Ultrasound for dogs: POCUS and A-FAST in emergency medicine

Introduction

Ultrasound has been used in veterinary medicine since the late 1970s. Back then, it was considered an “exotic” diagnostic technique, but it is now a routine procedure used in many clinical situations. It is widely agreed that ultrasound diagnosis should primarily fall to imaging specialists or vets with ample experience, or cardiologists in the case of echocardiography.

But in practice, however, this is not always the case. Clearly, the ultrasound diagnosis of complex conditions such as portosystemic shunt will always correspond to specialists. Nevertheless, if a general veterinary surgeon acquires basic ultrasound knowledge and follows a suitable protocol, they can draw valuable information from a quick ultrasound exam without having to resort to state-of-the-art equipment.

Ultrasound for dogs: what do POCUS and A-FAST stand for?

These terms have become commonplace jargon in recent years, especially in the context of ultrasound in emergency medicine.

• POCUS stands for point-of-care ultrasound and the principle is to provide an almost immediate answer to a clinical question based on ultrasound results.
• The concept includes A-FAST and T-FAST ultrasound, which mean abdominal and thoracic focused assessment with sonography for trauma.¹

In other words, the purpose of these ultrasound modalities is to obtain useful and relevant information for patient triage, diagnosis and monitoring while spending very little time on the procedure, which can be reliably performed by a general practitioner.²  

Ultrasound for dogs: indications for performing an A-FAST

A-FAST was initially considered a rapid, noninvasive procedure with a high sensitivity and specificity for assessing the presence of free fluid in the abdominal cavity in trauma patients.^1,3,4 However, there is now consensus that the A-FAST protocol ensures a rapid response to other injuries that could be key to the outcome of the case, such as identifying any hepatic, renal or splenic changes suggestive of a neoplastic process, or if there are neoplasms or uroliths in the bladder.²

Given the above, and in addition to its use in patients with abdominal trauma, A-FAST is also indicated in animals treated for collapse, hypotension, tachycardia, changes in mental status of unknown aetiology, treatment-refractory anaemia, and during postoperative monitoring of dogs at risk of bleeding or peritonitis.^1,3,4  

Only recently, POCUS protocols were published for animals treated in emergency departments with specific conditions in different body systems.^5–7

What does the A-FAST ultrasound protocol involve?

The A-FAST protocol is based on the exploration of four specific areas of the abdomen following a standardised procedure. Four predetermined quadrants are examined in a clockwise order. The procedure can be done with patient in right or left lateral decubitus and should only take 3–6 minutes.

Areas of exploration

• Subxiphoid, to assess the diaphragmatic/hepatic space and gallbladder.
• Left flank, to assess the splenorenal space.
• Area between the abdominal wall and spleen; caudally, to examine the cystocolic space and the cranial appearance of the bladder.
• Lastly, examination of the right flank to assess the hepatorenal space and the area between the intestinal loops, right kidney and abdominal wall.³

Assessment

• This examination permits a rapid assessment of the presence of free fluid of various origins (haemoabdomen, uroabdomen, septic exudate).
• Severity is classified on a scale of 0 to 4 and represents the number of quadrants where free fluid is detected. If the score increases over time, it indicates active effusion.
• It should be noted, however, that small amounts (< 3x3 mm) of free fluid can be detected in 1 or 2 quadrants in puppies and healthy dogs.^1,3
• The nature of the fluid is confirmed by ultrasound-guided abdominocentesis, which has a much greater diagnostic sensitivity than the blind technique. The A-FAST protocol has almost completely eliminated the need for diagnostic peritoneal lavage, which used to be a routine technique in emergency veterinary clinics.¹
• A-FAST can also help determine urine output in hospitalised patients by calculating the volume of the bladder contents, estimated from images obtained at the level of the cystocolic  junction. The calculation is based on the following formula:
• volume in cm³ = bladder length (longitudinal plane) x height (longitudinal plane) x width (transverse section) x 0.625.^2,3
• There is, however, a caveat: this formula tends to underestimate the volume in bladders at less than 10% capacity or with masses or other processes that alter bladder shape.
• A-FAST ultrasound is extremely sensitive in detecting free intra-abdominal air and returns information on gastrointestinal motility at the level of the stomach/proximal duodenum and jejunum, with 4–5 peristaltic waves/minute expected in the first case, and 1–3 in the second, when there is food in the gastrointestinal tract. This can be used in the diagnosis of ileus.²

Conclusions

The A-FAST protocol can provide extremely valuable information in the management of emergency patients. Although initially its main indication was to assess free fluid in patients with abdominal trauma, we now know that it offers many more advantages. Furthermore, given that it does not require ultrasound expertise or investment in highly complex equipment, it is a good option for use by small clinics and general veterinary surgeons. 

References

1.     Lisciandro GR. (2011) Abdominal and thoracic focused assessment with sonography for trauma, triage, and monitoring in small animals. J Vet Emerg Crit Care (San Antonio); 21:104–122.

2.     Lisciandro GR, Lisciandro SC. (2021). Global FAST for patient monitoring and staging in dogs and cats. Vet Clin North Am Small Anim Pract; 51: 1315–1333.

3.     Garcia-Fernández A, Aguilar-Gallego N. (2022). A-FAST y T-FAST (Parte I) – Ecografía abdominal y torácica en urgencias Clin Vet Peq Anim 2022, 42 (1): 7–13

4.     McMurray J, Boysen S, Chalhoub S. (2016). Focused assessment with sonography in non traumatized dogs and cats in the emergency and critical care setting. J Vet Emerg Crit Care (San Antonio); 26: 64–67.

5.     Mays E, Phillips K. (2021). Focused ultrasound of vascular system in dogs and cats-thromboembolic Disease. Vet Clin North Am Small Anim Pract; 51: 1267–1282.

6.     Cole L, Humm K, Dirrig H. (2021). Focused ultrasound examination of canine and feline emergency urinary tract disorders. Vet Clin North Am Small Anim Pract; 5: 1233–1248.

7.     Fulton RM. (2021). Focused ultrasound of the fetus, female and male reproductive tracts, pregnancy, and dystocia in dogs and cats. Vet Clin North Am Small Anim Pract; 5: 1249–1265.