 So, an atrial septal defect is an example of an asianotic heart condition. You can see an example over here on the right side of an atrial septal defect. It's, again, a left-to-right shunt through from the left atrium to the right atrium. And blood will naturally flow in this direction because there's increased pressure in the left atrium compared to the right atrium, so it's flowing down that pressure gradient. And we'll note that this is due to a failure of the interatrial septum to close. And you'll recall from embryology that there's two types of atrial septal defects. There's a succundum type, which is actually the more common type. That's due to incomplete growth of the septum succundum, or too much resorption of the septum premium. And then there's also the premium type of ASD, which is due to the failure of the septum premium to fuse with the endocardial cushions as it grows downwards. And how do we identify these patients? A fixed, widely-split S2 is going to be a very classic symptom of ASD. And let's explain physiologically why this happens. So you'll recall from normal physiology that in a normal person, when you take a deep breath, you have a drop in intratherastic pressure, and then you have, as a result of that, increased venous return through the vena cava into the right atrium. And as a result, you have increased venous return into the right ventricle, and the increased volume of blood being pumped out into the pulmonary artery. And that increased amount of blood will mean that it takes longer to pump out. And as a result, the pulmonic valve will close later than the aorta, and that will cause a splitting of the S2. Now, if we think about the case of an atrial septal defect, it's not only with breathing in that we have increased venous return, now that there is, with every beat, blood flowing down that pressure gradient from the left atrium into the right atrium with every beat. So now every beat, we have an increased amount of blood in the right ventricle and an increased amount of blood flowing out into the pulmonary artery. That means with every beat, now instead of just with breathing in, we have a splitting of the S2. And that's a very important symptom and a very important physiology concept that you should be aware of. Another thing to note, you'll also have a systolic ejection murmur. And why is that? It's because, again, because of this increased amount of blood flowing this way into the right ventricle, there's an increased amount of blood flowing past that pulmonic valve out of the right ventricle, and that causes that systolic ejection murmur best heard in the pulmonic area. And then how do we evaluate these patients? Again, ECHO is going to be the gold standard. Another concept that you should be aware of is thinking about O2 saturations in the different chambers and the setting of heart defect. So again, let's explain some physiology here. We have this nice oxygenated blood that's just come out of the lungs in the left atrium and now it's flowing again down that pressure gradient into the right atrium and mixing with deoxygenated blood. But since we have a little bit of oxygen from the blood coming on the left side, we'll actually note that there's an increased O2 saturation now in the right atrium. And that will remain true as the blood flows into the right ventricle in the pulmonary artery as well. So if you were to measure the O2 concentration in the right atrium or the right ventricle of an ASD patient compared to a normal patient, they would actually have an increased O2 saturation. So that's definitely something to be aware of. And how do we treat these patients? Luckily, most ASDs will resolve spontaneously without treatment. So if you ever get a question about a patient with an ASD asking about treatment, chances are that there's not going to be any treatment that you just need to watch and wait and make sure that the defect closes on its own. But there are some indications for surgery. Like I mentioned earlier, the patient being symptomatic, usually that's good enough to make you think that they might need surgery. But additionally, if they're less than a year old with pulmonary hypertension, that's an indication for surgery. And again, if they're symptomatic, you did the watch and wait approach, but they're now older child with a large defect, that's an indication for surgery as well.