Notes supplied by Nadja Sigrist, DrMedVet, FVH(Small Animals), DACVECC.

Introduction

FAST originally stood for focused assessment with sonography for trauma and has been introduced in human medicine to identify trauma patients with abdominal haemorrhage. Since then, FAST has become a common diagnostic tool in the human emergency room to scan for fluid accumulation in any body cavity (abdomen, retroperitoneal, pleural, pericardial space). Abdominal FAST has also been validated in traumatised dogs and shows a good sensitivity and specificity to detect abdominal fluid. Recently, thoracic FAST in dogs has been introduced to rule out pneumothorax. It has therefore been suggested that FAST should stand for focused assessment with sonography for trauma, triage and tracking, as the FAST SCAN will detect effusion of any origin in all body cavities as well as other abnormalities.

Indications

As a screening tool for abdominal or thoracic fluid accumulation, an abdominal FAST SCAN is indicated in all patients presenting with trauma, collapse or acute abdomen among others. Indications for thoracic FAST are respiratory distress to rule out pneumothorax, pleural effusion or pulmonary fluid accumulation and in patients with suspicion of pericardial effusion.

No contraindications exist, however, as FAST will most of the time only detect fluid and not the cause of fluid accumulation, it is not indicated in patients with obvious effusions.

Why do a Fast Scan?

 Ultrasonography is far more sensitive than radiography for detecting free fluid and is radiation sparing.

 FAST is a relatively inexpensive, rapid, safe, noninvasive imaging technique that can easily be learned by non-radiologist veterinarians.

 A FAST scan can be performed during stabilisation of the patient in the emergency room.

Equipment

FAST requires an ultrasound machine. Usually, any probe can be used and B-mode is used.

Technique

 Ideally the animal is placed in lateral recumbency (right lateral may be preferred); however, some animals have to be scanned in sternal recumbency or standing due to respiratory distress or other problems. Shaving is not required but may facilitate interpretation of results by the inexperienced operator. Skin contact is ensured by wetting the fur and skin with alcohol.

 The abdomen including the retroperitoneal space is scanned at four sites: just caudal to the xiphoid process (diaphragmatic-hepatic view), in the midline over the urinary bladder (cystocolic view), and over the left and right flank regions (splenorenal and hepatorenal views) (Figure 1). Each site is scanned in two planes at a 90-degree angle to each other.

 An abdominal fluid scoring system has been developed (grade 0–4, depending on the number of fluid-positive sites).

 Serial abdominal FAST exams may be indicated as some patients may develop fluid accumulation at a later time, and serial examination allows monitoring of the fluid score.

 The thorax is scanned at four to five sites: on both sides over the heart (pericardial view) and over the caudodorsal lung field (eighth to ninth intercostal space). Additionally, the pleural cavity may be scanned from the diaphragmatic-hepatic site used in the abdominal scan.

Figure 1. Abdominal FAST SCAN. From: Sigrist, N; Spreng, D. Erstversorgung von Traumapatienten. Schlütersche, Hannover, 2011.

Interpretation of Results

 Abdomen. Free fluid is black and is easily identified between and next to the organs that can be seen at the corresponding site.

 Thorax. Pericardial effusion is easily identified on the pericardial or diaphragmatic-hepatic view as fluid around the heart surrounded by the pericardium (white line). Normally, the lungs can be seen moving (gliding) between two ribs (glide sign). The visible glide sign rules out pneumothorax.

 Advanced thoracic FAST includes M-mode evaluation and provides more information about other types of pulmonary pathology. This requires advanced training and is not within the scope of this lecture.

It must be stressed that a FAST scan does not replace a full abdominal or thoracic scan, and whenever possible, a complete ultrasound examination should be performed once the patient has been stabilised.

References

1.  Boysen SE, Rozanski EA, et al. Evaluation of a focused assessment with sonography for trauma protocol to detect free abdominal fluid in dogs involved in motor vehicle accidents. Journal of the American Veterinary Medical Association 2004;225(8):1198–1204.

2.  Lisciandro GR. Abdominal and thoracic focused assessment with sonography for trauma, triage, and monitoring in small animals. Journal of Veterinary Emergency and Critical Care 2011;21(2):104–122.