As part of the focused assessment with sonography for trauma (FAST) protocol, PoCUS is used to detect abdominal free fluid to identify intra-abdominal bleeding. A similar PoCUS exam can identify ascites, chronic liver disease, portal hypertension, and bowel obstruction in patients with abdominal distention. These PoCUS findings are best detected using the machine’s abdominal mode and a curvilinear transducer. Although not optimal, a phased-array transducer with abdominal mode can also work. Abdominal views should always be labeled with location and transducer orientation, using detailed descriptions or simple acronyms like “RUQ long” for a longitudinal view of the right upper quadrant or “P trans” for a transverse view of the pelvis. Consider using a pictogram of the abdomen to depict the transducer's location and orientation. To evaluate for free fluid, chronic liver disease, portal hypertension, and distended fluid-filled bowel requires images from the right epigastrium, right upper quadrant (RUQ), left upper quadrant (LUQ), and pelvis (P). Overlying gaseous bowel or thick fatty tissue often impact image quality. Turning the patient to their side and applying slow, gradual pressure with the transducer can help displace these tissues and improve image quality. A full bladder is often a necessary acoustic window for imaging the pelvis.
Image 10.1 Common abdominal views include the right upper quadrant longitudinal view (A), left upper quadrant longitudinal view (B), pelvis longitudinal view (C), and pelvis transverse view (D).
Abdominal free-fluid accumulation, often termed ascites, may present with shortness of breath, early satiety, edema, abdominal distension, an elicitable fluid wave, and dullness to percussion in the flanks that shifts with turning. However, these findings are not sensitive and often only present after accumulating a sufficient fluid volume. Various factors, usually in combination, can cause fluid to accumulate in the peritoneal space. These causes include the following.
Abdominal free fluid appears on ultrasound as anechoic (dark) gaps between structures. Differentiate free fluid from a fluid collection, such as a cyst, where fluid is localized and walled off from surrounding free space. The anechoic areas may be obvious or subtle, and the size may be considered large, moderate, small, or trace. Like fluid collections, free fluid may be described as simple (uniformly anechoic) or complex (nonhomogeneous with echogenicities) due to blood clots, fibrous strands, septations, or floating debris.
Ultrasound can reliably detect less than 200mL of free fluid in the pelvis (the most dependent area) but not reliably less than 600mL when examining Morison's pouch (potential space between the liver and the right kidney).1,2 The phrenicocolic ligament prevents the direct flow of fluid between the pelvis and the left upper quadrant. Thus, perisplenic fluid may not be initially detected in the pelvis, and vice versa, as it has to pass through the right side to get there. Therefore, carefully examine the right and left upper quadrants and the pelvis to maximize free-fluid detection. Consider fluid present if detected in any of these areas, but absent only if undetected in all three areas. Consider the exam indeterminate if one area is not visible and free fluid isn’t seen elsewhere. A full bladder provides a good window into the pelvis. However, consider reducing the gain to compensate for bladder-related acoustic enhancement, which makes standard gain settings excessive and prevents fluid detection in the deep pelvis. The LUQ is usually the most challenging view, as the relatively small spleen provides the sonographic window and is located more posteriorly and superiorly than right-sided views.
Image 10.2 Abdominal views showing moderate and large free fluid accumulation, respectively. The right upper quadrant longitudinal view (A, B), the left upper quadrant longitudinal view (B, C), and the pelvis longitudinal view (C, D).
Image 10.3 Abdominal views may reveal hepatic veins (A) or fluid collections, such as a renal cyst (B), that can be confused with free fluid.
Image 10.4 Abdominal views may reveal complex free fluid, such as echogenic purulent fluid associated with a septic abortion (A) or ascites with extensive septations due to abdominal tuberculosis (B).
Despite a thorough exam, PoCUS to detect abdominal free fluid after trauma may not detect 1) minor or early injuries, 2) fluid constrained by intra-abdominal adhesions, 3) retroperitoneal hemorrhage, or 4) other intra-abdominal injuries. As a result, this exam is not sensitive enough to rule out significant injuries and is not a replacement for CT imaging in trauma patients. Use color Doppler mode to distinguish free fluid from vasculature if necessary. Also, avoid confusing the seminal vesicles (in males) or the psoas muscle with free fluid in the pelvis. Positive free fluid with hemodynamic instability after trauma is specific for intra-abdominal injury, and the patient should receive an urgent surgical evaluation.
