Robert Reardon, M.D.
I. Introduction and Indications
Many trauma patients have injuries that are not apparent on the initial physical exam. Patients can present with distracting injuries or altered mental status. Significant bleeding into the peritoneal, pleural, or pericardial spaces may occur without obvious warning signs. The purpose of bedside ultrasound in trauma is to rapidly identify free fluid (usually blood) in the peritoneal, pericardial, or pleural spaces.
Physicians in Germany and Japan began using routine bedside ultrasound for trauma patients in the 1970's.(1) In the United States emergency physicians started using this tool in the 1980’s (2,3) and it has now become the initial imaging test of choice for trauma care in the United States and is part of the Advanced Trauma Life Support (ATLS) protocol developed by the American College of Surgeons.
“FAST” is an acronym for Focused Assessment with Sonography in Trauma and has become synonymous with beside ultrasound in trauma.(14,15) The FAST exam, per ATLS protocoll, is performed immediately after the primary survey of the ATLS protocol. Ultrasound is the ideal initial imaging modality because it can be performed simultaneously with other resuscitative cares, providing vital information without the time delay caused by radiographs or computed tomography (CT). The concept behind the FAST exam is that many life-threatening injuries cause bleeding. Although ultrasound is not 100% sensitive for identifying all bleeding, it is nearly perfect for recognizing intraperitoneal bleeding in hypotensive patients who need an emergent laparotomy and for diagnosing cardiac injuries from penetrating trauma.(6-9) Recently, research studies have shown that bedside ultrasound is equivalent to, or better than, chest radiography for identifying a hemothorax or pneumothorax in trauma patients.(10-13) For this reason some trauma centers have begun performing an extended FAST exam (EFAST), evaluating for pneumo- and hemothorax in addition to intraperitoneal injuries.(10,11,16)
Emergency physicians are trained to practice in a variety of different settings. Depending on the skill of the operator and the practice setting, the FAST exam can be used in a variety of different ways to guide clinical decision-making.(16,17) Physicians should understand the potential applications of trauma ultrasound as well as the common pitfalls and their own technical limitations. It is important to recognize the imperfect nature of such exams, but sonographers who master this challenge, will find it an invaluable tool in the care of trauma patients.(16-18)
Penetrating Cardiac Trauma
Bedside ultrasound performed by emergency physicians significantly decreases mortality in patients with penetrating cardiac injuries.(9) 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 “classic” signs of Beck’s triad are not commonly present and are difficult to appreciate by physical exam alone. The key to managing penetrating chest trauma is to identify a developing pericardial effusion as early as possible, before tamponade and cardiac arrest occurs. All patients with penetrating chest injury should be “screened” for a potential pericardial effusion. If an effusion is present cardiac injury is assumed until proven otherwise and the patient should go directly to the operating room for a pericardial window or sternotomy.
Blunt Cardiac Trauma
Significant blunt cardiac injury is relatively uncommon. Most patients who suffer severe cardiac injury such as rupture of the free ventricular wall die quickly. One research report described patients with blunt cardiac rupture who were rapidly diagnosed and aggressively managed because of early bedside ultrasound. The authors stressed the importance of prompt cardiac ultrasound in all patients with significant blunt chest trauma.(19) 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.(19)
Blunt Abdominal Trauma
During the last decade, the most commonly studied use of the FAST exam and most of the trauma ultrasound research was performed on patients with blunt abdominal injuries. 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. The FAST exam is an ideal initial screening modality for early recognition of intraperitoneal blood since it is rapid, safe and sensitive and can be repeated if the patient’s status changes.
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.(4,23-25) In addition it can be used to help prioritize initial management in patients with multiple penetrating injuries or an unknown missile trajectory.(26) Within minutes it allows clinicians to know to concentrate initial efforts on a cardiac, chest and or intraperitoneal injury.
The sensitivity of the FAST exam for determining the need for laparotomy is only about 50%.(27) Bowel injuries are very common in penetrating trauma and the FAST exam does not detect most of these injuries. Some clinicians think that this low sensitivity makes the FAST exam less useful in penetrating trauma, but others advocate it as a valuable tool to help assess for significant hemoperitoneum and to help prioritize management when multiple penetrating injuries are present.(4,5,17,24,26)
Bleeding into the pleural space, called a hemothorax, is common in both blunt and penetrating trauma. It can usually be managed with placement of a simple chest tube.
