Point-of-care ultrasound (POCUS) is a focused ultrasound (US) examination performed and interpreted by the clinician at the patient’s bedside to answer a specific clinical question or guide an invasive procedure. Because the clinician can correlate findings with a patient’s signs and symptoms, POCUS can provide valuable diagnostic information to narrow the differential and guide management in real time [1, 2]. Integration of POCUS into clinical practice has been shown to increase diagnostic accuracy and expedite time to diagnosis and treatment, as well as reduce costs and length of stay and improve patient safety and satisfaction [3–8].

POCUS examinations are focused and operator-dependent; therefore, they tend to provide limited information. As such, POCUS does not replace comprehensive radiologist-performed diagnostic studies; CT is overall superior in evaluating patients with an acute abdomen (i.e., sudden onset of severe abdominal pain) [1, 8]. However, because POCUS is portable, noninvasive, radiation-free, and easily repeatable, it is especially valuable in high-acuity settings, where expedient answers and patient stability are a consideration. POCUS may not be the best definitive test, but it is an excellent initial diagnostic tool that can aid in critical decision-making [9]. The scope of POCUS has expanded significantly in recent decades to include a myriad of applications. Among these are targeted evaluations of acute intraabdominal pathology, where rapid recognition and prompt management are key. This includes hemoperitoneum, ectopic pregnancy, abdominal aortic aneurysm (AAA), obstructive uropathy, appendicitis, bowel obstruction, and pneumoperitoneum [9, 10]. This InPractice article provides an overview of the scope and utility of POCUS for the acute abdomen, focusing on commonly used applications in clinical practice.

Free Fluid

Intraperitoneal free fluid is the pathologic accumulation of fluid within the peritoneal cavity. This can result from various disease processes, including traumatic hemorrhage, ruptured ectopic pregnancy, AAA, bowel perforation or obstruction, and ascites. Although many of these pathologies usually warrant additional diagnostic imaging, POCUS is a valuable initial test that can quickly detect free fluid, which can influence workup and management [11].

Free Fluid in Trauma Setting

Evaluating intraperitoneal free fluid in the setting of trauma is among the most well-established uses of POCUS. First described in Europe in the 1970s and adopted in the United States by the 1990s, this examination is now known as “focused assessment with sonography in trauma” (FAST) [12]. The FAST examination includes a quick survey of key areas in the intraperitoneal cavity for free fluid, a sign of hemorrhage, and an indirect indication of organ injury; cardiac and thoracic components are also included. The abdominal component of FAST evaluates the right and left upper quadrants, focusing on the perihepatic and perisplenic spaces, respectively, along with the pelvis [11, 12] (Fig. 1).

The sensitivity and specificity of FAST for detecting intraperitoneal free fluid is 64–98% and 86–100%, respectively [13]. Though not perfect, FAST has greater accuracy compared with physical examination, laboratory tests, and radiography to detect intraabdominal injury. In hemodynamically unstable patients, the diagnostic accuracy increases significantly [14]. Because of its utility, the FAST examination has become the initial screening modality (replacing the previous standard of diagnostic peritoneal lavage) at most trauma centers nationwide and is included in the Advanced Trauma Life Support protocol [15]. A FAST examination is primarily indicated in the setting of blunt trauma but can help triage and prioritize further diagnostic testing and management in cases of penetrating abdominal trauma. A positive FAST—that is, the presence of free fluid—suggests an intraabdominal injury, whereas a negative FAST alone does not obviate additional testing for intraabdominal injury. This is because a FAST examination cannot reliably rule out injuries to solid or hollow organs [13]. 

The amount of free fluid detected at a specific point in time depends on the rate of accumulation, location, and the patient’s position [9]. Free fluid gravitates to the most dependent area, which is the right upper quadrant (RUQ) in a supine patient [16]. Within the RUQ, the hepatorenal recess (also known as Morison pouch) is a common area of interest; however, the caudal tip of the liver is where fluid tends to collect first. In the left upper quadrant, attention should be directed to the perisplenic area, particularly in the subdiaphragmatic space. In the pelvis, fluid tends to collect posterior to the uterus, known as the pouch of Douglas, in females and in the rectovesicular space or lateral to the bladder in males [17] (Fig. 2).

