By the latter half of the 1990s, for patients who had sustained blunt or penetrating abdominal trauma, the FAST examination's utility for detecting free intraperitoneal fluid had been universally recognized. While CT remained the gold standard for detecting specific intra-abdominal pathology, the FAST examination had gained acceptance as a rapid screening tool for identifying free intraperitoneal fluid.
During the 1980s, surgeons in Germany developed bedside utilization of ultrasonography for evaluation of trauma patients. Although excellent results were reported in early studies, with the sensitivity ranging from 84 to 100% and the specificity from 88 to 100%, these findings went largely unnoticed in the United States in part since the articles were not initially translated into English.2–7
In the 1990s, a number of prospective studies (with study sizes greater than 100 patients) had been reported on this issue in the English literature.1, 8–19 The majority of these studies focused on the FAST examination for the evaluation of free intraperitoneal fluid in blunt abdominal trauma patients only. These studies reported the sensitivity and the specificity to range from 69% to 90% and 95% to 100%, respectively.
Tiling et al. were the first investigators to suggest that the FAST examination could provide comprehensive evaluation for significant areas of hemorrhage, including pericardial, pleural, intraperitoneal, and retroperitoneal. Their prospective study of 808 blunt trauma patients found a sensitivity of 89% and a specificity of 99% for free intraperitoneal fluid. Their clinical algorithm incorporates the FAST examination during the initial patient evaluation (Figure 5-1).1
In one of the first North American trauma ultrasound studies, Rozycki et al. demonstrated the FAST examination to have an overall sensitivity of 79% and specificity of 95.6%. They concluded that appropriately trained surgeons could rapidly and accurately perform and interpret FAST examinations and that ultrasound was a rapid, sensitive, and specific diagnostic modality for detecting intraperitoneal fluid and pericardial effusion.10 In another study, they successfully used ultrasound as the primary adjuvant modality to detect hemoperitoneum and pericardial effusion in injured patients. In the FAST examinations of 371 patients, they had 81.5% sensitivity and 99.7% specificity. The authors stated that ultrasound should be the primary adjuvant instrument for the evaluation of injured patients because it was rapid, accurate, and potentially cost-effective.11
In 1995, Ma and Mateer prospectively demonstrated that the FAST examination could serve as a sensitive, specific, and accurate diagnostic tool in the detection of free intraperitoneal and thoracic fluid in patients who had sustained major blunt or penetrating trauma. Overall, the FAST examination had a sensitivity of 90%, specificity of 99%, and accuracy of 99%. In evaluating the subgroup of blunt trauma patients, which consisted of 165 of the 245 patients, the FAST examination was 90% sensitive, 99% specific, and 99% accurate. In evaluating the subgroup of penetrating trauma victims, which consisted of 80 of the 245 patients, the FAST examination was 91% sensitive, 100% specific, and 99% accurate.12 Since emergency physicians performed all the FAST examinations, it became the first prospective study to support that appropriately trained emergency physicians could accurately perform and interpret FAST examinations. The results reiterated that a FAST examination of the entire torso could successfully provide early and valuable information for the presence of free fluid in both the peritoneal and thoracic cavities. In addition, the FAST examination was found to be equally sensitive, specific, and accurate for both blunt and penetrating torso trauma. Penetrating trauma patients could benefit from the rapid and accurate information yielded by ultrasonography.18, 19, 24 The identification and localization of significant hemorrhage in penetrating trauma patients would allow physicians “to prioritize resources for resuscitation and evaluation.”10 Most studies have utilized a multiple view FAST examination for evaluation of trauma patients. Some investigators have employed a single view technique, examining only Morison's pouch for free intraperitoneal fluid.29–31 In one study, all patients were placed in the Trendelenburg position and the perihepatic (Morison's pouch) was the single area examined. The results of this technique were reported to be 81.8% sensitive, 93.9% specific, and 90.9% accurate.29
