ULTRASOUND EVALUATION OF THE ABDOMEN




Introduction



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The intensivist caring for the critically ill patient may often consider abdominal pathology when assessing for the causes of the patient’s ill health. Patient symptoms, vital signs, and laboratory tests together with physical examination may leave the clinician with a broad differential diagnosis. Imaging is a frequent tool used to refine the differential diagnosis. Plain film of the abdomen can show obstructive bowel gas pattern or free air, mass effect from organomegaly or ascites, but gives limited information about solid organs. Computerized tomography (CT) is a better test for retroperitoneum, bowel, or for solid organ injury, but is neither portable nor obtained as quickly as bedside ultrasound. Magnetic resonance imaging generally is a relatively lengthy examination, is contraindicated in patients with pacemakers or some other implantable devices, and is often not readily available. Focused ultrasound, however, is quickly available and can often help to identify the problem or exclude diagnoses and assist with therapy.



The American Institute of Ultrasound in Medicine (AIUM) lists 13 indications for abdominal ultrasound (Table 20-1). Many of these indications may be the leading cause of a patient’s illness; all can complicate other medical conditions. Causes of pain, for instance, may be assessed by ultrasound. Pain due to acute cholecystitis or ureteral obstruction, peritonitis due to abscess, or vascular occlusions may be detected. The cause of biliary obstruction or explanation for a rapid drop in hematocrit may be identified. Palpable abnormalities can be confirmed. Obstructive uropathy can be evident as a cause of renal insufficiency. Ascitic fluid can be followed by serial examinations. Liver metastases may be the etiology of elevated liver function tests or explain the increased risk for pulmonary embolism. Bladder distension can be assessed in the neurogenic bladder of a patient with spina bifida. A focused examination for free fluid in the trauma patient may affect operative decisions. The transplant organ can be evaluated for early complications. Ultrasound guidance can also decrease the risk associated with vascular and body interventions. For the critically ill, a point-of-care ultrasound examination that combines an evaluation of abdominal organs, vascular structures, and the peritoneal spaces for fluid may focus the physicians’ attention to the most likely causes of a patient’s ill health.




TABLE 20-1AIUM Indications for Abdominal and/or Retroperitoneal Ultrasonography




Transducer Selection and Supplies



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Transducer Selection



Two probes form the basis for evaluation of the abdomen. The low frequency curved 5.2 MHz transducer provides excellent penetration and a large far-field for abdominal organs and peritoneal spaces. A small footprint-phased array transducer (microconvex 4.2 MHz probe) is useful for intercostal spaces, in the obese, and in the area of bandages and other tight areas. A linear probe is useful for identifying the pleura or other small parts like the scrotum that are only a few centimeters or less from the probe. It will not provide the many centimeters of penetration needed to visualize through the liver.



Ultrasound Coupling Agent



Ultrasound gel applied to the probe or patient’s body will provide a clear acoustic window. It may be used in conjunction with a probe cover if contamination is likely. Probe sheaths in varying sizes are commercially available. First, apply gel generously to the footprint of the probe, cover the probe with the sheath, and then apply another layer of gel to the outside of the probe. Whenever sheathing, care should be taken to avoid air bubbles between the probe and sheath that will cause reflective artifact in the image. If they occur, they can be manually milked aside.



Cleaning Transabdominal Transducers



Transducers must be cleaned before use and between patients according to guidelines described for ultrasound transducers. The AIUM recommends that after each use probes should be cleaned with soap and water, quaternary ammonium sprays or wipes as directed by the manufacturer in the operating manual. Heavy contamination with blood or enteric contents may warrant additional cleaning. Probes should be sheathed if contamination is likely. The probe including the cord must still be cleaned after use even if sheathed because of reported leakage rates. Strict adherence to cleaning guidelines must be followed to prevent spread of infection from one patient to another and to ensure equipment longevity.



