Introduction

FAST stands for focused assessment with sonography for trauma and was first developed in the 1990s as a means for trauma surgeons to detect presence of intra-abdominal injury bedside during initial triage of trauma patients. Abdominal FAST is also called AFAST since the development of a thoracic FAST method. Multiple studies in human medicine have demonstrated that the method is very sensitive and specific for detection of peritoneal effusion secondary to blunt abdominal trauma, comparable to diagnostic peritoneal lavage or computed tomography. The method was then adapted and validated in our veterinary patients and provides a quick, standardized way of searching for free peritoneal fluid during initial patient assessment. The method is easy to learn and can be performed by persons with little other training in ultrasonography.

Method

Four ultrasonographic windows are used with the patient in either right or left lateral recumbence, depending on suspected pathology and patient comfort. Clipping of the haircoat is not necessary, a small area of hair is simply parted and the skin is sprayed with alcohol to ensure probe contact without air interference. For the sub-xiphoid view, the ultrasound probe is placed behind the xiphoid and angled cranially. Increasing depth allows a view of the pleural space and in smaller patients of the cardiac apex and pericardium. The ultrasound probe is then placed on the left flank to visualize spleen and left kidney, in this view retroperitoneal effusion can potentially be identified. A caudoventral midline view at the bladder apex is often the most successful window to detect presence of peritoneal effusion. Finally, a right flank view is used to evaluate the interface of the right kidney, body wall and small intestines.

The examination is usually performed in a clockwise fashion starting at the sub-xiphoid view, moving up to the non-gravity-dependent flank view, to the caudal midline (bladder) view and finally to the gravity-dependent flank view. In each location, the ultrasound probe should be left in place but fanned back and forth to visualize all the target organs. Too much movement of the ultrasound probe often results in hair interference if the haircoat was not clipped. A longitudinal view is typically used, but transverse views may be added if deemed helpful.

Presence of fluid in any of these locations should be recorded to determine progression or resolution of disease. The examination should be performed approximately every 4 hours, or more frequently if clinically indicated.

Findings

Fluid is hypo- or anechoic (dark) in an ultrasound image. Free fluid is typically multifocal in nature with small triangular areas of hypoechogenicity in between the abdominal organs, adjusting to the shape of their surface. If there is a large amount of fluid the abdominal organs may be displaced (typically dorsally) and compressed. Strands of mesenteric fat may extend through the fluid. In the caudal abdomen, free fluid has to be distinguished from a distended urinary bladder which is recognizable by the cranial curvilinear hyperechoic (bright bladder wall). Free fluid adjacent to the kidney may be retroperitoneal or peritoneal, as the peritoneal lining is very thin and the two compartments are not easily differentiated if only a small amount of fluid is present. Fluid also tends to accumulate between the liver lobes and appear as triangular or linear hypoechoic areas. Free fluid tends to accumulated in the gallbladder fossa and can look very similar to gallbladder wall edema. In the sub-xiphoid view cranial angling of the probe allows a view of the pleural space. Any fluid detected cranial to the diaphragm is consistent with pleural effusion. Pericardial effusion presents as a curvilinear hypoechoic rim around the heart, outlined by a thin hyperechoic line which represents the pericardium.

Presence or absence of a urinary bladder should be noted during the AFAST examination and if no bladder is identified, recent voiding, rupture or anuria are considerations. The clinical history of the patient as well as repeating the examination after fluid therapy usually clarifies these questions. Presence or absence of the gallbladder should also be noted during the examination, although it can be variable in position and size, and if gallbladder rupture is suspected an ultrasound examination should be performed by a radiologist.

Free peritoneal gas is more difficult to recognize but can also be detected with this method. Gas creates a reverberation artifact that has the appearance of multiple repetitive lines extending from the peritoneal surface distally into the abdomen with decreasing intensity, creating effectively a "shadow". Clinicians performing AFAST scans should familiarize themselves with the appearance of gas in the intestinal tract which creates the same artifact. Free gas is normally seen after surgery and can persist for up to 3 weeks after the procedure.

Pitfalls

There is a learning curve when using this method and frequent practice under supervision of a more experience clinician or radiologist is recommended. False positive diagnosis can result from interpreting large abdominal vessels or the gallbladder as free fluid when imaged at an oblique angle. The urinary bladder when distended can be misinterpreted as free fluid, as well as fluid within the colon, uterus or distended small intestines. False negative results can result if the scan is performed with too much pressure on the transducer, resulting in compression and displacement of small amounts of free fluid. Exudates can be very hyperechoic if very cellular, and can be mistaken for fat or a parenchymatous structure. Finally, in the presence of peritoneal effusion many abdominal organs are slightly different in position, echogenicity and appearance. For example, free fluid trapped between pancreatic lobules result in an apparent pancreatic enlargement and hypoechogenicity whereas the mesenteric fat often has a hyperechoic appearance in presence of fluid. Care should be taken not to over-diagnose abdominal conditions other than peritoneal effusion and a full ultrasound examination should be performed by an experienced ultrasonographer once the animal is stable if there are any questionable findings during the AFAST scan.

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 Anim Pract. 2013;43:773–797.

2.  Lisciandro GR. Abdominal and thoracic focused assessment with sonography for trauma, triage, and monitoring in small animals. J Vet Emerg Crit Care. 2011;21(2):104–122.

3.  McMurray J, Boysen S, Chalhoub S. Focused assessment with sonography in nontraumatized dogs and cats in the emergency and critical care setting. J Vet Emerg Crit Care. 2016;26(1):64–73.