FAST Scan (Focused Assessment with Sonography in Trauma)

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Indication: 
Penetrating Cardiac Trauma
Many patients with stab wounds to the heart don’t suffer significant blood loss because the wound in the pericardium seals, creating a pericardial effusion. Cardiac tamponade will usually develop, but may be delayed by several minutes or even hours. Prior to the development of tamponade patients will be relatively asymptomatic. When symptoms eventually develop, clinical decompensation occurs rapidly resulting in shock and then cardiac arrest. The key to managing penetrating chest trauma is to identify a developing pericardial effusion as early as possible, before tamponade and cardiac arrest occurs.

Blunt Cardiac Trauma
Most patients who suffer severe cardiac injury such as rupture of the free ventricular wall die quickly. Cardiac rupture causes a pericardial effusion, which will be easily recognized during the FAST exam. Severe global ventricular dysfunction may also be noted during the FAST exam, more likely the result of severe acidosis from hypovolemic shock than blunt cardiac injury. Although blunt cardiac rupture is rare, the cardiac portion of the FAST exam should still be performed on all patients with significant blunt chest trauma, especially those who are hypotensive.

Blunt Abdominal Trauma

Intraperitoneal bleeding after blunt trauma is common. It is usually the result of a spleen or liver injury and difficult to diagnose on physical exam.

Penetrating Abdominal Trauma

Although many studies limit analysis of the FAST exam to the setting of blunt trauma, it appears to be equally sensitive for detecting hemoperitoneum in patients with penetrating trauma.
The sensitivity of the FAST exam for determining the need for laparotomy is only about 50%.Bowel injuries are very common in penetrating trauma and the FAST exam does not detect most of these injuries.

Hemothorax
About 200 mL of pleural fluid is required before it can be detected with a plain CXR. Ultrasound is much more sensitive for detecting pleural fluid and can identify as little as 20mL in the pleural space.

Pneumothorax

Using ultrasound to evaluate for a pneumothorax is a relatively new concept but it is easy to learn. Pneumothoraces are common in trauma and more than half are missed on a supine chest radiograph.

Anatomy
Subxiphoid view of cardiac anatomy.


Parasternal long axis view of cardiac anatomy.



 
overview of potential intraabdominal and thoracic spaces.


Abnormal Findings
The purpose of the FAST exam is to find free fluid (usually blood) in the pericardial, pleural, or intraperitoneal spaces. Free fluid is jet black and tends to collect in the most dependant areas and surround the organs.

A good quality FAST can probably reliably detect about 200 mL of free intraperitoneal fluid. If good images of the pelvis are obtained, requiring more technical skill, even smaller volumes may be detected. Placing a patient in the Trendelenburg position improves the sensitivity for detecting free fluid in the Morison's pouch view.

Cardiac Views
Subxiphoid Four-Chamber View: Place the probe in the subxiphoid region with the marker-dot toward the patients’ right side or right shoulder. Angle the probe toward the left shoulder 

This view (Subxiphoid view) shows the right ventricle immediately adjacent to the left lobe of the liver 

















A pericardial effusion (black stripe between liver and right ventricle) will be easily recognized between the liver and the heart. Increasing the depth of the image and having the patient take a deep breath will improve chances of obtaining a good image.



Parasternal Long-Axis View: Place the probe just to the left of the sternum in about the 4th or 5th intercostal space, directly over the center of the heart, with the marker-dot toward the 4 o’clock position.
 

 This view shows the anterior and the posterior pericardium. Sliding the probe toward the cardiac apex (toward the 4 o’clock position) provides a good look at the apex. This view requires less depth and is easier to obtain in uncooperative patients.
 

Abdominal and Lower Thoracic Views: When a patient is in the supine position the most dependant area in the upper peritoneum is Morison's pouch (between the liver and right kidney) and the most dependant area in the lower peritoneum is posterior to the bladder in the male and the pouch of Douglas (posterior to the uterus) in the female

Right Coronal and Intercostal Oblique Views: The easiest abdominal view to obtain is the view of Morison’s pouch. To obtain this view place the probe in the mid-axillary line at about the 8th to 11th intercostal space with the marker-dot pointed cephalad. This gives a coronal view of the interface between the liver and kidney.
 

