|Focused assessment with sonography in trauma|
Focused assessment with sonography in trauma (commonly abbreviated as FAST) is a rapid bedside ultrasound examination performed by surgeons, emergency physicians, and paramedics as a screening test for blood around the heart (pericardial effusion) or abdominal organs (hemoperitoneum) after trauma.
The four classic areas that are examined for free fluid are the perihepatic space (including Morison's pouch or the hepatorenal recess), perisplenic space, pericardium, and the pelvis. With this technique it is possible to identify the presence of intraperitoneal or pericardial free fluid. In the context of traumatic injury, this fluid will usually be due to bleeding.
The extended FAST (eFAST) allows for the examination of both lungs by adding bilateral anterior thoracic sonography to the FAST exam. This allows for the detection of a pneumothorax with the absence of normal ‘lung-sliding’ and ‘comet-tail’ artifact (seen on the ultrasound screen). Compared with supine chest radiography, with CT or clinical course as the gold standard, bedside sonography has superior sensitivity (49–99% versus 27–75%), similar specificity (95–100%), and can be performed in under a minute. Several recent prospective studies have validated its use in the setting of trauma resuscitation, and have also shown that ultrasound can provide an accurate estimation of pneumothorax size. Although radiography or CT scanning is generally feasible, immediate bedside detection of a pneumothorax confirms what are often ambiguous physical findings in unstable patients, and guides immediate chest decompression. In addition, in the patient undergoing positive-pressure ventilation, the detection of an otherwise ‘occult’ pneumothorax prior to CT scanning may hasten treatment and subsequently prevent development of a tension pneumothorax, a deadly complication if not treated immediately, and deterioration in the radiology suite (in the CT scanner).
There are five components to the eFAST exam:
eFAST (extended focused assessment with sonography for trauma) allows an emergency physician or a surgeon the ability to determine whether a patient has pneumothorax, hemothorax, pleural effusion, mass/tumor, or a lodged foreign body. The exam allows for visualization of the echogenic tissue, ribs, and lung tissue. Few radiographic signs are important in any trauma and they include the stratosphere sign, the sliding or seashore sign, and the sinusoid sign.
Stratosphere sign is a clinical medical ultrasound finding usually in an eFAST examination that can prove presence of a pneumothorax. The sign is an imaging finding using a 3.5–7.5 MHz ultrasound probe in the 4th and 5th intercostal spaces in the anterior clavicular line using the M-Mode of the machine. This finding is seen in the M-mode tracing as pleura and lung being indistinguishable as linear hyperechogenic lines and is fairly reliable for diagnosis of a pneumothorax. Even though the stratospheric sign can be an indication of pneumothorax its absence is not at all reliable to rule out pneumothorax as definitive diagnosis usually requires X-ray or CT of thorax.
Seashore sign is another eFAST finding usually in the lungs in the M-mode that depicts the glandular echogenicity of the lung abutted by the linear appearance of the visceral pleura. This sign is a normal finding. In absence of a seashore sign or presence of a stratosphere sign, pneumothorax is likely. B-lines or "comet trails" are echogenic bright linear reflections beneath the pleura that are usually lost with any air between the probe and the lung tissue and therefore whose presence with seashore sign indicates absence of a pneumothorax.
Sinusoid sign is another M-mode finding indicating presence of pleural effusion. Due to the cyclical movement of the lung in inspiration and expiration, the motion-time tracing (M-mode) ultrasound shows a sinusoid appearance between the fluid and the line tissue. This finding indicates a possible pleural effusion, empyema, blood in pleural space (hemothorax).
FAST is less invasive than diagnostic peritoneal lavage, involves no exposure to radiation and is cheaper compared to computed tomography, but achieves a similar accuracy.
Numerous studies have shown FAST is useful in evaluating trauma patients. It also appears to make emergency department care faster and better.
FAST is most useful in trauma patients who are hemodynamically unstable. A positive FAST result is defined as the appearance of a dark ("anechoic") strip in the dependent areas of the peritoneum. In the right upper quadrant this typically appears in Morison's Pouch (between the liver and kidney). This location is most useful as it is the place where fluid will collect with a supine patient. In the left upper quadrant, blood may collect anywhere around the spleen (perisplenic space). In the pelvis, blood generally pools behind the bladder (in the rectovesicular space). A positive result suggests hemoperitoneum; often CT scan will be performed if the patient is stable or a laparotomy if unstable. In those with a negative FAST result, a search for extra-abdominal sources of bleeding may still need to be performed.