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There is a major difference that must be understood when applying AFAST to blunt and penetrating trauma. The difference lies in the behavior of blood. In blunt trauma, it generally doesn't matter where the patient bleeds as mesothelial cells lining the abdominal cavity, pleural cavity, and pericardial sac rapidly defibrinate blood, making it readily apparent on AFAST as free fluid (anechoic triangulations). In contrast, victims of penetrating trauma have ripping, tearing and crushing of tissue, especially with bite wounds, and hence blood clots. During AFAST, clotted blood is often difficult to discriminate from adjacent soft tissue and thus is often initially missed (Figure 7.6). Although there are no large veterinary studies (Lisciandro et al. 2008, 2009; Lisciandro 2012), the importance is likely similar to that in human medicine (Udobi et al. 2001), in that FAST is likely highly specific for surgical trauma but variably sensitive. AFAST (and Global FAST) should still be used as a first‐line screening test because of all the clinical questions that may be answered regarding the patient. However, in instances of a negative AFAST, serial examinations should be implemented as standard of care until it is clear whether the patient is surgical versus medical.

In time, clotted blood will defribinate and appear as free fluid, and when viscus organ injury is present, likely the organ will “leak” into the abdominal cavity. Thus, AFAST carries the potential to detect septic peritonitis and other developing complications earlier in their course. In fact, vascular injury, uroabdomen, bilioabdomen, gastrointestinal injury, and others may present over the next several days post injury. Parenchymal injury is also possible and the use of color flow Doppler can be helpful especially for screening for renal injury (Figure 7.7). The author recommends continually performing AFAST serial examinations (or the Global FAST approach) four, eight, 12, and 24 hours post admission, and even two, three, and five days post injury. In other words, repeat the AFAST (Global FAST) as many times as needed until a surgical problem is comfortably ruled out.

Serial FAST examination strategies have proven helpful in human medicine (Blackbourne et al. 2004; Kirkpatrick et al. 2004; Ollerton et al. 2006; Matsushima and Frankel 2011; Mohammadi and Ghasemi‐Rad 2012). Although abdominal radiographs are of variable yield for blunt trauma because of their unreliability in detecting free fluid and their inability to semiquantitate volume (Lisciandro et al. 2009), radiography is always indicated in penetrating trauma as part of patient evaluation because of its ability to effectively detect hard tissue (fractures, luxations), soft tissue (herniation), and viscus organ injury (free air).

Figure 7.6. Clotted blood adjacent to the left kidney at the SR view. In penetrating trauma, blood often clots and clotted blood appears like soft tissue and is easy to miss during AFAST. Where suspected, color flow Doppler may be used. Clotted blood will lack flow. The image at the SR view shows the left kidney and an adjacent large clot with no flow on color Doppler, unlabeled in (A) and labeled in (B). Serial exams are key because if the clot was missed initially, the opportunity to capture the condition remains when repeating AFAST. Clot is outlined with cursors (V). LK, left kidney.

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

Pearl: Although abdominal radiography is typically a low yield diagnostic test in bluntly traumatized patients for abdominal effusion, radiography should always be part of the standard work‐up in penetrating trauma.

Pearl: AFAST often misses clotted blood, which is a common feature of penetrating trauma, because the echogenicity of clots is similar to soft tissue. The use of color flow Doppler can be helpful because clotted blood has no blood flow.

Pearl: Serial AFAST exams increase sensitivity for the detection of intraabdominal injury in penetrating trauma suspects and should be routinely performed four hours post admission (sooner if unstable or questionable) and then as often as needed when surgical injury is still possible, including up to five days or more post injury.

Figure 7.7. Evaluating for blood flow of the left kidney at the (SR) view. In (A) is a B‐mode image followed by color flow Doppler in (B) showing pulsatile flow. In penetrating injury, especially bite wounds over the lumbar region, or in animals which were shaken by their attacker, renal injury and avulsion are possible. It is a good habit to apply color flow Doppler routinely in these at‐risk cases, especially prior to exploratory surgery to help decision making by the surgeon for salvaging or removing the kidney.

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