Get me some diagnostics - FAST!

The Emergency and Critical Care department at SARC sees its fair share of high need cases. However, even with the most complex conditions, most cases start off with simple diagnostics as the first step, regardless of where treatment ultimately ends up.

One of our most helpful tools in front line diagnostics is ultrasonography. Using an ultrasound for basic scanning is not as daunting as is often made out, and many practices have either upgraded or purchased ultrasound equipment in recent times thanks to taxation breaks.

We thought that it may be a good time look at the simple FAST (focussed abdominal sonogram in trauma) scan which benefits many of our patients. This easy scan can be performed in just a couple of minutes and can provide invaluable information.

The FAST scan is a focussed, goal directed, sonographic examination of the abdomen aimed at detecting the presence or absence of haemoperitoneum. It provides a viable alternative to other investigations in the blunt abdominal trauma patient, and can be integrated into the primary survey in patients with signs of haemorrhagic shock or suspicion of intra-abdominal injury. It has the additional advantages of being non invasive, reproducible, and is capable of being rapidly performed cage-side. Indeed, the FAST scan is often regarded as being a simple extension of your routine clinical examination. A standard 4 view examination can be completed in approximately 2 minutes.

Ultrasound in the ER

The FAST exam is performed immediately after the primary survey. Ultrasound is the ideal initial imaging modality because it can be performed simultaneously with other resuscitative procedures, providing vital information without the time delay caused by radiographs or requirement of GA with computed tomography (CT). The concept behind the FAST exam is that many life-threatening injuries cause bleeding. Significant bleeding into the peritoneal, pleural, or pericardial spaces may occur without obvious warning signs. The purpose of cage-side ultrasound in trauma is to rapidly identify free fluid (usually blood) in the peritoneal, pericardial, or pleural spaces. Although ultrasound is not 100% sensitive for identifying all bleeding, it is nearly perfect for recognising intraperitoneal bleeding in hypotensive patients and for diagnosing cardiac injuries from penetrating trauma.

The Scan

The FAST scan is a 4 site scan reliant on detecting the presence of fluid within the pericardium and most dependent zones of the peritoneum in the horizontal patient (in humans this is done with them lying on their back, but in animals we use lateral recumbency unless precluded by injury). It is capable of detecting more than 100-250ml of free fluid. The EFAST scan includes views of the thorax, to detect fluid and air within the pleural space. If you would like further information on the EFAST scan, please feel free to contact us.

The Views

To detect fluid in the abdomen, 2 ultrasonographic views (transverse and longitudinal) were obtained at each of 4 sites (just caudal to the xyphoid process, on the midline over the urinary bladder, and at the left and right flank regions). These sites represent the areas where fluid is most likely to accumulate, due to gravity.

Try to imagine the images below with fur and four legs.

 

 

1) Subcostal View (pericardial and pleural)

The transducer is placed caudal to the xyphoid process. Moderate pressure against the abdominal wall with the whole of the transducer may be required to direct the beam cranially in both longitudinal and transverse views to obtain the image. This should demonstrate both the liver and heart, in a 4 chamber view. The heart is easily recognisable, due to its characteristic motion. The heart will be surrounded by a rim of echogenic pericardium.

Any discrete blackness between this rim and the heart wall represents fluid in the pericardial sac. Free fluid cranial to the diaphragm can be seen in haemothorax.

2) Right Side View (perihepatic)

The transducer is positioned over the most gravity dependent area on the right side. Some panning of the beam in both transverse and longitudinal planes should demonstrate the liver, kidney and diaphragm. A black rim between the liver and kidney represents free intraperitoneal fluid.

3) Left Side View (perisplenic)

The transducer is positioned in the left flank over the most gravity dependent area. Views here demonstrate the spleen, kidney, some liver (with stomach and diaphragm). Rotating the transducer to obtain longitudinal and transverse planes should demonstrate the presence of any fluid between the spleen and kidney. This view may be marred by acoustic shadows from the stomach.

Any evidence of a black rim between these organs represents free intraperitoneal fluid. Gross injury to solid organs may sometimes also be seen, such as splenic ruptures.

4) Suprapubic View (pelvic)

The transducer is placed on the midline over the bladder area and angled caudally in to the pelvis. This demonstrates the bladder. The probe is then rotated through 90 degrees to move the beam providing a view of the bladder, rectum and rectovesical pouch. This pouch is immediately cranial to the bladder, alongside the rectal wall and even very small amounts of fluid can be detected here as a black area of the outside of the bladder wall.

 

 

Pitfalls

As with any investigation, FAST scanning has limitations. It is not as sensitive as CT in identifying solid organ injury, and relies on the surrogate indicator of free fluid within the peritoneum to identify significant haemorrhage. Haemoperitoneum is not present in all patients with abdominal visceral injuries, and certain injuries may not be initially detected on the FAST exam. These include perforation of a hollow viscus, bowel wall contusion, pancreatic trauma and renal pedicle injury.

Therefore, over-reliance on a single FAST scan may lead to false conclusions. The scan should be repeated during the secondary survey and also if the patient demonstrates clinical deterioration, since free fluid may have accumulated in the intervening time and now be visible on ultrasound. The quality of images obtained may also be a limiting factor with patient obesity and gas in the bowel leading to degradation in image quality.

Closing Thoughts

FAST scanning has become a beneficial diagnostic tool in a large range of cases. With simple training all veterinary staff can perform the scanning process and results are available immediately.

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