PoCUS can identify signs of liver disease and portal hypertension, which can help determine the cause of ascites. Still, newly diagnosed ascites should be sampled and analyzed for at least appearance, cell counts, and culture, especially if an infection is possible. Depending on the context, total protein, albumin, LDH, glucose, amylase, creatinine, and cytology may also be helpful, depending on the context. Ultrasound visualization of the needle insertion site before paracentesis can significantly reduce the risk of a ‘dry tap’ or bleeding complications.3,4 To do this, first, find a fluid pocket in a dependent location, such as inferior to the LUQ or RUQ views, with sufficient fluid depth regardless of slight variations in needle trajectory. Mark this spot and note the maximum depth before hitting deeper tissue and any debris or loculations that may impair drainage. Then, change to a high-frequency linear transducer to visualize the abdominal wall in detail. If no linear probe is available, reduce the depth to the lowest possible to see the entire abdominal wall. Once the detailed view is optimal, enter into color Doppler mode and move the color box around to ensure that this location is not over or adjacent to any detectable blood vessels. This will prevent accidental injury to the inferior epigastric, superficial epigastric, or superficial circumflex iliac vessels, which may vary in location. When inserting a guidewire to perform the Seldinger technique, use ultrasound to confirm the wire is correctly positioned before inserting the dilator.
Image 10.5 An ideal location for paracentesis is a dependent region with more than sufficient fluid depth (A). Avoid injuring underlying vessels by visualizing any potential location with a linear transducer before performing the procedure (B).
The liver is normally of medium echogenicity (slightly more echogenic than normal kidneys) with a smooth capsular surface and visible branches of blood vessels. The portal vessels, which bring blood into the liver from the portal vein and hepatic artery, typically have a thin hyperechoic outline that can help distinguish them from the hepatic veins, which drain the liver into IVC and lack a border or outline. The portal vein can be imaged with an oblique view in the epigastrium as it runs left to right just above the IVC after the superior mesentery vein and splenic vein join together.
Image 10.6 An oblique epigastric view shows the portal vein as it passes over the IVC (A). This part of the portal vein can be measured at its largest width (B). A measurement >1.3cm is associated with portal hypertension (C). Periportal fibrosis (D, E) is a specific (not sensitive) sign of chronic schistosomiasis, which is a leading cause of portal hypertension in endemic areas. Reduced flow and inflammation can lead to portal vein thrombosis (F), which worsens portal hypertension and liver perfusion.
Except in regions where schistosomiasis is endemic, portal hypertension is usually due to cirrhosis but can also result from elevated central venous pressure, veno-occlusive disease, or severe splenomegaly. Portal venous congestion leads to ascites, splenomegaly with hypersplenism, and esophageal varices, which are life-threatening when they bleed. PoCUS can identify high portal vein pressure by measuring the portal vein’s diameter at the widest point on its longitudinal axis as it passes over the IVC. Color Doppler showing venous flow can help distinguish the portal vein from a dilated bile duct. The portal vein diameter can increase with age and body weight. A portal vein diameter of 13mm or greater is associated with portal hypertension, especially in younger and thinner populations. However, a smaller diameter does not rule out portal hypertension. In schistosomiasis, adult blood flukes can chronically release eggs into the portal circulation, leading to portal hypertension. This creates a unique sonographic liver pattern called periportal fibrous. PoCUS users in areas where schistosomiasis is endemic should take the time to learn how to identify this pattern. These findings, portal vein dilation and periportal fibrosis, are only helpful if identified, as their absence cannot rule out clinically significant disease.
Image 10.7 Sonography of the liver may reveal various abnormalities, including hepatic steatosis (A) and cirrhosis (B). There may be subtle infiltration with scattered hypoechoic lesions resembling microabscesses of tuberculosis (C). Gallstones may be seen (D). Heterogeneous lesions may be infectious or malignancy and should not be overlooked (E, F, G, H). Lastly, dilated bile ducts may be seen as in this patient with jaundice (I, J)
In steatosis, liver cells accumulate excessive fat stores that increase the tissue’s echogenicity, blur the distinguishing outline of the portal vasculature, and increase attenuation, which impairs the liver’s utility as a sonographic window. Steatosis is often attributed to non-alcoholic fatty disease (NAFLD), which is associated with insulin resistance, gut dysbiosis, and obesity, and affects more than 25% globally.5 Alcohol intake, some forms of chronic hepatitis (hepatitis C, Wilson’s disease, and heritable metabolic diseases), and certain drugs, including glucocorticoids, amiodarone, methotrexate, tamoxifen, some antipsychotics, and certain antiretroviral agents, can also cause or worsen steatosis. In addition, persistent steatosis can progress to cirrhosis, the common end-point of chronic liver disease, including forms that are not associated with steatosis, like Hepatitis B or autoimmune hepatitis. Fortunately, steatosis is reversible if the underlying cause can be addressed and significant fibrosis has not yet occurred.