Despite its recognized utility, the FAST examination has several limitations. In addition to inability to exclude organ injury, it can also be limited by other factors, including operator experience, body habitus, and bowel gas. Free fluid can have varying appearances depending on the type and composition of fluid, and it can change over time. For example, blood is initially anechoic or black, but blood becomes more echogenic as it clots, which makes it difficult to identify and distinguish free fluid from surrounding organs, fat, or other structures. US cannot differentiate the type of fluid (i.e., blood, urine, ascites) and is unable to detect retroperitoneal hemorrhage [11, 12].

Free Fluid in Nontrauma Setting

The FAST examination is highly valuable when evaluating free fluid from nontraumatic hemoperitoneum (such as from ruptured ectopic pregnancy), bowel perforation or obstruction, ascites, and undifferentiated hypotension [11]. When evaluating or managing a patient with ascites, POCUS allows for quantification and distribution of the fluid and can provide real-time procedural guidance for paracentesis at the bedside, which improves success and reduces complications [3]. The other topics are covered separately in the subsequent sections.

Ectopic Pregnancy

Ectopic pregnancy is the leading cause of maternal mortality in the first trimester, and prompt recognition is key [18, 19]. US is the primary imaging modality throughout pregnancy, and the main goals of POCUS, especially in the first trimester, are to identify an intrauterine pregnancy (IUP) and evaluate for free fluid. Confirming an IUP essentially excludes ectopic pregnancy, whereas the absence of an IUP should raise concern for an ectopic pregnancy, especially if the patient has concerning signs or symptoms. Though not a goal of POCUS, it is possible to identify an extrauterine gestational sac containing a yolk sac or fetal pole, which is diagnostic of ectopic pregnancy. Additional nonspecific findings that may also be seen include a complex mass, tubal ring, and free fluid, but their absence does not rule out ectopic pregnancy [11].

In the setting of known or suspected ectopic pregnancy, free fluid in the RUQ is highly concerning for rupture, which can be life-threatening. This finding not only predicts the need for operative intervention, but it significantly expedites the time to diagnosis and definitive management [18–20].

Abdominal Aortic Aneurysm

AAA is a relatively common acute abdominal process with high mortality. A ruptured AAA requires rapid diagnosis and prompt surgical intervention. Clinicians often rely on classic signs and symptoms, including severe abdominal or back pain, syncope, hypotension, or a pulsatile abdominal mass, but these have poor sensitivity [21]. CTA is the preferred imaging study for ruptured AAA, and US is a well-established and validated screening modality [22].

POCUS has excellent ability to detect AAA, which is defined by a diameter exceeding 3 cm when measured from outer wall to outer wall (Fig. 3).

A systematic review and meta-analysis found POCUS to have a sensitivity of 99% and specificity of 98%, when performed by emergency medicine physicians [23]. Although POCUS can accurately determine the presence or absence of AAA, the ability to detect signs of rupture is poor, which is largely due to the limited ability to visualize the retroperitoneum. Findings that indicate rupture include deformation of aneurysmal shape, heterogeneity or focal discontinuity of the intraluminal thrombus, focal disruption of the outer wall, hypoechoic areas in the paraaortic region, and hemoperitoneum [24].

POCUS is a great initial imaging option for suspected AAA, especially in the emergency department (ED), but adequate visualization may be limited by body habitus or bowel gas. The latter can be mitigated by graded compression, whereby slow, sustained pressure is applied to the abdomen to displace loops of bowel (and associated intraluminal gas) to allow better visualization of underlying structures. Like the FAST examination, AAA assessment is one of the core POCUS applications and is a required part of residency training for nonradiology specialties [10].

Cholelithiasis and Cholecystitis

Cholelithiasis and cholecystitis are common biliary pathologies characterized by gallstones in the gallbladder and inflammation of the gallbladder, respectively. US is the reference standard for diagnosis. Biliary POCUS is well established for identifying cholelithiasis or cholecystitis [10].

