The interatrial septum can contain defects, like a patent foramen ovale or primum or secundum atrial septal defects.
- How do you distinguish between an ultrasound artifact and a real defect?
- What is the role of Doppler ultrasound in diagnosing septal defects?
- Can the appearance of the right heart provide any clues?
In this video, Cristiana Monteiro—a cardiac physiologist from the University of Oxford—explains the anatomy behind septal defects and how to diagnose them using ultrasound.
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At the end of this Medmastery lesson, you will be able to recognize and classify abnormalities in their interatrial septum, such as a secundum or primum atrial septal defect, and a patent foramen ovale. First things first, many physicians may be confused by an apical four chamber image that looks like this, an echocardiogram of the right atrium with a dark area in their interatrial septum. Is that a defect?
The first thing you should do is to look at the right heart and ask yourself, is it a normal size. If there were an atrial septal defect, the right heart would very likely be enlarged, as we can see in the heart on the right. So, here we can see that the right heart is a normal size. Instead, this dark area in the interatrial septum, is actually due to ultrasound drop out, this occurs when the ultrasound beam is parallel to the septum.
As a result, the returning signal is weak if it appears at all, but this is okay, dropout occurs with all equipment even when it is brand new. In this case, the ultrasound beam is parallel to the septum. And in this particular spot, the septum is so thin that it barely reflects any ultrasound back to the transducer. You may have guessed, this is actually the fossa ovalis and it is completely normal. But what happens when you do suspect a septal defect?
In this case, you must use a color box to assess the direction of the blood flow. In addition, if your cursor is perpendicular to the potential defect, you should also use Pulse Wave Doppler to measure the velocity of blood flow and confirm your suspicions. Use all views where you can see that interatrial septum that is that parasternal short axis of the valves, apical four chamber and subcostal views. To better understand septal defects, let's take a look at how the interatrial septum is formed.
The interatrial septum is actually made up of two thinner membranes, the septum secundum on the right atrial side here in blue, and the septum primum on the left atrial side here in red. You will notice they are both incomplete and each has a hole. But these holes are offset and covered by the opposing septum together creating a continuous wall. This area is called a fossa ovalis. When flow occurs between the two atria, from the right atrium to the left atrium, the hole is referred to as a foramen of ovale. This is a normal structure in the fetal heart, but it should close over after birth.
A secundum atrial septal defect is characterized by an interatrial septum only made up of the septum primum, the septum on the left atrium side. As a result, there will be a shunt in the middle of the interatrial septum. The blood flow moves from left to right, as the pressure in the left atrium is higher than in the right atrium. Conversely, a primum atrial septal defect is characterized by an interatrial septum only made up of the septum secundum the septum on the right atrium side.
Although both septum exists in the fetal heart, the septum primum recedes excessively in patients with this type of septal defect. This leaves a gap in the lower interatrial septum. Again, the blood flow moves from the left atrium to the right atrium. And lastly, a patent foramen ovale, or PFO. Remember, the foramen ovale is a normal structure in the fetal heart and it is essential to fetal circulation. However, if the foramen ovale does not close fully after birth, it may allow blood to flow from the right atrium into the left atrium when the right atrial pressures exceed the left. Well done, you can now recognize and distinguish between ultrasound dropouts, an arterial septal defect and the patent foramen ovale.