Point-of-Care Ultrasound Techniques for the Small Animal Practitioner. Группа авторов

artifacts result in loss of interpretive clarity by the ultrasound machine along any luminal borders, in this case the gallbladder, thus, falsely making the gallbladder appear to contain sediment, other intraluminal abnormalities, or a mass, as well as defects along its wall similar to the urinary bladder (see Figure 6.26) (see Chapters 3 and 5).

Figure 6.15. Pitfalls at the DH view related to the gallbladder, hepatic venous system, and stomach wall. In (A) the gallbladder (GB) appears triangulated, mimicking free fluid, but by fanning through the gallbladder in both directions, this mistake is less likely to occur. Moreover, free fluid does not have hyperechoic borders as the wall of the gallbladder does. In (B) branching of the hepatic venous system mimics free fluid. In real time and through repetition, the sonographer will get used to where fluid typically pools as well as incorporating the added information of performing the entire AFAST. In (C) the stomach is fluid filled in this patient and can mimic the gallbladder to the hasty sonographer; and its sonographically layered wall can mimic gallbladder wall edema. This area should be avoided by rocking the probe more cranial as shown in the many DH views within this chapter and Figure 5.10. GB, gallbladder; LIV, liver; ST, stomach.

      Source: Reproduced with permission of Dr Gregory Lisciandro, Hill Country Veterinary Specialists and FASTVet.com, Spicewood, TX.

      Pitfalls Creating False Positives

      There are several fluid‐filled or fluid‐associated structures within the liver that the sonographer must be aware of.

       Gallbladder and Biliary System

When imaged in certain planes, the gallbladder and common bile duct can appear as anechoic sharp angulations like free fluid. This pitfall, misinterpreted as a false positive, is easily avoided by fanning, tracing, and connecting the gallbladder to its biliary tree (Figures 6.15 and 6.16).

       Hepatic and Portal Veins

The liver's venous system can appear as anechoic sharp triangulations like free fluid. The venous system as a general rule is not obvious in normalcy when patients are examined in lateral, sternal and standing positions. This false positive is easily avoided because most free fluid is not as linear as the venous system, and the venous system in most instances can be traced and seen branching (see Figure 6.12) (see Chapter 8). Color flow Doppler may be used to distinguish the venous system from free fluid but is rarely needed. Hepatic veins are differentiated from portal veins in a couple of ways. First, portal veins have brighter, more echogenic (hyperechoic) walls when compared to hepatic veins and thus portal veins often appear as hyperechoic (bright white) equal [=] signs (see Chapters 8 and 39). Hepatic veins are darker walled sonographically and are generally traced as they drain into the CVC, especially when there is hepatic venous distension (see Figure 6.12F and Chapters 7, 20, and 36).

Figure 6.16. Additional examples of pitfalls at the DH view. (A) and (B) are the same image unlabeled and labeled. The gallbladder is bilobed, most commonly seen in cats. It appears similar to free fluid but through interrogation by fanning through the gallbladder in both directions, the rounding of the hyperechoic gallbladder wall differentiates it from free fluid. (C) and (D) are the same image unlabeled and labeled. Note the caudal vena cava (CVC) can mimic pleural effusion at the level of the diaphragm; however, the dynamic changes in CVC diameter with respiratory and cardiac cycles and its A‐lines through the far‐field differentiate the CVC from pleural effusion. In (E) and (F), the same image is unlabeled and labeled with a circle indicating the stomach wall that is anechoic and can mimic free fluid (and its associated edge shadowing artifact; see Figure 3.2). Note how consistent the diaphragm is within the images serving as a landmark for proper DH view image acquisition, which in and of itself (the proportionality shown) will prevent many DH view pitfalls. CVC, caudal vena cava; GB, gallbladder; LIV, liver; ST, stomach.

      Source: Reproduced with permission of Dr Gregory Lisciandro, Hill Country Veterinary Specialists and FASTVet.com, Spicewood, TX.

       Caudal Vena Cava

      The CVC can mimic pleural effusion in haste, especially at the level of the diaphragm (see Figure 6.16).

       Stomach Wall

      Typically, the sonographer should stay away from the area of the stomach during the DH view by directing the probe far cranially, striving for the proportionality shown (see Figures 6.15 and 6.16). The stomach has a sonolucent (dark or black) component to its wall, which typically appears linear in real‐time imaging but can appear as an anechoic triangulation like free fluid. The stomach wall is also subject to artifacts such as edge shadowing (see Figure 3.2). The best way to avoid being confounded by the stomach is to stay further cranially when interrogating the DH view, understanding that little is gained by imaging caudally toward the stomach during an AFAST (see Figure 5.10). Free intraabdominal fluid is most commonly, easily, and accurately seen between liver lobes along the diaphragm, or between the liver and diaphragm (see Figure 6.10).

      Pearl: Stay away from the stomach region because it is too far caudal for the DH view. Free fluid is better appreciated between the liver and diaphragm, between liver lobes more cranially along the diaphragm, and the liver and falciform fat.

      Minimizing False Negatives

      Repeat AFAST serially four hours post admission as standard of care (or sooner as clinical course dictates) and routinely repeat after resuscitation and rehydration (Lisciandro et al. 2009; Lisciandro 2011; Boysen and Lisciandro 2013). The four‐hour postadmission rule of thumb is supported in human medicine by the American College of Emergency Physicians (ACEP) guidelines (www.acep.org) as standard of care and has been shown to increase FAST sensitivity, improve patient management, and prevent the “crump factor” in people with hypotension (Blackbourne et al. 2004; Ollerton et al. 2006; Bilello et al. 2011). In the AFAST study, ~20% of dogs changed score on serial examination,