The interatrial septum is the fibromuscular wall between the heart’s two upper chambers, the right and left atria. In a developed heart, its primary job is to prevent blood from mixing between the two chambers. This separation is important for the circulatory system to operate efficiently.
Function and Development of the Interatrial Septum
The septum’s main purpose is to separate deoxygenated and oxygenated blood. The right atrium receives oxygen-poor blood from the body, while the left atrium gets oxygen-rich blood from the lungs. By preventing this mixing, the septum ensures the body’s tissues receive blood with the highest oxygen content for efficient delivery.
The septum’s structure transforms significantly from fetal life to after birth. During fetal development, the lungs are not functional, and a natural opening called the foramen ovale exists within the interatrial septum. This opening allows blood to be shunted from the right atrium directly to the left atrium, bypassing the pulmonary circulation.
This developmental process begins in the first and second months of fetal life with the growth of a crescent-shaped tissue called the septum primum. Following this, a second structure, the septum secundum, forms to the right of the septum primum. The layered formation of these tissues creates the foramen ovale, which is covered by a flap-like valve.
Shortly after birth, when a baby takes its first breath, the pressure in the left atrium increases, pushing this flap valve against the septum and effectively closing the foramen ovale. Over time, this flap usually fuses with the septum, leaving a small, shallow depression known as the fossa ovalis.
Atrial Septal Defects
An atrial septal defect (ASD) is a hole in the interatrial septum that persists after birth. This opening allows oxygen-rich blood from the left atrium to flow into the right atrium, mixing with oxygen-poor blood. This abnormal flow, called a left-to-right shunt, increases the blood volume the right side of the heart must pump to the lungs.
Over time, this increased workload can cause the right atrium and ventricle to enlarge and weaken. The extra blood flow can also lead to higher pressure in the lung’s arteries, a condition called pulmonary hypertension. Small ASDs may cause no symptoms for years, but larger defects can lead to fatigue, shortness of breath, and an increased risk of stroke or heart failure.
There are several types of ASDs, classified by their location on the septum:
- Ostium secundum defect: The most common type, located in the center of the septum in the area of the fossa ovalis.
- Ostium primum defect: Occurs in the lower part of the septum and can be associated with issues in the heart valves.
- Sinus venosus defect: Found in the upper part of the septum and often associated with abnormal connections of the pulmonary veins.
- Coronary sinus defect: A rare type involving a communication between the coronary sinus and the left atrium.
Diagnosing Septal Issues
The first indication of an atrial septal defect is often a heart murmur detected during a physical exam. A heart murmur is a whooshing sound from turbulent blood flow that a doctor hears with a stethoscope. The murmur associated with an ASD is caused by increased blood flow across the pulmonary valve, not the defect itself.
The primary tool for confirming a diagnosis is an echocardiogram. This non-invasive test uses sound waves to create detailed images of the heart, allowing doctors to see the hole in the septum. An echocardiogram can show the ASD’s size and location and use Doppler technology to measure blood flow between the atria.
In some cases, a transesophageal echocardiogram (TEE), where an ultrasound probe is guided down the throat, provides clearer images. Other tests like a chest X-ray or an electrocardiogram (ECG) can offer supporting information. These can show enlargement of the right side of the heart or detect abnormal heart rhythms.
Treatment for Atrial Septal Defects
Treatment for an ASD depends on the hole’s size, blood flow volume, and the patient’s symptoms. Small defects that do not cause the right heart to enlarge may not require intervention and can sometimes close on their own in children. In these instances, regular monitoring by a cardiologist is often sufficient.
For larger defects causing symptoms or right heart enlargement, closure is recommended to prevent complications. A common treatment is a minimally invasive catheter-based procedure. A doctor inserts a thin tube through a blood vessel, typically in the groin, and guides it to the heart.
A special closure device, like a small mesh plug, is then delivered through the catheter and deployed to seal the hole. Over time, the heart’s tissue grows over the device, making the closure permanent.
When a defect is too large or complex for a catheter procedure, surgical repair may be necessary. This involves open-heart surgery, where a surgeon stitches the hole closed or patches it with synthetic material or the patient’s tissue. While more invasive, surgical repair is highly effective and the standard for certain ASDs, like primum or sinus venosus defects.