Sonographically, gallstones typically appear as a hyperechoic structure with distinct posterior shadowing. Imaging findings of cholecystitis include thickened gallbladder wall, pericholecystic fluid, and sonographic Murphy sign, in addition to sludge or gallstones in most cases [11] (Fig. 4).

Like other abdominal POCUS examinations, body habitus and bowel gas can be limiting. For the latter, having the patient sit upright or move into a left lateral decubitus position can often help move the bowel loops away from the gallbladder to improve visualization. Additional pitfalls include misidentification of anatomy or mistaking normal physiologic changes with pathologic findings, such as a contracted gallbladder [11, 25].

Despite these limitations, emergency physician–performed POCUS for cholelithiasis and cholecystitis has sensitivity and specificity comparable to that of radiologist-performed US [25, 26]. Biliary POCUS alone has been shown to reliably inform surgical decision-making and can reduce length of stay in the ED [27, 28]; however, routine adoption in clinical practice has been limited. Although POCUS does not replace comprehensive imaging, it is a safe, efficient, and reliable diagnostic option for cholelithiasis and cholecystitis.

Obstructive Uropathy

Obstructive uropathy can result from intrinsic or extrinsic obstruction of the urinary tract system and can be unilateral or bilateral, depending on the etiology. Renal colic commonly results from nephrolithiasis, which can lead to hydronephrosis when obstruction occurs. CT is the preferred imaging study for suspected nephrolithiasis, but POCUS is appropriate for initial imaging examination [11].

The focus of renal POCUS is to detect signs of obstruction (i.e., hydronephrosis and urinary retention) rather than identifying the stone, itself. POCUS is less sensitive, but more specific for nephrolithiasis compared with CT and has test characteristics comparable to those of traditional US. Use of POCUS is associated with shorter length of stay in the ED [29]. POCUS is highly specific for nephrolithiasis with moderate hydronephrosis, but less accurate in cases of mild or no hydronephrosis [30] (Fig. 5).

Advanced signs of obstruction are associated with larger stones, which often require surgical intervention, rather than conservative management [31]. For younger patients in whom uncomplicated renal colic is suspected, POCUS is a favorable initial imaging modality and perhaps the only imaging study needed. In patients with less typical signs and symptoms, POCUS can influence clinical suspicion and help inform the need for further imaging [32].

Appendicitis

Appendicitis is the most common surgical emergency worldwide and can be complicated by perforation, which occurs in approximately 20% of cases [33]. Along with clinical and laboratory findings, multiple imaging studies play a role in diagnosing appendicitis. CT may be the most accurate, but radiologist-performed US is well established and especially favorable in children in whom radiation exposure is a concern [34, 35]. POCUS has emerged as a promising diagnostic tool with relatively high sensitivity and specificity for appendicitis [34].

Characteristic sonographic findings include target sign in the short axis, blind-ended pouch in the long axis, lack of compressibility, diameter greater than 6 mm, wall thickness greater than 3 mm, appendicolith, and hypervascularity. Indirect findings suggestive of appendicitis include periappendiceal free fluid or abscess, hyperechoic mesenteric fat, enlarged mesenteric lymph nodes, increased peritoneal thickness, and signs of small bowel obstruction (SBO) [35] (Fig. 6).

Benefits of POCUS examination include lack of radiation exposure, lower costs, and ability to help prioritize radiology studies or expedite surgical consult. It may be particularly useful in centers where radiologist-performed US is not continually available. Visualizing the appendix is often limited by body habitus, pain, retrocecal location, and operator skill and experience. In addition, because a normal appendix is often difficult to visualize, this may be a more challenging POCUS study to learn [9, 34]. POCUS is a promising adjunct diagnostic tool, but it has not been extensively studied as a stand-alone test for appendicitis [34].

