ASD) is a defect or hole in the wall separating the two upper receiving chambers (the right and left atriums) of the heart - see figure 2.1.

ASD is one of more common "hole in the heart" seen, and it can occur in isolation or in combination with other congenital heart defects. Cardiologists subclassify these defects according to their locations on the atrial septum. The secundum ASD is the more common defect to occur as an isolated lesion. It represents about 5 - 10% of all congenital heart defects seen and is more common in females than males.

Figure 2.1
On the left is a diagram of a heart with an atrial septal defect (ASD), compared to a normal heart on the right. Hover the mouse pointer over the diagram to see the description. LA - Left Atrium, RA - Right Atrium, LV - Left Ventricle, RV - Right Ventricle.

What Is the Effect of Atrial Septal Defect (ASD)?

The left atrium is normally at a higher pressure than the right atrium. The atrial septal defect allows blood to flow from the left atrium to the right atrium. The blood shunted across the ASD is re-circulated back to the lungs. As seen in figure 2.2, some red blood returning from the lungs into the right upper heart chamber (right atrium) now can flow across the ASD to mix with the blue blood, which is then re-circulated to the lungs. Compare this to the normal heart in figure 2.3, where there is no ASD and no mixing of the red and blue blood.

Figure 2.2	Figure 2.3
Blood circulation in a heart with an ASD. Note some red blood now crosses the ASD to mix with the blue blood.	Blood circulation in a normal heart. Note that the red and blue bloods do not mix during circulation through the heart.

The effects of having "extra" blood flowing across the ASD into the right atrium and subsequently into the right ventricle and then re-circulated to the lungs are:

• The right-sided upper heart chamber (right atrium) and right-sided lower heart chamber (right ventricle) become dilated to accommodate this extra blood volume.
• The lungs becomes more "wet" or even become "flooded" with this extra re-circulated blood.
• The blood pressure in the lungs increases.

The degree to which these effects occur would depend on the size of the ASD and the amount of blood that is re-circulated back to the lungs. This can range from insignificant to severe.

Most infants with ASD have no symptoms, and the ASD goes undiagnosed. The ASD can be an incidental finding when an echocardiogram is performed for other reasons.

The most common presentation of an ASD is that of a cardiac murmur. The murmur caused by the ASD is generally soft, and there can also be associated changes in the normal heart sounds. Less commonly, babies with large ASD can present with symptoms of heart failure such as rapid breathing and poor feeding.

The chest X-ray and electrocardiogram are often non-specific; they can be totally normal in small ASDs. In larger defects, the chest X-ray can show enlargement of the heart and increased blood vessel markings in the lungs. The electrocardiogram can sometimes shows subtle changes that lead a doctor to suspect the presence of an ASD.

The investigation of choice when an ASD is suspected is the echocardiogram. This will not only confirm the presence of the ASD, it will also allow the cardiologist to assess the number and sizes of the ASD(s), as well as it's effects on the heart. The echocardiogram can also shows any other associated cardiac defects and well as the suitability of the various treatment options. Figure 2.4 is a still frame taken during an echocardiographic study and shows the presence of an ASD. Hover your pointer over the picture for labels, or click on the picture for a short movie clip of color flow mapping showing blood flowing across the ASD.

Figure 2.4
Echocardiogram showing the presence of an atrial septal defect. Hover pointer to see labels, click to see color flow study showing blood flowing across the defect.

What is the appropriate treatment for a particular child with an ASD will depend on many factors such as the number, size and location of the ASD(s), any associated cardiac defects and the effects of the ASD on the heart. The suitability of the various treatment options will vary from person and person and should be discussed with your cardiologist. These options would include:

• Continued Outpatient Observation - Small ASD with insignificant haemodynamic effect need not be treated. Many of these small defects, especially those discovered in infancy, close spontaneously with time. However, they should be followed-up by a cardiologist or your family physician.
  
• Surgical closure - This is an open-heart surgery performed under cardiopulmonary bypass. The surgeon can close the ASD directly with sutures or with a patch.
  
• Transcatheter device closure - ASD can now be closed using special devices delivered using cardiac catheters (long, thin tubes). With this technique, the patient need not undergo cardiopulmonary bypass and there is no surgical scar. The most commonly used device here is the Amplatzer Septal Occluder. The ASD must be carefully assessed prior to recommending this option as not all ASDs are suitable for transcatheter device closure.