AFAST, part 1

AFAST = Abdominal Focused Assessment with Sonography for Trauma

In the emergency setting, focused ultrasound examinations are increasingly being used as extensions of the physical examination.

The goal is the identification of life-threatening conditions – such as internal bleeding or organ rupture – but can also assist in tracking conditions over a period of time. This is through obtaining an abdominal fluid score (AFS).

An AFS is based on the number of AFAST fluid-positive quadrants identified using a 4-point scale – from AFS 0 (negative for fluid in all quadrants) to AFS 4 (positive for fluid in all quadrants).

This article will aim to provide an introduction to performing and interpreting the AFAST examinations.

How to perform an AFAST

Patient stability at the time of presentation to the emergency service will determine the scan position. Lateral recumbency is preferred, with no difference between free fluid detection rates between patients positioned in left or right lateral recumbency.

If the patient has a very long, thick coat, shaving will be necessary, otherwise generally the patient is not shaved for the examination. Isopropyl alcohol and/or acoustic coupling gel should be used.

Four acoustic windows are assessed in a complete AFAST examination:

1. CC = cystocolic
2. SR = splenorenal
3. DH = diagphragmatic-hepatic
4. HR = hepatorenal

Cystocolic window

The primary landmarks for the cystocolic window are the urinary bladder and the ventral surface of the descending colon.

Place the probe caudally along the midline, with the notch oriented cranially, to create an image along the long axis of the patient. Be aware that too much pressure on the abdomen can displace a small-volume urinary bladder.

The appearance of effusion in the cystocolic window should not be confused with the rupture of the urinary bladder, in which the urinary bladder will be collapsed.

Peritoneal effusion will present itself as anechoic material adjacent to the bladder.

Splenorenal window

Place the probe on the left side of the abdomen, caudal to the 13th rib and ventral to the lumbar hypaxial musculature. While maintaining a long axis image, fan medially to search for the left kidney. As the kidney is centered, fan laterally and cranially to identify the spleen.

Peritoneal effusion will present itself as anechoic material adjacent to renal or splenic parenchyma, between loops of small intestine, or within mesenteric fat.

Diaphragmatic-hepatic window

Place the transducer in a subxiphoid location, with the notch oriented cranioventrally. Visualize the hyperechoic, curvilinear diaphragm-lung interface with the hepatic parenchyma.

Fan between the patient’s left and right side, following the contour of the diaphragm, while maintaining a sagittal image along the long axis of the patient. In a normal patient, the liver and diaphragm are in direct contact. If peritoneal effusion is present at this window, it may appear as anechoic material between the liver and diaphragm or between the liver lobes.

Visualize the gall bladder. In normal dogs and cats, the gall bladder appears as a thin-walled, ovoid structure containing anechoic material. Usually the gall bladder can easily be found from the original subxiphoid position by fanning the probe toward the patient’s right side.

The combined presence of a large volume of organised, non-mobile gall bladder contents (for example, mucocele), anechoic surrounding fluid, and hyperechoic adjacent fat should raise concern for gall bladder leakage, rupture and peritonitis.

Hepatorenal window

Place the probe caudal to the right 13th rib, ventral to the lumbar hypaxial musculature, and fan dorsomedially to search for the interface of the right kidney with the liver.

Relative to the left kidney, the right kidney is generally in a more cranial and lateral position; therefore, angle the probe cranially before fanning.

In the hepatorenal window, anechoic material between the hepatic parenchyma and the kidney represents a retroperitoneal or peritoneal effusion.

Conclusion

The AFAST examination is a powerful diagnostic tool used in a variety of patient settings. If ultrasound is available, this examination can provide immediate information at minimal risk to the patient. Furthermore, AFAST does not require a lot of ultrasound experience, and most veterinarians can achieve proficiency within a short period of time.

But remember, AFAST examinations should never be prioritised over immediate life-saving therapy.