The single-view (perihepatic) imaging technique was compared with the multiple-view technique of the FAST examination for the identification of free intraperitoneal fluid in patients who had sustained major blunt or penetrating torso trauma. For detecting free intraperitoneal fluid, when comparing the multiple-view FAST examination of the abdomen to the gold standard, the multiple-view FAST examination technique had a sensitivity of 87%, a specificity of 99%, and an accuracy of 98%. When comparing the perihepatic single view of the abdomen to the gold standard, the single view FAST examination technique had a sensitivity of 51%, a specificity of 100%, and an accuracy of 93%.13 Based on this and other studies, the more sensitive and accurate FAST examination method for detecting free intraperitoneal fluid was determined to be the multiple-view technique.13
Detection of Solid Organ Injury
The concept of contrast-enhanced ultrasound may assist clinicians to identify specific organ injuries on the FAST examination.32–35 Contrast enhanced ultrasound is the application of ultrasound contrast agents to complement or augment traditional sonography. Ultrasound contrast agents are microbubbles that are administered intravenously. The microbubbles vibrate strongly at the high frequencies used in diagnostic ultrasonography, which makes them several thousand times more reflective than normal body tissues. This characteristic allows microbubbles to enhance both grey scale images and flow-mediated Doppler signals. Microbubbles, which are filled with an inert gas, have been found to be as safe as conventional agents in radiography and magnetic resonance imaging.36 Preliminary reports have demonstrated contrast-enhanced ultrasound as a promising tool for detecting solid organ (liver and spleen) injuries after blunt abdominal trauma. When an ultrasound contract agent was administered immediately before performing the traditional FAST examination, studies have demonstrated that the examination correlated appreciably better than unenhanced sonography for detecting hepatic and splenic injuries and estimating the extent of their injuries.32–34 Contrast enhanced ultrasound may potentially guide clinicians with the nonoperative management of patients with solid organ injuries after blunt trauma.
Clinical pathways and protocols have been derived from the use of the FAST examination and incorporated with other diagnostic methods commonly used for trauma evaluation in North America (Figure 5-2). An ultrasound-based key clinical pathway has been shown to reduce the number of diagnostic peritoneal lavage procedures and CT scans required to evaluate blunt abdominal trauma without increased risk to the patient. Using the key clinical pathway, diagnostic peritoneal lavage procedures were reduced from 17 to 4%, and CT scans reduced from 56 to 26%. The injury severity score increased from 11.6 to 21.5 for diagnostic peritoneal lavage patients and from 4.6 to 8.3 for CT scan patients. FAST examinations were used exclusively in 65% of the patients. This ultrasound-based key clinical pathway was found to result in significant reductions in the utilization of diagnostic peritoneal lavage and CT scanning in the evaluation of blunt abdominal trauma without increased risk to the patient (Figure 5-3). The investigators estimated cost savings of $450,000 per year, using this key clinical pathway.37 The issue of cost savings of the FAST examination has also been addressed in another study. For blunt trauma patients, the FAST examination was found to be more efficient and cost-effective than CT scanning or diagnostic peritoneal lavage. There was a significantly shorter time to disposition at approximately one third the cost in the ultrasonography group.38
Suggested algorithm for the use of ultrasonography in the evaluation of the patient with blunt abdominal trauma. (From Rozycki GS, Shackford SR: Ultrasound: What every trauma surgeon should know. J Trauma 40:2, 1996, with permission.)
Key clinical pathway for the evaluation of blunt abdominal trauma. (From Branney SW, Moore EE, Cantrill S, et al.: Ultrasound-based key clinical pathway reduces the use of hospital resources for the evaluation of blunt abdominal trauma. J Trauma 42:1086–1090, 1997, with permission.)
