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.
-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.
-Physiologic reasons such as an ovarian cyst rupture.
-Pathologic reasons secondary to an inflammatory process in the abdomen or pelvis such as inpatients 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.
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.
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.
1. Right Flank
Place probe on the anterior-axillary line with the indicator to the patient’s head. Your knuckles should be on the bed.
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.
This view is further explored in the pericardial effusion section of this website.
Right Upper Quadrant - Right Flank
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.
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.
Left Upper Quadrant - Left Flank
This view uses the spleen as a window to interrogate four different potential spaces: pleural space above the diaphragm, subphrenic space below the diaphragm, splenorenal space and the inferior pole of the left kidney which is a continuation of the left paracolic gutter. Free fluid in the hemithorax will be identified as an anechoic or black area above the diaphragm. Scanning caudad allows visualization of the inferior pole of the left kidney and the left paracolic gutter. Isolated free fluid in the left upper quadrant is rare and will most likely be found in the subdiaphragmatic space due to the phrenicocolic ligaments and only on rare occasions, when large amounts of fluid are present, will free fluid occur between the spleen and the kidney. The phrenicocolic ligament restricts the flow of free fluid between the left paracolic gutter to the left flank, so fluid actually spreads across the midline into the right flank.
The following images display a normal sonographic examination and a positive exam for free fluid. This is a hypoechoic fluid surrounding the spleen.
Pelvic View- Suprapubic
Assessment of the most dependent space in the peritoneum for free fluid occurs here. Here we evaluate the recto vesicular space in male and the rectouterine space in female. Interrogation may be necessary with a fluid-filled bladder. When empty, free fluid may still be seen but is less reliable at ruling out the presence of smaller amounts of free fluid. When free fluid is present, it is noted most often posterior or superior to the bladder and uterus. The bladder should be scanned in its entirety in both the sagittal and transverse planes.
The following images display a normal sonographic examination. On the left a transverse veiew of the bladder and on the right a sagittal view:
The following images display fluid behind the bladder which can be appreciated as a hypoechoic fluid deeper than the posterior wall of the urinary bladder. On the right left, the transverse view of the urinary bladder. On the right the sonographer is going from the transverse to the sagittal view.
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