Point-of-Care Ultrasonography in the Critical Care Setting: Abdominal POCUS

Before discussing specific POCUS studies and pathology, the reader should review the common approaches to evaluating the abdomen with ultrasonography.

Common pitfalls limiting POCUS evaluation can include pain tolerance to the required abdominal pressure by the ultrasonography probe¹ ; artifacts created by food, intraluminal gas, or stool; and lack of adequate ultrasound penetration in those with central obesity. In most cases, imaging can be enhanced with improved pain control, changes in patient positioning, and proper probe selection. A slight but firm increase in downward pressure with the probe will often displace intraluminal gas to either side, eliminating the artifact.

Except where specifically noted, a curvilinear transducer should be used to produce adequate depth of ultrasound penetration and a wide footprint.²  Most abdominal studies will be performed with the patient in the supine position; however, the left lateral decubitus position may improve image acquisition in biliary tract studies. If the patient is able, flexion of the knees will help relax the muscles of the abdominal wall, improving examination quality.¹  The patient and ultrasonography machine should be positioned so as to avoid excessive bending, reaching, or twisting on the part of the examiner.

The most common indication for biliary POCUS is the evaluation of right upper quadrant (RUQ) pain and/or jaundice when there is suspicion of mechanical biliary obstruction or biliary tract stone disease.

The gallbladder is a pear-shaped pouch that lies just under the liver and is connected to a series of ducts. Bile is made in the liver and stored in the gallbladder until it is secreted via these ducts into the gastrointestinal tract. The cystic duct connects the gallbladder with the common hepatic duct, which drains from the liver, becoming the common bile duct (CBD), draining into the duodenum (Figure 1).

The CBD can vary in length, but for adults younger than 50 years, it is generally between 6 and 9 cm with a diameter of less than 7 to 8 mm.^2,3  The diameter of the CBD normally increases by 1 mm for every decade of life after age 50 years. In patients who have previously undergone cholecystectomy, a diameter of less than 10 mm is considered normal.^3,4 

As with any ultrasonography examination, obtaining a thorough history prior to POCUS evaluation is important. A number of conditions, including chronic cholangitis, prior liver transplant, gastrointestinal oncological processes, and cirrhosis, can decrease the sensitivity and specificity of evaluating the CBD for abnormalities. These patients may benefit from additional radiographic evaluation where clinically appropriate, such as computed tomography or magnetic resonance cholangiopancreatography.^2,5 

Point-of-care ultrasonography images of the biliary system are seen best when the patient has been without anything to eat or drink for at least 8 hours so the gallbladder is inactive and intraluminal bowel contents are lessened.²  The patient should be positioned in either the supine or a left lateral decubitus position. Images can be improved by having the patient inhale deeply and hold their breath.^1,5 

Intercostal or subcostal approaches are both adequate for obtaining the longitudinal and transverse planes required.^1,2  However, the subcostal image is suggested to be the examiner’s starting point.

Position the probe at the abdominal midline with the indicator facing the patient’s head. Sweep the probe along the subcostal margin down in the direction of the right flank. Just before or just after the midclavicular region, depending upon the size of the patient’s liver, the gallbladder, hepatic artery, portal vein, and CBD (ie, the portal triad) can be seen in short axis. Fanning of the probe may be necessary to adequately identify the gallbladder.^1,2,5 

The view of the portal triad in Figure 2 is often termed the “Mickey Mouse sign,” with the larger portal vein forming Mickey’s head and the hepatic artery and CBD forming the right and left ear, respectively.^4,6  A normal variant has the hepatic artery and CBD reversed, which may cause confusion. The use of color Doppler ultrasonography is helpful to differentiate the 2, because the CBD will not demonstrate color flow.^4,6,7  Rotating the probe 90° will obtain a longitudinal view of the portal vein and CBD.^1,2,6 

If the examiner is having difficulty finding the gallbladder or CBD in the subcostal view, an alternative method is the intercostal approach. Because of the small space between ribs, a phased array probe should be used for this method. Place the probe approximately 7 cm lateral to the xiphoid process (the “X minus 7” method). The gallbladder should be located just posterior to the liver directly beneath the probe. It may be necessary to rock the probe from side to side to obtain adequate views.⁶ 

The presence of stones within the CBD or the gallbladder itself confirms the diagnosis of cholelithiasis (Figure 3). Stones are hyperechoic but may be too small to easily locate on ultrasonography. In this case, the presence of acoustic shadowing is beneficial because these small stones will cast a long shadow, which enables the examiner to trace the stone back to its point of origin.

