Developed for the evaluation of injured patients, the Focused Assessment for the Sonographic Examination of the Trauma Patient (FAST) is a rapid diagnostic examination to assess patients with potential injuries to the torso. The test sequentially surveys for the presence or absence of fluid in the pericardial sac and in the dependent abdominal regions, including Morison’s pouch region in the right upper quadrant (RUQ), the left upper quadrant (LUQ) behind the spleen and between the spleen and kidney, and the pelvis posterior to the bladder. Surgeons can perform the FAST during the primary or secondary survey of the American College of Surgeons Advanced Trauma Life Support19 algorithm and, although minimal patient preparation is needed, a full urinary bladder is ideal to provide an acoustic window for visualization of blood in the pelvis.
Blood, as any fluid, will accumulate in dependent regions of the abdomen.20 In the supine position, this corresponds to Morison’s pouch, the splenorenal recess, and above the spleen as well as in the pelvis posterior to the bladder. All these regions may be visualized rapidly and dependably with the FAST. Furthermore, ultrasound is an excellent modality for the detection of intra-abdominal fluid, having been shown to detect ascites in small amounts.21,22 Although the exact minimum amount of intraperitoneal fluid that can be detected by ultrasound is not known,23 most authors agree that it is a sensitive modality.
The FAST is performed in a specific sequence for several reasons. The pericardial area is visualized first so that blood within the heart can be used as a standard to set the gain (Table 16-1). Most modern ultrasound machines have presets so that the gain does not need to be reset each time the machine is turned on. Occasionally, if multiple types of examinations are performed with different transducers, the gain should be checked to ensure that intracardiac blood appears anechoic. This maneuver ensures that a hemoperitoneum will also appear anechoic and will be readily detected on the ultrasound image. The abdominal part of the FAST begins with a survey of the RUQ that is the location within the peritoneal cavity where blood most often accumulates and is most readily detected with the FAST. Indeed, investigators from four Level I trauma centers examined true-positive ultrasound images of 275 patients who sustained either blunt (#220) or penetrating (#55) injuries.24 They found that regardless of the injured organ (with the exception of those patients who had an isolated perforated viscus), blood was most often identified on the RUQ image of the FAST. This can be a time-saving measure because when hemoperitoneum is identified on the FAST examination of a hemodynamically unstable patient, that image alone, in combination with the patient’s clinical picture, is sufficient to justify an immediate abdominal operation.24 In a stable patient, following the exam of the RUQ, the LUQ and pelvis are visualized as discussed below.
Ultrasound transmission gel is applied on four areas of the thoracoabdomen, and the examination is conducted in the following sequence: the pericardial area, RUQ, LUQ, and the pelvis (Fig. 16-2).
Schematic diagram of transducer positions for FAST: pericardial, right upper quadrant, left upper quadrant, and pelvis.
A 3.5-MHz convex transducer is oriented for sagittal or longitudinal views and positioned in the subxiphoid region to identify the heart and to examine for blood in the pericardial sac. The normal and abnormal views of the pericardial area are shown in Fig. 16-3. The subxiphoid image is usually not difficult to obtain, but a severe injury to the chest wall, a very narrow subcostal area, subcutaneous emphysema, or morbid obesity can prevent a satisfactory examination.25 Both of the latter conditions are associated with poor imaging because air and fat reflect the wave too strongly and prevent penetration into the target organ.14 If the subcostal pericardial image cannot be obtained or is suboptimal, a parasternal ultrasound view of the heart should be performed (Figs. 16-4 and 16-5).
(Left) Sagittal view of pericardial area showing pericardium as single echogenic line (normal). (Right) Sagittal view of pericardial area showing separation of visceral and parietal areas of pericardium with blood (arrow) that appears anechoic.
Transducer position for left parasternal view of heart.
Normal (left) and abnormal (right) heart, parasternal view.
Next, the transducer is placed in the right anterior or midaxillary line between the 11th and 12th ribs to obtain sagittal images of the liver, kidney, and diaphragm (Fig. 16-6) and determine the presence or absence of blood in Morison’s pouch and in the right subphrenic space. Next, attention is turned to the LUQ. With the transducer positioned in the left posterior axillary line between the 10th and 11th ribs, the spleen and left kidney are visualized and the presence or absence of blood between the two organs and in the left subphrenic space is determined (Fig. 16-7). The splenic window is often the most difficult window to adequately visualize and the probe should be placed significantly more posterior (posterior axillary line) and superior (one to two rib spaces higher) than with the RUQ window.
(A) Normal sagittal view of liver, kidney, and diaphragm. Note Gerota’s fascia is hyperechoic. (B) Abnormal sagittal view of liver, kidney, and diaphragm. Note fluid (blood) in between liver and kidney (arrows).
(Left) Normal sagittal view of spleen, kidney, and diaphragm. (Right) Abnormal sagittal view of spleen, kidney, and diaphragm with fluid (blood) in between spleen and kidney and above the spleen in the subphrenic space.
