How to identify ascites on an ultrasound
Learn how to distinguish between free fluid and abdominal structures on an ultrasound image.
Fluid in the abdomen can be easy to spot on ultrasound, but how do you know whether you're looking at the bladder or free fluid swimming around in the peritoneal space? In this video, you'll learn how to identify pockets of fluid appropriate for paracentesis and be able to distinguish them from other structures in the abdomen.
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[00:00:00] Throughout this lesson, we will be looking for water but maybe not in a way that you've done before. As you know, fluid on ultrasound appears anechoic or black. Ascites is no different. Here, is the bladder to the left and ascites to the right, here. It can be really hard to tell the difference. One easy way to remember is that an ascites and other free fluid in the abdomen, are not bound by specific borders.
[00:00:30] This means the fluid will have sharp edges and appear more like a crescent than round. This is because the fluid is surrounding other structures and filling crevices. Unlike fluid seen in vessels or the bladder, that have contained borders, ascites fills potential spaces in the abdomen and pelvis. Based on how fluid travels in the abdomen, the most likely place to find fluid is in the right upper quadrant and pelvis. Fluid ascends via the right paracolic gutter to more Morison's pouch and the right subphrenic space. The phrenicocolic ligament
[00:01:00] restricts flow going up and to the left. I like to think of the free fluid in the abdomen like puddles, it tends to collect in the divets, in low spots. But first, we should review the conventional views of seeing free fluid in the abdomen. Here, is where you'll typically apply the probe to get a coronal view of the right upper quadrant, looking for fluid. It's the midaxillary line in the 7th to 9th intercostal space. Just as a reminder, this is what the normal right upper quadrant looks like. Here, you can see the liver and the kidney. And here, is
[00:01:30] what the same anatomy looks like on ultrasound. The probe is pointing up towards the head, so the left side of the image is what is closest to the head, while what is on the right side of the image is closest to the patient's feet. You can see the liver, kidney, and diaphragm here. Train your eye to look carefully at the interface between the liver and the kidney, right here. It is a potential space called Morison's pouch. And this is what ascites looks like on ultrasound, on this view. You can see the black area on the ultrasound, free-flowing,
[00:02:00] not contained by any borders. You can see it filling the potential space between the liver and the kidney. Here, is where you would typically apply the probe to get a view of the left upper quadrant. It is a little more posterior and a little bit superior compared to the placement for the right upper quadrant. This is because the spleen is smaller than the liver and tucked up more into the rib cage. And this is a reminder of what the normal left upper quadrant looks like, with the relationship of the spleen and kidney. Here, is what a normal left
[00:02:30] upper quadrant appears like on ultrasound. You can see the spleen, kidney, and diaphragm. Because the spleen is smaller, fluid can surround the entire spleen. You can see it either between the spleen and the kidney or above the spleen, adjacent to the diaphragm. Here, is a left upper quadrant with fluid. Again, the fluid appears free-flowing and not contained by any hyperechoic border. It has sharp edges as it fills the potential space,
[00:03:00] between the kidney and the spleen. On to the pelvis. There are two main ways to look at the pelvis. In sagittal and transverse planes. Here, is what the pelvis appears like in transverse. Our probe would go to the patient's right side, in a horizontal orientation. Remember, that the pelvis is kind of like a bowl, so you'll likely need to tilt the probe face inferiorly with the tail of the probe pointing more towards the patient's head.
[00:03:30] Sometimes, you'll need to use the suprapubic bone as an inferior landmark. Here, is what a normal transverse bladder looks like. Typically, kind of like a rectangle. The indicator is towards the patient's right, so what is closest to the dot is on the patient's right side. On the other side of the screen correlates with the patient's left. This slide illustrates the cross-sectional anatomy, in the body, when the probe is placed in a sagittal plane, in the pelvis. And here, is the expected anatomy in sagittal plane for a male and female pelvis.
[00:04:00] Here, you can appreciate the different structures that can be seen on ultrasound in the male and female pelvis. Most notably the uterus in the female pelvis. Here's, what a normal female sagittal pelvis looks like. Here, you can see the uterus to the left side of the bladder. Remember, because the probe indicator is pointing towards the patient's head, the structures closest to the dot will be more cephalad, thus, this uterus is anteroflexed, with the bladder located more towards the feet. You are looking for fluid
[00:04:30] between the uterus and the bladder, in this interface here. Here, you can see free fluid in the female pelvis, black, filling up the potential space, without any sharp borders. Here is the bladder, and here is the fluid. You can see the uterus, for a second, in this clip right there, as the solid structure. But again, your eyes should train itself to look posterior to the bladder
[00:05:00] for free fluid. Sometimes, you can see fluid posterior to the uterus. This can sometimes be physiologic. However, if you ever see fluid between the uterus and the bladder, like this clip, you should be concerned that there is something pathological occurring. Moving on to the male pelvis. This is what a typical normal sagittal bladder looks like in the male pelvis. Typically, like a rounded triangle. And now, here is what free fluid looks like in the sagittal male pelvis. Fluid tends to collect either
[00:05:30] posterior behind or superior to the bladder. Great. You now have a good understanding of what fluid looks like in the abdomen and how the important structures of the abdomen look on ultrasound. This is important information to know when performing an ultrasound-guided paracentesis.