Atrial Septal Defect Explained

Learning medicine is hard work! Osmosis makes it easy. It takes your lectures and notes to create
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much more. Try it free today! The atrial septum separates the right and
left atrium of the heart, and an atrial septal defect describes when there’s some opening
in a baby’s heart between these two upper chambers. But how does this opening form? Well, let’s first run how the septum develops
in the first place. When the heart’s first developing, a tissue
called the septum primum between left and right atria grows downward, slowly creating
two separate chambers by closing this gap, or ostium primum, which means the “first
opening”. This septum primum then fuses with the endocardial
cushion and closes the gap completely. Meanwhile, a hole appears in the upper area,
called the ostium secundum, or “second opening”. Now, we also have the septum secundum which
grows downward just to the right of the septum primum, and covers the ostium secundum like
a curtain, leaving a small opening called the foramen ovale, and essentially creating
a makeshift valve that allows blood to go from right atrium to left atrium, but not
the other way. The developing fetus gets oxygenated blood
from the placenta, which goes from the umbilical vein over to the right atrium, which is different
because after development, only deoxygenated blood goes to the right atrium, which is then
sent to the lungs to pick up some oxygen. In the developing fetus though, it’s already
oxygenated, so instead of going to the right ventricle and to the lungs and back to the
left atrium, it just bypasses the right ventricle and lungs through the foramen ovale and into
the left atrium. The oxygenated blood then goes to the left
ventricle to be pumped to the body. At birth, the septum secundum and septum primum
slap shut and then fuse and close off this foramen ovale, so we can rely on our own lungs
for oxygen. Most ASD cases are due to the ostium secundum
which can happen when the secundum septum doesn’t grow enough during development This
actually accounts for about 10-15% of all congenital heart defects and is the most common
congenital heart defect in adults. Fewer ASD cases are due to the ostium primum,
where the “first opening” or ostium primum, doesn’t make it all the way down, again
leaving an opening between right and left atria. This primum type of congenital defect is found
in around 25% of patients with Down syndrome. Atrial septal defects, in general, are also
commonly associated with fetal alcohol syndrome. Okay, so there’s this opening in the septum,
now what? Well you know how pressure’s higher on the
left side than on the right? Well this pressure difference causes blood
to be shunted from the left atrium to the right atrium, meaning more oxygenated blood
gets over to the right side, which is usually deoxygenated blood, right? This type of defect is an acyanotic heart
defect, which means “not blue”, since oxygenated blood is going into pulmonary circulation
and essentially taking “an extra” trip to the lungs. In contrast, a cyanotic heart defect allows
deoxygenated blood to bypass the lungs and make it out to the body, causing a blue or
purplish discoloration of the skin—which is not the case here. That being the case, you’ll see an increase
in oxygen saturation of blood in the right atrium, right ventricle, and pulmonary artery. This extra blood volume passing by the pulmonic
valve also causes a delay in the closure of the pulmonic valve relative to the aortic
valve closure. This slight delay can be heard via auscultation
as a splitting of the S2 sound, as well as a systolic murmur in some cases. One potential complication for patients with
an ASD is if the patient develops an embolus (let’s say a blood clot because that’s
the most common type) that makes its way through the ASD. So if we take the example of a person with
a deep vein thrombosis, part of that blood clot might break loose and become an embolus
and usually this guy would travel into the right atrium, then right ventricle, then be
pumped off to the lungs and lodge somewhere in the lung. It’s possible that the embolus crosses over
to the left side, in which case it can enter the systemic circuit and potentially lodge
in the brain. This situation is called paradoxical embolism,
paradoxical because the embolus starts off in the right side, but crosses over into the
left side. Sometimes children with ASDs will be monitored
for a little while, since smaller openings may close on their own. In other cases, though, especially for larger
openings, surgery will be performed to prevent complications in the future, which typically
involves plugging or patching the opening. All right, as a quick recap… An atrial septal defect is a congenital heart
defect where the septum between the right and left atrium doesn’t close up all the
way and remains open after birth. This causes oxygenated blood to go into pulmonary
circulation. One complication from this condition is a
paradoxical embolus which crosses from the right to the left side via the ASD.


  1. Very clear video! Now we are up to date again regarding atrial septal defects! Thanks osmosis 👍

  2. I like this channel but I wish you would create playlists to group like videos together. e.g. mental illness/disorders.

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