The diagnosis of cholecystitis may be initially detected by identifying the presence of the sonographic Murphy’s sign: tenderness elicited by the downward pressure of the ultrasonography probe directly over a visualized gallbladder. Additional findings include the presence of gallstones, thickening of the anterior wall of the gallbladder (>3 mm), and occasionally, distension of the gallbladder itself. Gallbladder dimensions greater than 4 cm transversely and greater than 9 cm longitudinally are suspicious for cholecystitis (Figure 4). Percutaneous drainage of the gallbladder may be performed under ultrasonography guidance; however, the technique for performing this procedure is beyond the scope of this article.

Ascending cholangitis is characterized by a thickening of the CBD walls. The CBD may also appear dilated and may have stones or purulence (in the form of hypoechoic debris) within it. See the Table for tips for abdominal POCUS by area examined.

Documentation of findings should include the indication for the examination, views obtained, impression, and significant findings. When evaluating the gallbladder and CBD, specific measurements should be obtained and documented rather than using vague terminology such as “enlarged” or “thickened.”

Patients with RUQ pain or abnormal liver function tests may benefit from a liver ultrasonography examination. There are a number of ultrasonography liver studies available to critical care providers, but only 3 relatively simple and straightforward examinations will be discussed in this article: assessment of hepatomegaly (associated with hepatitis and cirrhosis), identification of fatty liver disease, and identification of a liver abscess.

The liver is a large, multilobar organ located in the RUQ of the abdomen. On ultrasonography, the liver should be hypoechoic, with a similar echogenicity to that of the renal cortex. The contour of the liver should be smooth and round.

Start by placing the transducer along the patient’s midaxillary line at approximately the 10th to 11th intercostal space. The probe marker on the transducer should initially be oriented toward the patient’s head. Depending on the patient’s anatomy, ribs may obscure the view, so a slight rotation and movement of the probe may be necessary to obtain the best view. The liver should be readily visible as a large hypoechogenic structure that dominates the screen.

The long-axis length of the liver should be measured for the evaluation of hepatomegaly (Figure 5). The size of the liver will vary according to patient age, sex, and body size; however, generally, a healthy adult liver should be less than 16 cm long.⁸  General appearance, contour, and echogenicity of the liver should be observed. As noted, the echogenicity of the liver should be relatively homogenous throughout and similar to that of the renal cortex. In cases of fatty liver disease, the liver appears more hyperechoic than the renal cortex or the spleen. In cirrhosis, the echogenicity is more heterogeneous than normal.

Hemangiomas are common benign tumors with a prevalence between 1% and 20%.⁹  These are typically not concerning; however, further evaluation with magnetic resonance cholangiopancreatography may be warranted if the diagnosis is new or the lesions are atypical in appearance. These are normally hyperechoic and well defined but may also be hypoechoic with hyperechoic borders.

Pyogenic liver abscesses are relatively uncommon in the United States, with a hospitalization rate of only 3.6 per 100 000. However, the incidence is increasing at a rate of approximately 4% per year.¹⁰  This is likely due to the increase in risk factors among Americans, which include diabetes, cardiopulmonary disease, chronic renal failure, immunosuppression, history of abdominal surgery, and malignancy. The majority of cases arise in those with preexisting liver disease or trauma, and these abscesses should be considered as a source of sepsis in such patients. These abscesses are typically poorly demarcated, hypoechoic to anechoic structures with variable amounts of hyperechoic debris.¹¹ 

Thorough documentation following a liver ultrasonography examination should include the size and echogenicity of the liver as well as the location, size, and characteristics of any abnormalities such as hemangiomas or abscesses.

Ultrasonography examinations of the renal and urinary system are commonly performed to evaluate oliguria, anuria, or development of an acute kidney injury. The main pathology the examiner is assessing in a POCUS evaluation of the kidney is the presence and degree of hydronephrosis. Renal calculi or other obstructions of the kidney, ureter, bladder, or urethra may also be observed. By using color or power Doppler ultrasonography, the patency and adequacy of renal vasculature may also be assessed, but this topic is not discussed in this article.