It is important to follow the lower edge of the liver caudally until a good view of the tip is obtained.

Free fluid is usually seen in Morison’s pouch or along the lower edge of the liver and around the lower tip of the liver. Rib shadows may be prominent when the marker-dot is pointed directly cephalad. Shadows can be minimized by rotating the probe very slightly counter-clockwise, so the marker-dot is pointed toward the posterior axilla and giving anintercostal oblique view.


Slide the probe cephalad to obtain a view of the diaphragm and look for pleural fluid 

Pleural fluid will appear as a jet black triangle just superior to the diaphragm
 

Also, this view may reveal free intraperitoneal fluid superior to the liver
 

between the liver, diaphragm and around the liver tip


Left Coronal and Intercostal Oblique Views: This is often the most difficult abdominal view to obtain. Place the probe in the posterior-axillary line at about the 6th to 9th intercostal space with the marker-dot pointed cephalad, producing a coronal view. 


From this position the interface between the spleen and left kidney can be found. Free fluid is rarely seen between the spleen and the kidney but rather surrounding all other parts of the spleen or between spleen and diaphragm. To get rid of rib shadows, and to get a better view of the spleen, slide the probe cephalad and rotate it very slightly clockwise, producing an intercostal oblique view, so that the spleen (not the kidney) is seen


The marker-dot will be pointed toward the posterior axilla. This view will allow good images of the lower tip and superior surface of the spleen, where intraperitoneal free fluid is most likely to collect. The diaphragm will also be seen in this view, just superior to the spleen


Pelvic Views: Pelvic views are not as easy to obtain as right upper quadrant views, but since the pelvis is the most dependent part of the peritoneal space, it is the most likely place to see abdominal free fluid. It is a good idea to obtain both longitudinal and transverse views of the pelvis. If the longitudinal view is performed first, it is often easier to understand the anatomy and obtain good images. Place the probe in the midline just cephalad to the pubic bone with the marker-dot pointed cephalad.



Make sure the probe position is correct by actually placing the probe on the pubic bone and noting a bone shadow on the image. From this position sliding the probe slightly cephalad will produce a good longitudinal pelvic view. The bladder will be found just cephalad to the pubic bone, and can usually be found even if it is nearly empty. A full bladder will be triangular in shape. The lower angle of the bladder marks the border between the intraperitoneal space (left side of the image) and the true pelvic structures (right side of the image).
In a male, free fluid will be seen along the intraperitoneal portion of the posterior to the wall of the bladder


In a female, the body of the uterus sits in the intraperitoneal space just posterior to the bladder


so free fluid will be seen just posterior to the uterus. This space is often called the pouch of Douglas and sometimes just small amounts can be detected


Free fluid may also be seen completely surrounding the edges of the uterus


If the bladder is empty, it is very difficult to recognize pelvic free fluid in a male. In a female, the pouch of Douglas may still be identifiable, even when the bladder is empty.

To obtain transverse views, simply rotate the probe 90 degrees, pointing the probe marker to the patients' right side


In transverse pelvic views, free fluid may be seen posterior to the bladder or uterus, or adjacent to the corners of the full bladder


Anterior Thoracic Views: When using ultrasound to evaluate for a pneumothorax, the probe is usually placed on the anterior chest in the 3-4th intercostal space and midclavicular line. This is a starting point and a likely place to find a pneumothorax when the patient is in the supine position. Subsequent imaging can be done on any part of the chest wall if there is concern for a very small or loculated pneumothorax.
A high frequency vascular/small parts probe can be used for this exam, but a standard curvilinear abdominal probe will also work well. The most important part about this exam is decreasing the depth setting, so that the ultrasound image shows a maximum depth of about 4 cm. The probe is placed in a longitudinal position with the marker-dot pointed cephalad.
In this orientation rib shadows can be used to find the pleural plane. It is best to adjust the probe linearly until two ribs are apparent, one on each side of the image. Between the ribs the pleural interface will be apparent at the posterior border of the ribs. It is important to anchor the probe and hold it very still while looking for the sliding motion of the visceral pleura against the parietal pleura. If the “sliding sign” is not present, a pneumothorax is suspected. Comparing one side of the chest to the other may be helpful.
  

Source

http://www.sonoguide.com/FAST.html


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