Liver hyperechogenicity is a reliable, qualitative marker of steatosis and may be described as severe if the portal vessels cannot be distinguished from the hepatic vessels.6 Nonetheless, this is a subjective finding, dependent on technique, and it is not well studied in the PoCUS context. Therefore, it’s prudent for PoCUS users to interpret fatty liver with caution and obtain consultative imaging if attempting to exclude liver disease or the findings would trigger further workup or screening for hepatocellular carcinoma, which can occur after cirrhosis develops.
Traditional ultrasound imaging cannot rule out mild to moderate fibrosis, which requires a biopsy or specific fibrosis biomarker tests and elastography. However, PoCUS can identify severe cirrhosis, which causes the liver to shrink and its smooth surface to become nodular and irregular. Widespread metastatic disease can mimic or coincide with severe cirrhosis and usually causes liver expansion and a more heterogeneous parenchymal pattern. Obtain consultative imaging if these abnormalities are suspected. In settings where other imaging and testing are unavailable or impractical, the liver can be considered cirrhotic when PoCUS reveals a shrunken, nodular, hyperechoic liver in a patient with other findings consistent with cirrhosis.
Regardless of whether liver disease is expected, if PoCUS reveals liver steatosis, review the medication list for potential culprit drugs and evaluate for signs of liver dysfunction and associated disorders like diabetes and hypertension. Always address specific causes and give healthy lifestyle recommendations, including 1) get enough exercise and sleep, 2) reduce intake of alcohol, added sugars, non-calorie sweeteners, room-temperature hard fats, and other processed foods, and 3) increase intake of whole, high-fiber foods.7–9 Also consider testing for and immunizing to prevent viral hepatitis. Sharing or discussing abnormal liver images can help illustrate the importance of the liver and its role in overall health.
PoCUS of the liver may reveal fluid collections, hyperechoic lesions, or wider-spread heterogenous irregularities, representing cysts, focal tissue hypertrophy, abscesses, dilated bile ducts, hemangiomas, or a variety of malignancies including hepatocellular carcinoma which is often associated with cirrhosis. These abnormalities should be evaluated with consultative imaging because diagnosing liver lesions is beyond the scope of most PoCUS users. Still, it’s helpful to understand that a simple cyst that’s asymptomatic, uniform, thin-walled, with no internal echoic structures, and <4 cm is less likely to be clinically significant, while a lesion associated with fever, abdominal pain, or anorexia should not be overlooked as it may represent an abscess or malignancy.
Small bowel obstruction (SBO) may present with abdominal pain, distention, nausea, vomiting, and constipation and may lead to hypovolemia, bowel necrosis, perforation, and peritonitis. The history and physical may be nonspecific, but making the diagnosis can have specific time-sensitive treatment implications. Fortunately, PoCUS can often image distended bowel because fluid usually displaces or replaces gas over time, especially in dependent areas such as the flanks. This PoCUS application is especially helpful when CT scanning is unavailable, impractical, or delayed.
Image 10.8 Sonography can confirm the diagnosis of small bowel obstruction if >10 cm of small bowel is dilated >3cm (A), especially when contents do not appear to move forward after prolonged observation (B).
The bowel is normally soft, compressible, moves with peristalsis, and is difficult to image with ultrasound because it's often empty or filled with gas. SBO is less likely when the small bowel diameter is less than 2.5-3 cm, and there is good single-direction peristalsis. If a significant portion (>10cm) of the small bowel is dilated >2.5-3 cm, then SBO is more likely, especially when contents move back and forth equally. Specificity increases with larger measurements. Borderline measurements should not be used alone to rule in the diagnosis, but only as an extra bit of data. These measurements do not apply to the stomach or large bowel, which can be distinguished from the small bowel by the presence of haustra. Compared to plain radiography, PoCUS for SBO is as sensitive or more, but less specific.10
Image 10.9 Sonography can estimate stomach filling. An empty stomach (A) looks like a unique column of bowel with a hypoechoic wall (A). As the stomach fills, the stomach appears more distended with contents (B). Finding gastric distention in the setting of suspected obstruction signals the need for decompression (C).
If the stomach and small bowel do not appear dilated, gastric decompression is likely unnecessary, and SBO is less likely. However, if a patient with symptomatic abdominal distention has obvious dilated small bowel >3cm, then consider prompt gastric decompression in addition to supportive measures, consultative imaging, and surgical evaluation.