Finally, the transducer is directed for a transverse view and placed about 4 cm superior to the symphysis pubis. It is slowly swept inferiorly to obtain a coronal view of the full bladder and the pelvis examining for the presence or absence of blood (Fig. 16-8).
(Left) Normal coronal view of full urinary bladder. (Right) Abnormal coronal view of full bladder with fluid in pelvis. (Note the bowel floating in fluid.)
Improper technique, inexperience of the examiner, and inappropriate use of ultrasound have long been known to adversely impact the accuracy of ultrasound imaging. More recently, the etiology of injury, the presence of hypotension on admission, and select associated injuries have also been shown to influence the accuracy of this modality.2,3,8 Failure to consider these factors has led to inaccurate assessments of the accuracy of the FAST by comparing it inappropriately to a CT scan and not recognizing its role in the evaluation of patients with penetrating torso trauma.26,27 Both false-positive and -negative pericardial ultrasound examinations have been reported to occur in the presence of a massive hemothorax or mediastinal blood.4,8,10,28 Repeating the FAST after the insertion of a tube thoracostomy improves the visualization of the pericardial area and decreases the number of false-positive and -negative studies. While false studies may occur, a rapid focused ultrasound survey of the subcostal pericardial area is a very accurate method to detect hemopericardium in most patients with penetrating wounds in the “cardiac box.”4,10 In a large study of patients who sustained either blunt or penetrating injuries, the FAST was 100% sensitive and 99.3% specific for detecting hemopericardium in patients with precordial or transthoracic wounds. Furthermore, the use of pericardial ultrasound has been shown to be especially helpful in the evaluation of patients who have no overt signs of pericardial tamponade. This was highlighted in a study in which 10 of 22 patients with precordial wounds and a hemopericardium on an ultrasound examination had admission systolic blood pressures >110 mm Hg and were relatively asymptomatic. Based on these signs and the lack of symptoms, it is unlikely that the presence of cardiac wounds would have been strongly suspected in these patients and, therefore, this rapid ultrasound examination provided an early diagnosis of hemopericardium before the patients underwent physiologic deterioration.
The FAST is also very accurate when it is used to evaluate hypotensive patients who present with blunt abdominal trauma. In this scenario, ultrasound is so accurate that when the FAST is positive, an immediate operation is justified.4,8,10,29
Because the FAST is a focused examination for the detection of blood in dependent areas of the abdomen, its results should not be compared to those of a CT scan because the FAST does not readily identify intraparenchymal or retroperitoneal injuries. Therefore, select hemodynamically stable patients considered to be at high risk for occult intra-abdominal injury should undergo a CT scan of the abdomen regardless of the results of the FAST examination. These patients include those with fractures of the pelvis or thoracolumbar spine, major thoracic trauma (pulmonary contusion, lower rib fractures), and hematuria. These recommendations were based on the results of two studies by Chiu et al. in 199730 and Ballard et al. in 1999.31 Chiu et al. reviewed their data on 772 patients who underwent FAST examinations after sustaining blunt torso injury. Of the 772 patients, 52 had intra-abdominal injury but 15 (15/52 = 29%) of them had no hemoperitoneum on the admitting FAST examination or on the CT scan of the abdomen. In other work conducted by Ballard et al. at Grady Memorial Hospital, an algorithm was developed and tested over a 3.5-year period to identify patients who were at high risk for occult intra-abdominal injuries after sustaining blunt thoracoabdominal trauma.31 Of the 1,490 patients admitted with severe blunt trauma, there were 102 (70 with pelvic fractures, 32 with spine injuries) who were considered to be at high risk for occult intra-abdominal injuries. Although there was only 1 false-negative FAST examination in the 32 patients who had spine injuries, there were 13 false negatives in those with pelvic fractures. Based on these data, the authors concluded that patients with pelvic fractures should have a CT scan of the abdomen regardless of the result of the FAST examination. The lower accuracy of the FAST in patients with pelvic fractures was again noted in a recent series published by Friese et al., in which an initial FAST examination had an 85% positive predictive value but only a 63% negative predictive value in 146 patients with pelvic fractures.32 These studies have helped provide guidelines to decrease the number of false-negative FAST studies, but, as with the use of any diagnostic modality, it is important to correlate the results of the test with the patient’s clinical picture. Suggested algorithms for the use of FAST are depicted in Fig. 16-9A and B. Indeed, the FAST exam has been included in the most recently published evidence-based guidelines for the evaluation of patients with blunt abdominal trauma from the Eastern Association for the Surgery of Trauma (EAST) with reported accuracy rates of 96–98%.33
(A) Algorithm for the use of ultrasound in patients with penetrating chest wounds. (B) Algorithm for the use of ultrasound in patients with blunt abdominal trauma.