The renal system is composed of the kidneys, ureters, urinary bladder, and urethra (Figure 6). The kidneys are located in the retroperitoneal space bilaterally, around the level of T12 to L3. The right kidney is slightly inferior and anterior as compared with the left, because of the large size of the right lobe of the liver.

The outer layer of the kidney is called the renal cortex; it surrounds the renal medulla and is seen as a number of pyramidal structures (Figure 7). From these medullary pyramids, the minor and major renal calyces drain into the renal pelvis and then into the ureter. The ureters then connect the kidney to the urinary bladder.

The bladder is unique in that its shape and size not only vary from person to person but may change depending on its urinary volume. This presents a challenge to the examiner because it is impossible to have a single image in mind of what the bladder should look like. The ureters enter the bladder at the trigone, which is located at the base of the bladder, near the urethra. From the midpoint of the lower bladder, the urethra drains downward toward the urinary meatus, where urine is expelled from the body (Figure 8).

Surrounding the renal anatomy are a number of other important structures. As previously mentioned, the liver abuts the right kidney and can serve as an excellent sonographic landmark. On the other side of the abdomen, the spleen serves a similar purpose when imaging the left kidney. The spleen appears similar in echogenicity to the liver; however, it is much smaller and shaped more like a slightly elongated sphere.

In the pelvis surrounding the bladder are the rectum and pubic bone. The uterus lies adjacent to the bladder in female patients. In male patients, the prostate may often be visualized because it lies inferior to the bladder, surrounding the urethra.

When scanning the renal system, 3 separate structures are actually examined: the right kidney, the left kidney, and the bladder. A thorough examination requires imaging of all 3 structures, although there may be certain clinical indications for a limited examination. This limited examination is common when evaluating the bladder for the purposes of troubleshooting a urinary catheter or stent. In general, it is good practice to perform a complete evaluation when examining renal pathology. For POCUS studies of the renal/urinary system, the examiner should use the curvilinear transducer. All 3 structures should be scanned in both the longitudinal and transverse planes. The examiner may need to position themselves in different stances depending on the amount of space available at the patient’s bedside.

When scanning the right kidney, the examiner should start by finding the liver as described above. Sliding the probe caudally will help center the kidney in the screen. The major anatomic features described above should be visible (Figure 7). The overall size of the kidney should be assessed. A normal kidney in this view is 10 to 11 cm in length. Fan the probe back and forth slowly to evaluate the whole kidney. Additionally, the examiner may need to rock the probe side to side to ensure both renal poles are visible and no structural abnormalities are missed.

In addition to the overall size of the kidney, the size of the renal calyx should be assessed. This is important in the diagnosis and grading of hydronephrosis. Hydronephrosis is an accumulation of fluid in the kidney, which is caused by a backup of urine. There are several causes of hydronephrosis, and clinical correlation is needed to determine the exact cause. Figure 9 shows the progression of hydronephrosis from mild to moderate to severe using a I to IV grading scale. When the examiner is evaluating a patient for hydronephrosis, it may help to remember the blockage is distal to the kidney and, as urine backs up, the distal structures will become enlarged first.

After evaluating the right-side kidney in the longitudinal view, rotate the transducer 90° so the probe marker is now pointed posteriorly. The examiner should aim to keep the kidney centered in the screen as it is rotated because this helps prevent losing the kidney and having to locate it again once the rotation is complete. At this point, the examiner will have the transverse view of the kidney. As in the longitudinal view, the examiner should assess the size of the kidney and the structures of the cortex, medulla, and calyces.

The process for examining the left-side kidney is similar to that of the right. The examiner should recall the left kidney is more superior and posterior than the right because of the relatively smaller size of the spleen compared with the liver, and so the scan should begin between the 8 th and 10th intercostal space on the left. Additionally, rather than beginning at the midaxillary line, the transducer is best placed at the posterior axillary line. This is sometimes referred to as the “knuckles to the bed” approach because the examiner’s knuckles should be touching the bed to obtain this view. The examiner may even find it necessary to apply firm downward pressure on the bed to position the probe posterior enough.