Base unit



The ideal ICU ultrasound unit should be portable, easy to use, highly reliable, relatively indestructible, and inexpensive. Ultra-small units, although intuitively attractive, particularly in locations with limited space, may be taken from the ICU and then be unavailable when needed. Their image resolution remains limited. Even larger ultrasound units, particularly if easy to use and with good resolution, can be easily removed from the ICU. In our ICU, we resolved this problem by assigning the less expensive equipment to simpler tasks (i.e., vascular access) and leaving the more sophisticated machines for torso imaging. Most focused ICU abdominal ultrasound examinations do not rely on the use of Doppler imaging. The addition of reliable Doppler capability to the ultrasound unit may complicate the issues of size, expense, and require additional training.



Data capture



Portable printers can be added to ICU ultrasound units to capture selected images for documentation purposes. Images and videos can be captured through video cards. Digital information can alternatively be stored through the use of a cloud-based electronic health record or picture archiving system without the need for physical records that can be easily lost or destroyed.




Image Orientation and Anatomic Correlation



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Scanning in a focused ICU abdominal ultrasound assessment is performed in transverse and longitudinal planes. By convention, in a transverse plane, the indicator on the probe is pointing to the patient’s right side and this corresponds to the left side of the display screen just like the convention used with CT scanning. In a longitudinal plane, the indicator on the probe is pointed toward the patient’s head and corresponds to the left side of the display screen with the toes oriented toward the right. In actual practice, off-axis imaging where the probe is oriented in a transverse or longitudinal plane relative to the structure being imaged (and not necessarily lined up with the sagittal sinus or true transverse plane through the patient) is frequently used when locating the common bile duct or evaluating the kidneys in long axis. The patient is typically positioned supine and the sonographer stands at the patient’s right side, scanning with his or her right hand. The room lights should be dimmed to improve the visualization of the screen.




Terminology



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Abdominal structures are described in terms of echotexture. A black or anechoic appearance can represent simple fluid, such as ascites, bile, or urine. A long artifact from a shadow behind a rib or stone can also be anechoic. Blood or pus will be hypoechoic, or darker than the surrounding structures, due to fluid but with cellular material causing low level echoes; sludge in the gallbladder or debris in the urinary bladder can also cause low level echoes. The liver and spleen are generally described as having medium echotexture. The kidneys are normally lower in echotexture than the liver. Fat such as in the retroperitoneum or portal triads will be echogenic, or brighter echotexture than surrounding structures, but will not shadow. Highly sound-absorptive structures, like the ribs, kidney stones or gallstones, will appear brightly echogenic and behind them the sound will be attenuated causing a shadowing artifact; gas in the bowel is reflective and shows “dirty shadowing” due to a mix of fluid, which transmits sound waves and gas and leads to streaky shadows.



Focused Assessment with Sonography for Trauma (FAST) Examination



The FAST protocol originated as a limited ultrasound examination to rapidly assess for abdominal free fluid or pericardial effusion as a noninvasive alternative to diagnostic peritoneal lavage (originally the Focused Abdominal Sonography in Trauma). This highly sensitive test has expanded to include an evaluation of the pleural space as the extended FAST exam, or eFAST, for pneumothorax. In addition to making these important findings, the eFAST can reduce the time to necessary interventions, such as thoracostomy tube or emergent surgical exploration.



The eFAST protocol is classically performed with the patient supine. Ultrasonography practiced in this fashion has high sensitivity for the detection of free fluid; as little as 100 cc of fluid (and likely less with machines of higher quality) may be detected. Changes in patient position may shift expected findings and Trendelenburg position increases the sensitivity of the test for the detection of abdominal free fluid.



A high-frequency linear probe is used first to evaluate the pleural space in the midclavicular lines (or the highest point in the chest if the patient is not supine) to evaluate for pneumothorax with the probe marker directed toward the head. The low-frequency probe can be used if it is more expedient. Begin with the probe in the second interspace and slide caudally viewing the interspaces between ribs. Normally, the visceral pleura is visible sliding past the parietal pleura beneath the probe with occasional comet tails (reverberation artifact from pleura). With a pneumothorax, there is a loss of normal pleural sliding (see Chapter 19).