About 200 mL of pleural fluid is required before it can be detected with a plain CXR.(29)
Ultrasound is much more sensitive for detecting pleural fluid and can identify as little as 20mL in the pleural space.(30) It was found to be equivalent to CXR in detecting hemothoraces in trauma and also showed to be a much quicker procedure, taking about 1 minute versus 15 minutes for chest radiography.(12,13)
Chest radiographs are still necessary in trauma patients to evaluate the mediastinum, lung parenchyma and several other anatomic features. Ultrasound can be used during the initial minutes of the trauma evaluation to determine if urgent chest tube placement is necassary. A chest radiograph can then be obtained after chest tube placement. This approach saves valuable time when managing an unstable multiple-trauma patient.(13,17,28)
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.(31) Bedside ultrasound has been shown to be equal or more sensitive than CXR for detecting this lung injury.(10,11,31-34)
Using ultrasound to look for occult pneumothoraces is most important in situations where missing one could result in significant deterioration, especially patients requiring positive pressure ventilation or helicopter transport.
Computed Tomography (CT) has many advantages over the FAST exam.
CT of the abdomen is better than ultrasound for showing parenchymal injury and the source of intraperitoneal bleeding. CT is useful to differentiate solid organ injury from bowel injury or other causes of hemoperitoneum and it is far superior demonstrating retroperitoneal bleeding. Modern CT scanners can also use abdominal/pelvic images to reconstruct bone windows and rule-out fractures of the spine and pelvis. Unfortunately, CT is very expensive, exposes patients to radiation and usually requires a bolus of IV contrast material. Because of these problems ultrasound will always have certain advantages over CT.
When is Trauma Ultrasound Most Useful?
Since CT has better accuracy for diagnosing torso injuries, the FAST exam is most useful in situations where CT is not practical due to time constraints or when CT scan can be reasonably avoided.
Clinical scenarios where the FAST is most useful:
- Hemodynamically unstable patients, when the cause of hypotension is unclear.
- Patients who need an emergent bedside procedure.
- Patients at a community hospital who require transfer to a trauma center. Consider pericardiocentesis if a pericardial effusion is found, consider early blood transfusion for significant hemoperitoneum, and consider a chest tube if a hemothorax or pneumothorax is discovered, especially if aeromedical transport is planned
- Intoxicated patients who can be observed and re-examined.
- Patients with penetrating trauma with multiple wounds or unclear trajectory, especially with wounds in upper abdomen or lower chest
- Patients with a concerning mechanism of injury but no indication for CT. Consider a period of observation and serial FAST exams.
See illustrations 1a,b and 2 for overview of anatomical structures examined with the FAST scan:
Illustration 1a: Subxiphoid view of cardiac anatomy. Illustration 1b:
Parasternal long axis view of cardiac anatomy.
Illustration 2a & Illustration
This shows an overview of potential intraabdominal and thoracic spaces.
These spaces are examined during the FAST exam to detect blood from organ or vascular injuries.
IV. Scanning Technique and Normal Findings
The FAST exam is often the first ultrasound exam that a novice clinician-sonographer will learn. Of course it is important to know relevant anatomy and have a good understanding of the standard scanning planes. Modern grayscale (B-mode) ultrasound images are 2 dimensional representations. Comprehensive ultrasound studies require scanning of every organ in 2 different planes, each plane at a 90-degree angle to the other. Fortunately, the FAST exam can be effectively performed with limited scanning planes, since we are only trying to find free fluid and not do a comprehensive survey of the involved organs. This approach makes the FAST exam easier to learn and less time consuming. The exam is performed in the supine position, normal findings show regular anatomy and no intraperitoneal or intrathoracic fluid.
V. 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 Learning to perform the FAST exam simply involves learning how to visualize the heart, diaphragms, liver, spleen and bladder. Interpretation of the FAST exam involves learning where free fluid commonly collects adjacent to these organs.