This view should look similar to the right kidney in the longitudinal axis, with the smaller spleen taking the place of the liver (Figure 10). The examiner should be able to identify all the same structures in the left kidney as in the right. The examiner should rotate the probe 90° to evaluate the left kidney in the transverse axis. Again, the anatomy should be similar to that of the right kidney.

After thorough evaluation of both kidneys, place the transducer on the midline of the patient’s abdomen, just above the pubic symphysis. The probe indicator should be aligned to the patient’s right side and the probe should angle downward, pointing into the pelvic cavity. This position will obtain the transverse view of the bladder. Recall that the shape of the bladder may vary greatly; however, a full bladder in this view is often seen to be trapezoidal (Figure 11).

An empty bladder may be difficult to visualize. A common complication in the critically ill patient is the presence of a urinary catheter, which continuously drains urine, making the bladder itself difficult to see. The balloon on the tip of the catheter may be readily recognizable on ultrasonography, however, and can be used as a landmark (Figure 12). Rotation of the probe 90° will obtain the longitudinal view of the bladder (Figure 13).

The bladder should be evaluated for distention and/or urinary retention. This can best be assessed by calculating the bladder volume, which is a more accurate measurement than that obtained with commercially available bladder scanners, particularly in patients with ascites. In the transverse view, measure the width and depth of the bladder in centimeters, then measure the height in the longitudinal view (Figure 14). The volume may then be calculated by using the following formula: volume = height × depth × width × 0.52.¹²  The 0.52 is a coefficient that varies depending on the exact shape of the bladder. However, determining the precise shape of the bladder can be difficult for the casual examiner. For most purposes, a precise calculation of volume is not important, so 0.52 may be used universally and will provide a volume that is sufficiently accurate. Volume before and after voiding may be compared to determine if there is adequate bladder emptying.

In the transverse view, presence of ureteral jets should be assessed. This is done by placing a color Doppler window over the posterior wall of the bladder and observing for flashes of color indicating the injection of urine into the bladder (Figure 15). This occurs at regular intervals, but may take up to 10 minutes to observe. Unilateral absence of the ureteral jet typically indicates obstruction of the ureter, such as from renal calculi. Bilateral absence may indicate anuria from another cause.

When documenting an ultrasonography evaluation of the renal system, the examiner should be sure to include the grading of hydronephrosis, if it is present. This documentation should include the location, size, and character of any unusual structures, such as cysts, and a comparison of the right and left kidneys. It should also include the volume of retained urine, including postvoid residual, as well as the presence or absence of bilateral ureteral jets.

One of the most useful applications of abdominal POCUS in the ICU is for the detection of intra-abdominal free fluid. This is an important evaluation in the patient with abdominal tenderness, increasing abdominal girth, and hemodynamic instability. Common causes of abdominal free fluid are ascites and blood.

There are a number of common locations in the abdomen for the accumulation of fluid. The hepatorenal space, also known as the Morison pouch or Morison space, is a potential space located between the inferior border of the liver and the right kidney. Because it is one of the most dependent parts of the abdomen, it is the most common location for the accumulation of free fluid (Figure 16).

In the left upper quadrant (LUQ), fluid may accumulate in the splenorenal space (Figure 17). Similar to the Morison pouch, the splenorenal space is a potential space located between the inferior aspect of the spleen and the left kidney. Also, in the LUQ, the subdiaphragmatic space superior to the spleen may accumulate fluid.

In the lower abdomen and pelvis, the rectovesical pouch in male patients and the rectouterine pouch in female patients are common places for fluid to accumulate. The rectovesical pouch is located posterior to the bladder and prostate, between these structures and the rectum. The rectouterine pouch is located between the uterus and rectum in female patients, again, posterior to the bladder.

Assessing the presence of free fluid in the abdomen involves using the scanning techniques previously discussed. To evaluate the Morison pouch, the examiner should use the techniques described above for the evaluation of the liver and right kidney. Under normal conditions, the Morison space should contain no fluid and, therefore, exist only as the border where the liver meets the kidney. Free fluid will appear as an anechoic substance in this space and may extend superior to the tail of the liver (Figure 16).

