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FAST Evaluation

Focused Assessment with Sonography for Trauma (FAST) ultrasound examination is an integral component of trauma resuscitation. It is a proven method for the presence of fluid in the abdomen in anatomically dependent areas in hemodynamically stable or unstable patients after blunt trauma and may also be helpful in penetrating injury. The presence of fluid is suggestive of injury in the peritoneal or thoracic cavity.

The primary indication for FAST is for patients presenting with acute trauma to the chest or abdomen. FAST is thus a screening test that is both sensitive and specific in the identification of free intraperitoneal fluid. Trendelenburg or sitting positions may increase the sensitivity of the ultrasound examination for visualizing abnormal fluid. The sonographic appearance of blood depends on time of insult. Initially, the free fluid is sonolucent (black). Clot forms in 2 to 4 hours and becomes more echogenic (more gray), and it returns to being more sonolucent with fibrinolysis over 12-24 hours. These fluid collections may also appear complex with a combination of hypo echoic, or even isoechoic to surrounding structures.

Pitfalls of FAST

Limitations to FAST include the ability to detect free fluid in some injured children, patients with mesenteric, diaphragmatic, or hollow viscous injury, and patients with isolated penetrating injury to the peritoneum. FAST examination is also limited in identifying retroperitoneal hemorrhage.

The FAST exam is a single component of an ongoing resuscitation effort. Since it is a focused examination, FAST does not identify all abnormalities resulting from truncal trauma. Like other tests, it does not replace clinical judgment and should be interpreted in the context of the entire clinical picture. If the findings of the FAST are equivocal, repeat evaluation and additional diagnostic testing may be indicated.

 

False-negative examinations:

-Peritoneal free fluid is identified by FAST when at least 100-500mls is present. A negative exam does not exclude the presence of small amounts of free fluid.
-Injuries that do not give rise to free fluid including contained solid organ injuries, mesenteric vascular injuries, hollow viscus injuries, and diaphragmatic injuries.
-FAST does not identify solid organ injury.​

 

False-positive examinations:

-Physiologic reasons such as an ovarian cyst rupture.
-Pathologic reasons secondary to an inflammatory process in the abdomen or pelvis such as in
patients with a history of ascites or severe polycystic disease.
-Other situations in which the exam is unreliable includes patients with: ventriculoperitoneal shunts, peritoneal dialysis or recent peritoneal lavage.

A FAST exam provides a picture of a patient’s condition at one moment in time. It never eliminates the possibility of injury or fluid collections that are below the detectable threshold of an examination.

FAST Pitfalls

FAST Exam

The objective of the FAST exam is to analyze the peritoneal cavity for free fluid. This requires examination of the abdominal 'windows' to the peritoneum, pericardium and pleural spaces. The subcostal window  is viewed separately.  The ability to denote free fluid in the pelvis is aided by the presence of a fluid-filled bladder. Subtle changes in transducer angle and position can help improve interrogation of a given area. 

FAST examinations should be conducted using a curvilinear probe and the highest clinically appropriate frequency used. For adults, mean frequencies of 3.5 and 5 MHz are most commonly used. Gain settings should be adjusted so that the diaphragm and renal sinus fat appear white and known hypoechoic structures (such as the inferior vena cava, gallbladder, and renal vein) appear black.

FAST Exam

Scanning Windows- An Overview

The diagram below is a graphical representation of the location where you will find the FAST windows. Pay special attention at the direction of orientation of the probe marker denoted by the arrows. The 3D anatomic model further down this section will assist us in understanding how to place the probe accurately.

1. Right Flank

Place probe on the anterior or middle axillary line with the indicator to the patient’s head. Your knuckles should be indenting the bed.

torso fast_edited.jpg

4. Suprapubic

The probe is placed in the transverse plane immediately cephalad to the pubic bone. Fanning along the bladder is necessary. The probe is then rotated into a sagittal plane. 

2. Left Flank

Place probe on the posterior axillary line or even more posteriorly. Note the higher location of this probe as compared to 1. 

3. Subcostal

This view is further explored in the pericardial effusion section of this website.

3D FAST Model Overview

Here we present an anatomic model showing the position of the probe relevant to the area interrogated with the use of FAST. Bear in mind that the subcostal view is not included here since it is part of the heart assessment. We will explore each of this locations in detail. Notice that the suprapubic exam has two scanning positions. Pause the clip if you would like to analyze the position closely. We will be looking at each of these areas separately.

FAST 3D model showing curvilinear probe position. The subcostal view has not been included here since it has been covered in the cardiac segment of this website. The 3D model is used with permission and modified from Z-anatomy.

3D FAST Model for the Right Upper Quadrant Examination and Probe Placement

Probe position with scanning sector shown.  Probe is on the posterior axillary line with the indicator to the patient’s head. The probe should be between the 8th and 11th rib space. The probe should be moved towards the anterior axillary line while scanning to evaluate the caudal edge of the liver. Due to patient position, the evaluation at the posterior axillary line may not be possible. Layers have been removed to improve understanding of the anatomic model. Our objective here is to see the right hemithorax , the hepato-diaphragmatic area, the hepatorenal recess and the paracolic gutter.

Clip 1 thorough 3 show the same probe position and sector. The views changes to enhance anatomic relations of the ultrasound sector. Notice the close proximity with ribs and pleura which create an ultrasound artifact.

  1.  

  2.  

  3.  

FAST 3D model showing curvilinear probe position. The subcostal view has not been included here since it has been covered in the cardiac segment of this website. The 3D model is used with permission and modified from Z-anatomy.

Gradient

Right Upper Quadrant - Right Flank Sonography

Four potential spaces for the accumulation of free fluid are examined in this region: the pleural space, the subphrenic space, the hepatorenal space (Morison’s pouch), and the inferior pole of the kidney, which is a continuation of the right paracolic gutter​. Morison’s pouch is the collection site for an abdominal injury since the paracolic gutters empty here. Blood initially develops at the tip of the liver and then progresses to separate the liver and the kidney.

Following the identification of the kidney and liver and diaphragm, the sonographer can rock or slide the probe cephalad to evaluate above the diaphragm. Free fluid in the hemithorax will be identified as an anechoic or black area above the diaphragm. Careful examination between dome of the liver and diaphragm is necessary to identify free fluid that may accumulate there. Small amounts of free fluid tend to collect around the caudal tip of the liver. Caudal probe movement allows visualization of the inferior pole of the right kidney as well as the right paracolic gutter for free fluid assessment.

The following images display a normal sonographic examination and a positive exam for free fluid. 

Liver

Kidney

Lets go further and give other examples. The following images display a positive right upper quadrant FAST. In these images we appreciate hypoechoic fluid between the kidney and the liver and around the liver.