The low-frequency probe is then placed in the subxiphoid position with the probe marker positioned toward the patient’s right side (Figure 20-1). Using the liver as an acoustic window, the probe is angled superiorly to interrogate the heart for a pericardial effusion (Figure 20-2). Fluid will appear as anechoic or a hypoechoic stripe between echogenic pericardium closer to the probe and medium echotexture and beating myocardium further away from the probe (see Chapter 10).




Figure 20-1


Probe position for four standard FAST views.






Figure 20-2


Subxiphoid view of the heart using liver (L) as acoustic window. RV, right ventricle; LV, left ventricle.





The low-frequency probe is then turned with the probe marker directed toward the head to begin an evaluation of the abdomen. The hepatorenal space (Morison’s pouch) is evaluated for fluid between the liver and right kidney in the longitudinal plane in the anterior axillary line at the 7th to 9th ribs, the most dependent part of the peritoneal cavity in the supine position. Fluid will appear anechoic, separating the liver from right kidney (Figure 20-3). Sliding the probe superiorly one or more rib interspaces will identify the thick echogenic diaphragm and allow for evaluation of pleural effusion above the liver and beneath the more echogenic and shadowing lung. The probe may need to be moved to avoid shadowing ribs.




Figure 20-3


Fluid (arrow) in hepatorenal space between liver and right kidney.





Additional longitudinal imaging is then made in left upper quadrant evaluating the splenorenal space between the left kidney and spleen in the mid or posterior axillary line (e.g., in more of a coronal plane) between the 10th and 11th ribs to detect fluid (Figures 20-4 and 20-5). A more posterior approach is necessary on the left to avoid shadowing gas in the stomach. Once again sliding cephalad one or more rib interspaces will identify the echogenic diaphragm and allow for an evaluation of the left pleural space for effusion.




Figure 20-4


Left upper quadrant showing splenorenal space between spleen (S) and left kidney (LK); diaphragm (arrow) cephalad to the spleen.






Figure 20-5


Free fluid around spleen (*).





Finally, attention is turned to the most dependent portion of the pelvis, the rectovesicular space and anterior vesicouterine space in females. The probe is placed above the pubic bone and pelvis interrogated in transverse and longitudinal planes (Figure 20-6). Fluid in the urinary bladder will provide an acoustic window to evaluate for the presence of anechoic or hypoechoic free fluid in the peritoneal spaces (Figure 20-7).




Figure 20-6


Anechoic urine in the bladder (B) in transverse plane.






Figure 20-7


Hypoechoic fluid (*) above urinary bladder in sagittal plane.





A pneumoperitoneum can also be identified by focused FAST ultrasound. Analogous to the lack of pleural sliding with pneumothorax, there is a lack of peritoneal sliding with pneumoperitoneum. A massive pneumoperitoneum may even be associated with an inability to obtain the traditional FAST views.



The eFAST exam can be readily repeated if negative or if the patient deteriorates. It should not replace CT as a more sensitive test for the detection of solid organ injury or for the evaluation of the retroperitoneum, which is not well evaluated due to shadowing from bowel gas. It should also not delay surgical intervention if emergent abdominal exploration is indicated. In addition, imaging can be suboptimal in certain patients, such as the morbidly obese, uncooperative patients, in patients with subcutaneous emphysema, or dressings that limit visualization. In these patients, CT may be indicated instead of the eFAST. The inability of FAST to distinguish fresh blood from ascites makes the use of focused abdominal ultrasound less helpful in the ICU patient. In addition to the abdominal views, the gutters should also be assessed for fluid in the ICU (Figure 20-8).


Jun 27, 2019 | Posted by in CRITICAL CARE | Comments Off on ULTRASOUND EVALUATION OF THE ABDOMEN
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