Focused Abdominal Sonography for Trauma (FAST) can be used in both blunt and penetrating trauma.
It is most useful in undifferentiated hypotensive patients.
FAST will determine whether there is a large amount of free fluid in the abdomen. This is presumed to be blood from trauma and decisive measures to control the bleeding (generally laparotomy) can be undertaken. FAST includes a subcostal view of the heart to assess for pericardial fluid. The presence of a haemothorax can also be determined from the traditional FAST views.
Trauma patients are generally in the supine position and may be uncooperative. The FAST examination is included in the primary survey, but should not interrupt resuscitation.
The Four Views
Right Upper Quadrant
This view is is a longitudinal view of Morison's pouch, the recess in the right upper quadrant between liver and kidney. In the supine position dependent fluid will collect here. A potential space exists, and if there is free fluid a hypoechoic (black) strip is evident here.
Looking above the diaphragm will enable haemothorax detection (see EFAST)
Left Upper Quadrant
Dependent fluid also gathers in the left upper quadrant between spleen and kidney, although it can also gather above the spleen. Again a longitudinal view usually intercostally demonstrates the presence of absence of free fluid.
The pelvis is explored in transverse and longitudinal section. A full bladder is required to image the pelvis adequately. Free fluid adjacent to the bladder (posterior and above) appears black, and outlines the bladder and surrounding bowel.
The probe placed in the epigastrium and angled up towards the left should will usually demonstrate the subcostal view of the heart. Occasionally bowel / stomach prevent this view. A rim of hypoechoic fluid around the heart is pericardial fluid.
TIPS AND TRICKS
1. Ultrasound is a dynamic process - always fan through the area of interest rather than just taking one still picture. Fluid may collect in unusual areas - it may not have read the books!
2. If a patient is hypotensive from intra-abdominal bleeding 85% will have a positive RUQ view.
3. Placing the patient 5 degrees head down makes the upper abdominal views more sensitive.
4. For the RUQ view you can sometimes look subcostally, but usually an intercostal view is required.
5. If a patient has ascites the scan will be positive - put a needle into the fluid under guidance, and if it is blood stained then the scan is a true positive.
6. The LUQ view is more difficult. Place the probe almost on the bed and a rib space higher than on the right and you may be successful.
7. Free fluid has "corners". It lies between other rounded organs and leaves sharp edges. Fluid inside organs tends to be rounded - and in the bowel peristalsis will be evident.