An ultrasound-based scoring system has been developed to quantify the amount of intraperitoneal blood in blunt abdominal trauma patients and to assess the need for therapeutic exploratory laparotomy. Scores ranged from 0 to 8. The system assigned two points for significant fluid collections ≥2 cm and one point for fluid collections ≤2 cm. A score of 3 correlated with 1000 mL of fluid. In the study, of those patients who had a score of 3 or more, 24 of 25 patients (96%) required therapeutic laparotomy. Of those who had a score of less than 3, therapeutic laparotomy was required in only 9 of 24 patients (38%). The FAST examination was found to be a useful adjunct in helping to make clinical decisions during the resuscitation period.39 In another study evaluating the role of the FAST examination in determining the need for therapeutic laparotomy, none of the patients with negative FAST examination results died or sustained identifiable mortality as a consequence of their negative scans.40
Detection of Pericardial Fluid
In the hypotensive patient who has sustained penetrating trauma to the torso, the echocardiographic portion of the FAST examination may prove to be the most beneficial aspect. Echocardiography remains the gold standard diagnostic procedure for detecting pericardial effusions. The classic physical examination findings of acute cardiac tamponade—distended neck veins, hypotension, and muffled heart tones—are present in less than 40% of patients with surgically proven cardiac tamponade.41 Timely emergency department procedures and expeditious transportation of the patient to the operating room may be accomplished by ultrasound diagnosis of hemopericardium.
In 1992, Plummer and coinvestigators evaluated the effect of bedside echocardiography performed by emergency physicians on the outcome of 49 patients with penetrating cardiac injuries over a 10-year period. Compared to a retrospective control group, the use of bedside echocardiography significantly reduced the time of diagnosis and disposition to the operating room from 42.4 ± 21.7 minutes to 15.5 ± 11.4 minutes while the actual survival improved from 57.1 to 100%.42
The accuracy of emergency ultrasound has been evaluated after it was introduced into five Level I trauma centers for the diagnosis of acute hemopericardium. Surgeons or cardiologists (four centers) and technicians (one center) performed pericardial ultrasound examinations on patients with penetrating truncal wounds. By protocol, patients with positive examinations underwent immediate operation. In 261 patients, pericardial ultrasound examinations were found to have a sensitivity of 100%, specificity of 96.9%, and accuracy of 97.3%. The mean time from ultrasound to operation was 12.1 ± 5 minutes. This further demonstrated that ultrasound should be the initial modality for the evaluation of patients with penetrating precordial wounds because it is accurate and rapid.43
Over the years, numerous studies have examined the role of echocardiography in blunt cardiac trauma. The utility and role of ultrasound, particularly with the diagnosis of cardiac contusion, remain unclear (see chapter 5, “Cardiac,” for a comprehensive review of this topic).
Detection of Pleural Fluid
Since patients who have sustained major trauma routinely present to the emergency department immobilized on a long spine board, clinicians may have difficulty identifying bilateral hemothoraces or a small unilateral hemothorax on the initial supine chest radiograph. The FAST examination can detect hemothorax before the completion of a chest radiograph or can be used as additional information when the chest radiograph is equivocal.
Of the six anatomic areas scanned by the FAST examination, only two are required to identify the presence of free pleural fluid in the two pleural cavities. Thus, tube thoracostomy for trauma patients may be expedited with the use of ultrasonography.
Ma and Mateer demonstrated that the FAST examination could serve as a sensitive, specific, and accurate diagnostic tool in detecting hemothorax in major trauma patients. When comparing the FAST examination and the chest radiograph to the criterion standard definitions, both diagnostic tests had an equal sensitivity (96.2%), specificity (100%), and accuracy (99.6%) for detecting pleural fluid. They concluded that ultrasonography was comparable to the chest radiograph for identifying hemothorax.44
Ultrasonography can detect smaller quantities of pleural fluid than the chest radiograph. It is estimated that an upright chest radiograph can accurately detect a minimum of 50–100 mL of pleural fluid45 and that a supine chest radiograph can detect a minimum of 175 mL of pleural fluid.46 By contrast, it is estimated that ultrasonography can detect a minimum of 20 mL of pleural fluid.9 Also, ultrasonography can help differentiate between pleural fluid and pleural thickening or pulmonary contusion when the supine chest radiograph is equivocal.
