Author
Lars Grimm, MD, MHS
House Staff
Department of Internal Medicine
Duke University Medical Center
Durham, North Carolina
Disclosure: Lars Grimm, MD, MHS, has disclosed no relevant financial relationships.
Editor
Eugene C. Lin, MD
Consulting Radiologist, Virginia Mason Medical Center
Clinical Assistant Professor of Radiology, University of Washington
School of Medicine
Seattle, Washington
Eugene C. Lin, MD, has disclosed no relevant financial relationships.
The focused assessment with sonography for trauma, or FAST, is a limited bedside ultrasound examination that seeks to quickly detect free intraabdominal fluid or cardiac complications. The extended FAST, or E-FAST, expands the examination to assess for pneumothorax. The primary indications for performing a FAST are blunt or penetrating trauma, trauma in pregnancy, or hypotension of unclear etiology. A FAST helps determine which patients require emergent laparotomy and which can be monitored or await slower, more definitive studies. Image courtesy of Wikimedia Commons.
The benefits of a FAST examination compared with the traditional diagnostic peritoneal lavage or abdominal computed tomography scan are numerous and listed above. Performance of a FAST should not delay more definitive treatment options when necessary. A positive FAST examination is usually an indication for laparotomy, but unfortunately a negative FAST cannot rule out the need for laparotomy.
The drawbacks of a FAST examination are listed. The specificity for the detection of free fluid from published meta-analyses ranges from 98%-100%, but sensitivity ranges are lower from 73%-88%. These results are typically lower for children, with a specificity of 94% and sensitivity of 58%. As a result, if clinically suspected, a negative FAST examination may still warrant additional intervention.[1,2]
The philosophy behind the FAST examination is that fluid will pool in the most dependent areas. As a result, the FAST examination includes 3 views that can detect pooled blood (red lines) and one to evaluate the heart: the hepatorenal recess, the perisplenic view, the subxiphoid pericardial window, and the suprapubic window. A standard 3.5-MHz convex probe is typically used.
To evaluate the hepatorenal pouch, or Morison pouch, the transducer probe should be placed in the right upper quadrant at the mid-axillary line between the ninth and 11th ribs (shown). The liver is used as an acoustic window and helps avoid the air-filled bowel. The probe can be moved inferiorly along the liver to obtain better views of the right kidney. Both oblique and coronal views should be used.
Pointing the probe toward the right posterior axilla allows visualization of the liver (L), kidney (K), diaphragm (D), and Morison's pouch (arrow). Morison's pouch is the space between the liver and right kidney. In supine patients, free intraperitoneal fluid from the right upper quadrant will accumulate first in Morison's pouch. In a normal examination, the pouch is free of fluid and appears as hyperechoic (arrow shown). Getting a proper window for viewing may be difficult in patients who are obese or who have subcutaneous air.
In cases of hemoperitoneum, blood may appear as an anechoic stripe (arrow shown). Fluid volumes of 200 mL have been reported to be detectable via ultrasound. Findings on a FAST examination must be placed in the appropriate clinical context because the detected fluid may be blood, ascites, urine, or peritoneal dialysate. L = liver; K = kidney.
The perisplenic view is obtained by placing the probe over the left flank, lateral to the spleen along the posterior axillary line between the ninth and 11th ribs (shown). Sliding the probe superiorly and inferiorly will help detect free-fluid above the spleen and along the spleen tip. If the ribs interfere with the imaging, then a linear array probe may be more useful.
In this view, the diaphragm, spleen, and left kidney can all be seen. The spleen itself is used as an acoustic window. This can make depiction of the left kidney more difficult because the spleen is smaller than the liver. A normal perisplenic view is shown with the spleen (S) and kidney (K) identified and no anechoic blood or fluid identified.
In the left perisplenic view, fluid in the subphrenic space and splenorenal recess can be detected. The image shown demonstrates blood (arrow) between the spleen (S) and diaphragm (D). Although not a focus of the FAST examination, solid organ injury may be identified in the course of looking for free fluid, but typically needs to be confirmed with alternative methods.
The pelvis is the most dependent part of the peritoneal cavity. To obtain the suprapubic view, the probe should be placed just above the pubic symphysis and directed inferiorly. The will allow visualization of the rectovesicular space, or the rectouterine Pouch of Douglas in women. The bladder should be left full if possible to provide a better window, thus insertion of a Foley catheter should be delayed. Both sagittal and transverse views should be obtained.
In the suprapubic view, the bladder and prostate or uterus can all be identified. A normal transverse suprapubic view is shown with the bladder (B) filled with urine. No fluid is seen adjacent to the bladder. A partially filled bladder can be distinguished from free fluid by draining the bladder and repeating the examination. Unfortunately, a FAST scan cannot demonstrate the retroperitoneum, which may hide a significant volume of hemorrhage.
The subxiphoid view is obtained by placing the probe in the subxiphoid space directed toward the left shoulder. This will allow viewing of the diaphragm and a 4-chamber view of the heart. This view may be uncomfortable for many patients because it requires significant pressure on the upper abdomen to perform.
The subxiphoid view allows for imaging of all chambers of the heart and the pericardium. The right ventricle (RV), left ventricle (LV), right atrium (RA), left atrium (LA), tricuspid valve (TV), and mitral valve (MV) are all labeled in the image shown. If the subxiphoid view is difficult to obtain because of bowel gas, then gentle, graded compression with the transducer may help to move the bowel out of the way.
A number of different cardiac parameters can be quickly assessed via the subxiphoid view on a FAST examination. Volume status can be assessed by measuring the diameter of the inferior vena cava. Left ventricular function can be assessed by estimating the ejection fraction. Pericardial effusions can be detected by identifying an anechoic stripe between the epicardium and the pericardium. The subxiphoid image shown demonstrates a traumatic tamponade (arrow) overlying the heart, leading to collapse of the right ventricle.
The E-FAST allows for the detection of pneumothorax by looking for sliding between the pleural layers during inspiration. The parietal pleura (Pp) and visceral pleura (Pv) appear as echogenic lines. During inspiration they should glide back and forth, termed the gliding sign. If there is a pneumothorax, the gliding sign will be absent. This must be viewed in real time. The images shown were from a normal lung, with the lung (L) and ribs (arrowheads) also noted. The sensitivity for the detection of pneumothorax with ultrasound is 86%-98%, which is better than that for supine anteroposterior chest radiographs (28%-75%). The specificity for both approaches 100%.[3]
Author
Lars Grimm, MD, MHS
House Staff
Department of Internal Medicine
Duke University Medical Center
Durham, North Carolina
Disclosure: Lars Grimm, MD, MHS, has disclosed no relevant financial relationships.
Editor
Eugene C. Lin, MD
Consulting Radiologist, Virginia Mason Medical Center
Clinical Assistant Professor of Radiology, University of Washington
School of Medicine
Seattle, Washington
Eugene C. Lin, MD, has disclosed no relevant financial relationships.
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