The volume of intraperitoneal blood that can be detected using the FAST exam depends on the skill of the operator and which views are obtained. To optimize sensitivity to detect the smallest amount of free fluid possible, it is important to obtain good images of multiple intraperitoneal sites.(20) A good quality FAST can probably reliably detect about 200 mL of free intraperitoneal fluid.(1) If good images of the pelvis are obtained, requiring more technical skill, even smaller volumes may be detected.(20,21) Placing a patient in the Trendelenburg position improves the sensitivity for detecting free fluid in the Morison's pouch view.(22) Trendelenburg positioning is reasonable when the pelvic view is indeterminate or difficult to visualize. Overall, the FAST exam is about 90% sensitive for detecting any amount of intraperitoneal free fluid. (4) As noted previously, and most importantly, the FAST exam is nearly perfect for detecting intraperitoneal bleeding that causes shock and requires an emergent laparotomy.(6,7)
There are two different cardiac views that can be performed with the FAST exam. One of the two is usually sufficient to evaluate for a pericardial effusion. It is important to learn both views, because one of the views may be easily obtained and the other impossible in any given patient.
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 (Figure 1). This view shows the right ventricle immediately adjacent to the left lobe of the liver (Figures 2, video clip 1). A pericardial effusion will be easily recognized between the liver and the heart (Video clip 2). Increasing the depth of the image and having the patient take a deep breath will improve chances of obtaining a good image.
Video clip 1
Figure 1: Position of the ultrasound
probe for the subxiphoid view.
Figure 2: Subxiphoid view. Video clip 1: Subxiphoid view.
Video clip 2: Pericardial effusion (black stripe between liver and right ventricle).
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 (Figure 3). This view shows the anterior and the posterior pericardium (Video clip 3). 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.
Video clip 3
Figure 3: Parasternal long axis-view. Video clip 3: Parasternal long axis-view.
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 (see also illustration 2).
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 (Figure 4). This gives a coronal view of the interface between the liver and kidney (Figure 5). It is important to follow the lower edge of the liver caudally until a good view of the tip is obtained (Figure 6).
Figure 4: Shows probe position. Figure 5 and 6: Morison’s pouch view with focus on the liver tip (6).
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 (Figures 7-9 and
video clip 4). 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 an intercostal oblique view.
Figure 7 – 9 and video clip 4: Right upper abdominal view with fluid in Morison’s pouch.
Slide the probe cephalad to obtain a view of the diaphragm and look for pleural fluid (Figures 10, 11). Pleural fluid will appear as a jet black triangle just superior to the diaphragm (Figure 12). Also, this view may reveal free intraperitoneal fluid superior to the liver (Figure 13), between the liver, diaphragm and around the liver tip (Figure 14).
Figure 10: Probe position for left sided pleural fluid evaluation. Figure 11: Normal view right pleura and lung.
Figure 12: Pleural fluid (red). Figure 13: Positive FAST scan with fluid between superior aspect of liver and diaphragm. Figure 14: Positive FAST with fluid at superior, anterior and inferior margin of the liver.
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 (Figure
15 shows the probe position, figure 16 a normal left upper quadrant - LUQ
- FAST view).
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 (Figure 17). A pleural effusion will appear as a jet black stripe or triangle just superior to the diaphragm (Video clip 5).
Figure 15: Probe position for LUQ FAST. Figure 16: Normal perisplenic view.
Video clip 5
Figure 17: Fluid surrounding the spleen. Video clip 5: Pleural effusion next to left diaphragm and 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.
Figure 18: Probe position for longitudinal view of the bladder.
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 (Video clip 6).
Video clip 6: Significant amount of free fluid posterior to the bladder.
In a female, the body of the uterus sits in the intraperitoneal space just posterior to the bladder (Figure 19), 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 (Figure 20). Free fluid may also be seen completely surrounding the edges of the uterus (Figure 21). 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.
Figure 19: Normal longitudinal view of bladder and uterus.
Figure 20: Small amount of free fluid in pouch of Douglas. Figure 21: Large amount of free fluid (black) surrounding the uterus.
To obtain transverse views, simply rotate the probe 90 degrees, pointing the probe marker to the patients' right side (Figure 22). In transverse pelvic views, free fluid may be seen posterior to the bladder or uterus, or adjacent to the corners of the full bladder (Figure 23).
Figure 22: Probe position for transverse pelvic view. Figure 23: Small amount
of free fluid posterior to the 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.
Video clip 7 shows this comparison between a normal “sliding sign” and an abnormal anterior thoracic view without pleural movement. Occasionally one may visualize the lead point of a pneumothorax, with visceral and parietal pleural movement inferior to an area without movement (Video clip 8).
Video clip 7
Video clip 8
Video clip 7: Shows normal “lung sliding” in its first part. The second part of the clip shows an abnormal chest view without lung sliding, suspicious for a pneumothorax. Video clip 8: Visceral and parietal pleural movement shows the lead point of a pneumothorax.
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