Although the FAST examination cannot completely replace the chest radiograph, it can complement chest radiograph findings by rapidly identifying hemothorax in the supine patient. By utilizing the FAST examination to initially identify hemothorax, the standard chest radiograph of the trauma patient can be performed after tube thoracostomy, thereby sparing the patient an additional chest radiograph.
Detection of Pneumothorax
Not only can the FAST examination detect a hemothorax, it can also identify a pneumothorax before the completion of a chest radiograph. This is especially relevant since the reported proportion of pneumothoraces that are occult compared to those actually seen on the chest radiograph ranges from 29 to 72%.47–52 The concept of using ultrasound to exclude the presence of a pneumothorax relies on the simple premise that if the two pleural surfaces are normally in apposition, then an intrapleural collection of air (pneumothorax) cannot be separating them. The focused goal is to identify the contiguity of the visceral and parietal pleura using simple sonographic signs to exclude the presence of a pneumothorax. This diagnostic test is considered to be an extended FAST (EFAST) examination.51 With experience, a pneumothorax can be expeditiously excluded in the vast majority of cases.53
Detecting a pneumothorax with ultrasound may initially appear to be paradoxical since air has the highest acoustic impedance of normal body substances, with almost complete reflectance of sounds waves at commonly used frequencies.54 Thus, only artifacts are normally seen deep to the pleural interface in the normal lung.55 Both hemothoraces and pneumothoraces, however, are superficial pleural-based diseases and, therefore, lend themselves to sonographic examination.
Unless there are pleural adhesions from previous disease or injury (a condition that reduces the risk of pneumothorax), normal respiration is associated with a physiologic sliding of the two pleural surfaces upon one another. The most common normal sign on sonography, which in essence excludes the presence of a pneumothorax, is lung sliding.48, 51, 53 For this physiologic “lung sliding” along the visceral–parietal pleural interface to be seen, both surfaces must be accessible for imaging and must be either contiguous or separated by a layer of fluid.50, 51, 56 This movement is better visualized at the lung bases and less so at the apices.53 A notable exception is when subcutaneous emphysema superimposes itself between the skin and parietal pleura, a clinical situation that is associated with a specificity of 98% for underlying occult pneumothorax.57 Examining the pleural interfaces with the color power Doppler mode can enhance the depiction of this sliding movement due to power Doppler's ability to detect motion, a finding that has been designated the “Power Slide.”49 Power Doppler is superior to conventional color Doppler in determining the presence or absence of flow at the expense of direction and speed information and thus has the ability to identify low-velocity and low-volume flow (or motion).49, 58, 59 It also documents a real-time physiologic process as occurring in a single image, allowing for simpler archiving and tele-transmission. In a similar way, the use of M mode imaging documents either the presence of lung sliding (“seashore sign”) or its absence when a pneumothorax is present (“stratosphere sign”) since the pleural movement will normally generate a homogenous granular pattern in this mode.53, 60 An intermediate sign that is best documented in M mode is the “lung point” sign, which is visualized when the lung intermittently contacts the parietal pleura with inspiration, thus alternating between the seashore and stratospheric signs.
Another normal sign is the comet tail artifact, which is a reverberation artifact that arises from distended water-filled interlobular septae under the visceral pleura. Comet tail artifacts are presumed to be the ultrasound equivalents of Kerley B lines seen on a chest radiograph.53, 61 Being related to the visceral pleura, comet tail artifacts can be seen only when the visceral pleura is in apposition to the parietal pleura. When a pneumothorax is present, the intrapleural air will separate the two pleural surfaces, and the “normal” signs will not be seen. Instead, the only images that will appear to be deep to this level are horizontal reverberation artifacts, which are often seen as a “mirror image” of the chest wall.
