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.