Fast and accurate triage is crucial in the admission of critically ill patients. As there is only one health, improving the speed and accuracy of diagnostic testing represent the core of veterinary and human medicines research efforts. Point-of-care ultrasound (POCUS), defined as a goal-directed ultrasonography evaluation performed by a non-specialist to answer specific diagnostic questions, guide management, or help to perform technical procedures has been shown to achieve these goals. While its importance in cardiovascular and respiratory assessment has been now widely accepted, within critical care at least, abdominal ultrasonography is not yet so.

Probably because of so many potential differential diagnoses and the fact that the abdomen is often the private game of diagnostic imaging. Often regarded as adversaries, the key difference between diagnostic imaging specialists and intensivists is that the first aim to define a diagnostic, if possible, while the second is more focused to (1) answering a specific clinical question or (2) assess a (dys)function (rather than a disease). In veterinary emergency and critical care medicine, admission of unstable patients is common, while concise or absent anamnestic information are common, and will represent a diagnostic challenge. Using ultrasound-based evaluation protocols enhance the speed of initial assessment, is available at the cageside, achievable during the resuscitation phase, and reduce the exposure of ionizing radiation. POCUS in the unstable patient allows for rapid and accurate differentiation between the distributive, cardiogenic, hypovolemic, and obstructive shock types.

In this presentation, the aim is to present an overview of abdominal evaluation with POCUS and expected findings in veterinary medicine. Among the ultrasound-based protocols, the famous AFAST, modified from the original FAST study, requires 4 views:

• diaphragmaticohepatic (DH)
• splenorenal (SR)
• cystocolic (CC
• hepatorenal (HR)

AFAST is performed with the patient in standing or sternal position. Dorsal recumbency should be avoided, as it increases the risk of cardiovascular decompensation. When free fluid is detected, the patient is placed in lateral recumbency when safe, followed by waiting 3-minutes to allow ascites to redistribute into gravity dependent regions before scoring. Originally, each respective view was scored as 0 (negative all 4 views) to 4 (positive all 4 views). However, more recent research has shown that scoring smaller positives as 1/2 either by measurement or a visual approach better categorizes animals with smaller amounts of fluid. This approach scores 1/2 if the fluid measurement is less than 5 mm and 1 if more. Based on total AFS, patients are categorized as small-volume bleeder/effusion if AFS <3, and large-volume/bleeder effusion if over. In a clinical setting, small-volume bleeders are not expected to become anemic directly from their intra-abdominal hemorrhage. Thus, if a patient has an AFS less than 3 and is anemic, then major rule-outs in the acute setting include:

1.  Bleeding elsewhere

2.  Preexisting anemia

3.  Hemodilution

4.  Laboratory error

POCUS can detect up to 10 ml free intraperitoneal fluid (IPF) by experienced operators. In veterinary medicine, recent data suggest a sensitivity and specificity around 80 to 90%.

Diaphragmatic-Hepatic (DH) View

The DH view rapidly images the peritoneal cavity, liver, gallbladder, caudal vena cava (CVC) and its associated hepatic veins, the thorax, pleural space, heart, and lung along its pulmonary-diaphragmatic interface. On the ultrasound machine, the operator should set with enough depth to visualize the triad of gallbladder, caudal vena cava, and the diaphragm. Gallbladder wall edema is referred to as the double rim sign and halo sign, recognized as sonographic striation, and represents a marker of anaphylaxis shock or right sided congestive heart failure. When GBWE is present, the clinician should integrate physical and other variables to differentiate congestive heart failure from anaphylaxis, as therapeutic interventions differ.

Splenorenal (SR) View

The splenorenal view images both the retroperitoneal space and the peritoneal cavity by imaging the left kidney and the spleen, respectively. In some small veterinary patients, the operator may be able to image the opposite (right) kidney. In right lateral recumbency, this region can be used for the detection of pneumoperitoneum, by the presence of free gas in the abdomen. Usually described as an enhanced peritoneal stripe sign, this has been highly associated with digestive tract rupture, unless an abdominocentesis or abdominal surgery has already been done. Physiologic air can be seen in the lumen of the bowel as small stars. Larger air bubbles can appear as hyperechoic stripes generating comet tail artefacts (these are rare in the small bowel but frequent in the large bowel), much like a linear view of the lung would look. In the splenorenal view, measurement of CVC and CVC:aorta ratio can be done directly during the FAST exam. The evaluation of volaemia by measuring the caudal vena cava (CVC) diameter is a common practice in adult humans, whereas the evaluation of the CVC:aorta (Ao) ratio is preferred for paediatric patients as it is considered to be independent of bodyweight and size. After identification of the left kidney, the probe needs to be moved caudally and then oriented dorsally to allow identification of the CVC and Ao, with the caudal part of the left kidney in the same field. Then, shift the probe 90° to obtain a transverse view of the CVC and Ao, with visual assessment of the best cross sections of both vessels (Cambournac). Diameters are measured in B-mode using lines perpendicular to both vessel walls. A CVC:Ao ratio of 0.9 to 1.2 is considered normal, whereas lower measurement is suggestive of hypovolemia and higher ones of fluid overload or congestive states (Cambournac, unpublished data).

Cystocolic (CC) View

On the CC view, the focus is on the bladder. Clinicians should aim for free fluid and remember that direct visualization of a bladder does not rule out leakage or rupture. In fact, the bladder can leak when distention is among a specific volume. Urinary bladder measurements may be used to estimate urinary bladder volume. This could be done either by using a formula and a set of two perpendicular measurements or by integrated software calculation directly in the newest probe. If the latter option is used, validation of measurement and formula should have been done before.

Hepatorenal (HR) View

POCUS fluid scoring should end at the HR view, being most gravity dependent where fluid would accumulate. While some authors advocate the use of focused spleen evaluation, this could be really confusing in younger clinicians with limited experience. In fact, differentiating malignancy from benign changes in splenic ultrasound appearance is not possible. By seeing a “mass” on the POCUS, younger clinician may conclude to a neoplasia, which could lead to altered owner consent.

Conclusion

POCUS has become an integral part of emergency medicine, widely considered as the emergency clinician stethoscope. While POCUS applications are permanently expanding, veterinary clinicians should remember that utilization and interpretation of such techniques requires very little training and experience, not to overinterpret results.

References

1.  Boysen SR, Lisciandro GR. The use of ultrasound for dogs and cats in the emergency room AFAST and TFAST. Vet Clin North Am Small Animal Pract. 2013;43(4):773–797. doi:10.1016/j.cvsm.2013.03.011

2.  Smallwood N, Dachsel M. Point-of-care ultrasound (POCUS): unnecessary gadgetry or evidence-based medicine? Clin Med. 2018;18(3):219–224. doi:10.7861/clinmedicine.18-3-219