Bowel Obstruction

Bowel obstruction is a common acute abdominal process that needs timely diagnosis and management to avoid complications, such as ischemia, perforation, and necrosis. Obstruction is defined by impaired flow of bowel contents with varying degrees, ranging from partial to complete obstruction [36]. CT is commonly used to diagnose SBO but can be time-consuming, expensive, and involve radiation exposure. Radiography is often used as an initial imaging study, but sensitivity and specificity are poor and outperformed by POCUS [37]. A systematic review and meta-analysis found POCUS to have a high sensitivity and specificity for diagnosing SBO, rivaling that of CT, with the added benefit of saving time and potentially radiation [38]. The high diagnostic accuracy of POCUS is primarily for complete obstructions; POCUS is less reliable for partial obstruction [39]. Like other POCUS applications, evaluating SBO is highly operator-dependent. Although it may be easy to learn and demonstrate competency, US fellowship training is associated with significantly increased diagnostic accuracy [37–40].

Sonographic findings of SBO include dilated bowel with a diameter of greater than 2.5 cm, fluid-filled loops of bowel, decreased peristaltic movement, increased wall thickness, prominent valvulae conniventes, and a collapsed colonic lumen [40, 41] (Fig. 7).

Free fluid in the peritoneal cavity is associated with higher-grade obstruction and predicts need for operative intervention [42]. A transition point is often difficult to visualize, but POCUS can locate and identify the potential cause of obstruction, such as hernia, intussusception, masses, and signs of ischemia [43]. Evaluating SBO is not a core application of POCUS; it is a suitable modality for initial imaging evaluation and early management of SBO.

Pneumoperitoneum

Pneumoperitoneum is free air in the peritoneal cavity [44]. Free air suggests perforation of a hollow viscus organ, which can result from weakening of the bowel wall, ischemia, foreign body, bowel obstruction, or infection and has high morbidity and mortality [45]. CT is the imaging study of choice. Radiography is often used as an initial diagnostic study for pneumoperitoneum, but sensitivity is generally poor, especially when the amount of air is small. An upright lateral view has greater sensitivity compared with posterior-anterior or supine views, but this is not always possible, especially in critically ill patients [45, 46]. POCUS has proven value as an initial diagnostic test that can expedite recognition and timely management of pneumoperitoneum [44, 47]. US cannot identify the site or extent of perforation like CT, but POCUS has greater sensitivity than radiography with comparable specificity and PPV [48].

On US, free air is evaluated by focusing on the least dependent areas in the peritoneal cavity, where the air tends to migrate. In a supine patient, air moves anteriorly toward the interface between the peritoneal cavity and anterior abdominal wall and is often best identified in the RUQ over the liver [9, 49]. The highly reflective surface of air produces increased echogenicity of the peritoneal line, which is referred to as the enhanced peritoneal stripe sign. Often accompanying this is reverberation artifact, consisting of repeating hyperechoic horizontal lines directly below the enhanced peritoneal line. “Dirty” shadowing may be seen, obscuring the underlying organs (Fig. 8A). Because air can move freely within the peritoneal cavity, these findings change with patient position, known as shifting phenomenon [9, 47, 49]. Similarly, the scissors maneuver can help detect free air and visualize its movement. This technique consists of applying pressure with the probe over the liver to displace the free air and associated artifacts, making these findings less prominent. When the compression is released, the free air returns and associated artifacts become visible again [50].

An important potential pitfall is mistaking free air for intraluminal bowel gas. To differentiate between the two, it helps to focus on the hepatic region where bowel gas is minimal and consider position change or compression to help further identify pneumoperitoneum. Free air moves independent of respiration and peristalsis, unlike intraluminal air in the bowel [9, 47].

Gastric contents in the peritoneal cavity may accompany pneumoperitoneum in cases of bowel perforation. On US, this will appear as free fluid with echogenic debris in the dependent areas of the abdomen [9, 47, 49] (Fig. 8).

POCUS is a valuable diagnostic tool that can be applied at the bedside to quickly answer focused clinical questions. With a broad and continually expanding scope, POCUS is used by many specialties and is incorporated in medical education and training. Although POCUS does not replace comprehensive imaging in patients with an acute abdomen, it has value as an initial imaging modality that can rapidly provide key diagnostic information. POCUS can influence clinical suspicion, guide decision-making, and expedite the diagnosis and treatment by enabling providers to correlate clinical and imaging findings with the patient in real time.

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