Similarly, the assessment of the LUQ involves using techniques described previously for examining the left kidney. Although fluid commonly accumulates in the splenorenal space (Figure 17), the examiner should take care, because of the increased risk of fluid accumulating between the diaphragm and the spleen, to slide the probe superior until the entire spleen and diaphragm are visualized. Pelvic fluid, which may be located in the rectovesical pouch and rectouterine pouch, can be assessed using the techniques described above for evaluation of the bladder in both the longitudinal and transverse planes.

Retroperitoneal hemorrhage or hematomas may occur in patients who are anticoagulated or receiving antiplatelet therapy and who undergo procedures involving puncture of the femoral vessels, such as angiography or left-sided heart catheterization. When scanning the right lower quadrant (RLQ) and left lower quadrant, sliding the probe more posterior and lateral may help visualize the retroperitoneal space and potentially identify retroperitoneal hemorrhage or hematoma.¹³ 

Finally, in patients with liver disease, ascites tends to accumulate in dependent portions of the abdomen and may be easily observed by scanning the RUQ and LUQ to either side of the midline. Paracentesis may be performed with real-time guidance or by using ultrasonography to identify a pocket of ascites and marking the skin, then performing the paracentesis without direct visualization.

Documentation should include characteristics of the fluid noted, including echogenicity, location, and general amount. If using POCUS for paracentesis, note whether the procedure was performed under real-time guidance or simply to identify a pocket of ascites before the procedure.

Appendicitis is rarely diagnosed in the ICU, but it is a potential and often overlooked source of sepsis, so its diagnosis via POCUS bears mentioning. It is one of the most common reasons for the onset of acute abdominal pain and why patients present to the emergency department.¹⁴  Despite these facts, it is often misdiagnosed and a common cause of peritonitis. Diagnosing appendicitis using POCUS can be challenging because the organ is often difficult to locate, making sensitivity somewhat low at 84%. However, ultrasonography is highly specific for appendicitis (96%) and so its use in evaluating the unstable septic patient in the ICU may be beneficial.¹⁵ 

The appendix is a blind-end pouch located in the RLQ of the abdomen. It is attached to the cecum; however, its position can be highly variable, including lying retrocecally. This unique positioning makes locating the appendix challenging and accounts for the relatively low sensitivity of ultrasonography for the diagnosis of appendicitis.

The linear probe may be used for children and thinner adults, but for larger patients, the curvilinear probe will be necessary to give adequate depth for locating the appendix. If the patient is able to communicate the location of maximal tenderness, the examiner should begin scanning where the patient indicates. Many patients in the critical care arena are unable to verbalize pain so the examiner may need to use a systematic approach to scanning the RLQ. Pain management could be necessary when performing this examination for both verbal and nonverbal patients to optimize ultrasonography findings.

With the probe in the transverse position, the examiner should sweep systematically across the RLQ, locating the ascending colon and tracing it downward toward the cecum. Once the psoas muscle and the iliac artery have been identified, the appendix is likely to lie just medial to these landmarks.

A normal appendix should be compressible, with a diameter of less than 6 mm and a wall thickness of less than 3 mm (Figure 18). Noncompressibility of the appendix, enlargement greater than 6 mm in diameter, and increased wall thickness are all signs of appendicitis. Sometimes, an appendicolith may be noted as a hyperechoic structure casting a shadow (Figure 19). If the appendix has ruptured, it may be compressible; however, the examiner may also note free fluid in the periappendiceal area and asymmetric thickening of the appendix wall.

Documentation should include a description of the appendix, including compressibility, diameter, wall thickness, presence or absence of appendicolith, and presence or absence of periappendiceal free fluid. Because of the relatively poor sensitivity of ultrasonography in diagnosing appendicitis, a follow-up computed tomography scan should be obtained if it remains high on the differential diagnosis list and the appendix is unable to be well visualized with ultrasonography.

The use of abdominal POCUS can facilitate the diagnosis and treatment of patients in the ICU. Most abdominal POCUS evaluations may now be performed as a screening examination by POCUS-trained critical care providers and complemented or confirmed by an ultrasonographer in a radiology department. Additionally, the field of abdominal POCUS continues to expand and techniques are continually being developed to replace traditional imaging modalities. The authors fully expect to see the development and proliferation of an increasing number of ultrasonography techniques in the near future.

Special thanks to Aaron Inouye, PA-C, Leon Chen, ACNP, and Abhilash Koratala, MD, for supplying some of the images in this article.