Lichtenstein and coworkers have described a standardized, but hierarchal thoracic examination whose scope depends on the clinical status and mobility of the patient. They designate the A-line as a brightly echogenic line between the rib shadows recurring at an interval that exactly replicates the interval between the skin and pleural line, and represents the horizontal reverberation artifact generated by the parietal pleura.53
If both lung sliding and comet tail artifacts are present, then the clinician can confidently exclude the presence of a pneumothorax. If lung sliding and comet tail artifacts are not visible, then the examiner should suspect the presence of a pneumothorax, a suspicion further heightened by the presence of the horizontal reverberation artifact (A-line).62, 63 In 200 consecutive undifferentiated ICU patients who went on to CT scanning, Lichtenstein and coworkers were able to note absent lung sliding in all patients with occult pneumothoraces; 41 of 43 patients had an A-line sign. The absence of lung sliding alone had 100% sensitivity, but only 78% specificity for diagnosing an occult pneumothorax. When an A-line was seen with absent lung sliding, there was 95% sensitivity and 94% specificity for diagnosing an occult pneumothorax.60 The presence of a lung point had 100% specificity for an occult pneumothorax. Kirkpatrick and coinvestigators prospectively evaluated a handheld ultrasound device in the real-time resuscitation of critically ill patients.51 This study focused on the most difficult-to-diagnose subset of pneumothoraces, as any patient with a clinically evident pneumothorax was treated without any imaging and those patients with clear-cut pneumothoraces on chest radiograph were excluded as well. In the remaining clinically stable patients, when comparing EFAST directly to chest radiography, the EFAST examination was more sensitive for the detection of occult pneumothoraces after trauma (49% versus 21%). Using CT corroboration, there were 22 false negative studies with EFAST compared to 34 with chest radiography.51 Blaivas and coworkers studied 176 patients by systematically examining four thoracic locations, ranging from the 2nd intercostal space in the midclavicular line to the 6th intercostal space at the posterior axillary line. They searched for lung sliding, supplemented by color power Doppler when lung sliding was not easily detected, and assessed the relative size of the pneumothorax by correlating the relative topography of lung sliding. Overall, they noted the sensitivity and specificity of ultrasound to be 98% and 99%, respectively, compared with 76% and 100%, respectively, for chest radiography in this setting.64
Reflecting on basic principles, the EFAST technique has an inherent advantage over chest radiography due to the physiologic behavior of pneumothoraces in the supine patient. Because of the effect of gravity, the supine lung tends to hinge dorsally, with free air collecting anteromedially.65–69 Supine pneumothoraces are reported to be most commonly located at the anterior (84%), apical (57%), basal (41%), medial (27%), lateral (24%), and never posterior (0%) lung locations.70 The standard imaging anatomic sites for the EFAST were chosen to correspond to the recommended auscultatory locations from the Advanced Trauma Life Support course.71
Despite the fact that pneumothoraces are often dynamic processes, clinical management is often based on the perceived size of a pneumothorax. Allowing for other factors such as the need for transport and positive pressure ventilation, many small pneumothoraces are managed expectantly, whereas large ones are drained. Thus, it is pertinent to ask whether ultrasound may be of help in determining the size of pneumothoraces. While it was once believed that sonography was of no use in determining the volume of a pneumothorax,55 it is now suggested that sonography may actually have utility in determining not only the presence, but actual extent of a pneumothorax.68
The presence of a “lung point” sign is not only 100% specific for pneumothorax, but the location of this sign roughly correlates with the radiographic size of the pneumothorax.63 A “partial sliding” sign has been described to represent the same phenomenon whereby smaller or occult pneumothoraces might be detected.72 Also, a good correlation between the estimates of pneumothorax size and CT findings using the relative thoracic topography of lung sliding has been noted.64