The amount of blood detected on the abdominal CT scan34 or in the DPL aspirate (or effluent) has been shown to predict the need for operative intervention.35 Similarly, the quantity of blood that is detected with ultrasound may be predictive of a therapeutic operation.36,37 Huang et al. developed a scoring system based on the identification of hemoperitoneum in specific areas such as Morison’s pouch or the perisplenic space.36 Each abdominal area was assigned a number from 0 to 3, and the authors found that a total score of ≥3 corresponded to more than 1 L of hemoperitoneum. This scoring system had a sensitivity of 84% for determining the need for an immediate abdominal operation. Another scoring system developed and prospectively validated by McKenney et al. examined the patient’s admission blood pressure, base deficit, and the amount of hemoperitoneum present on the ultrasound examinations of 100 patients.37 The hemoperitoneum was categorized by its measurement and its distribution in the peritoneal cavity, so that a score of 1 was considered a minimal amount of hemoperitoneum but a score of >3 signified a large hemoperitoneum. Forty-six of the 100 patients had a score >3, and 40 (87%) of them underwent a therapeutic abdominal operation. This scoring system had a sensitivity, specificity, and accuracy of 83%, 87%, and 85%, respectively. The authors concluded that an ultrasound score of >3 is statistically more accurate than a combination of the initial systolic blood pressure and base deficit for determining which patients will undergo a therapeutic abdominal operation. Although the quantification of hemoperitoneum is not exact, it can provide valuable information about the need for an abdominal operation as well as its potential to be therapeutic.
Recent Advances and Organ Specificity
As surgeons have become more facile with ultrasound exams and as technology has improved, extensions of the FAST exam have been described. Again, it is noted that the standard FAST exam is designed to accurately answer two simple questions: Is there fluid in the peritoneal cavity and is their fluid in the pericardial sac? The use of ultrasound for more complex diagnostic interventions is described below, but these areas are less well studied and beyond the purview of the traditional FAST exam.
A more recent prospective, multicenter trial conducted by the Western Trauma Association reported on the use of ultrasound to serially evaluate patients with documented solid organ injuries (SOI) after trauma.38 The so-called BOAST exam, or the bedside organ assessment with sonography after trauma, was performed by a limited number of experienced surgeon sonographers in 126 patients with 135 SOI in 4 American trauma centers. This study, performed over nearly 2 years, was designed to be a more thorough abdominal ultrasound examination with multiple views obtained of each solid organ (kidneys, liver, and spleen). Criteria for enrollment included normal hemodynamics, absence of peritonitis or other need for urgent laparotomy, and lack of excessive blood transfusion in the attending physician’s judgment. All patients were victims of blunt trauma with a mean Injury Severity Score (ISS) of nearly 15.
Overall, only 34% of injuries to solid organs were seen with BOAST yielding an error rate of 66%. None of the 34 grade I injuries were identified and only 13 (31%) of the grade II injuries were identified. Sensitivities for grade III and IV injuries ranged from 25% to 75% and only one grade V injury (to the liver) was examined and positively identified. Eleven patients developed 16 intra-abdominal complications (8 pseudoaneurysms, 4 bilomas, 3 abscesses, and 1 necrotic organ), and 13 (81%) were identified by the sonographers. This study emphasizes that ultrasound, in most surgeons’ hands, should not be considered a reliable modality for diagnosis and grading of SOI although it may be acceptably accurate in the diagnosis of post-traumatic abdominal complications in patients with SOI managed nonoperatively.
In Europe, preliminary work using Power Doppler ultrasonography to identify specific organ injuries has been published in recent years.39,40 Many of these exams include the use of a sonographic contrast agent injected peripherally during the scan. In one study, the authors were able to document extravasation of contrast in 20 of 153 patients (13%). Extravasation was seen not only from the spleen, liver, and kidney after trauma but also in postoperative patients (aortic aneurysm repair, postsplenectomy) and in a patient with a ruptured aortic aneurysm. In 9 of 20 patients, CT scan was performed and all 9 confirmed extravasation of contrast. In the 133 patients without extravasation, the absence of active bleeding was inferred by a subsequent CT scan in 82 patients, surgical data in 13 patients, and clinical follow-up in 38 patients, with no cases of active bleeding missed by ultrasound. Thus, the addition of an ultrasonic contrast agent and Power Doppler may be of some benefit in the diagnosis of specific injuries. It should be emphasized, however, that the FAST exam in most American trauma centers is used simply as a screening tool to identify the presence or absence of hemoperitoneum or hemopericardium in a trauma patient.
A focused thoracic ultrasound examination was developed by surgeons to rapidly detect the presence or absence of a traumatic hemothorax in patients during the ATLS secondary survey.9 This focused thoracic ultrasound examination employs the ultrasound physics principles of the mirror image artifact and tissue acoustic impedance as presented in Table 16-1. A test that promptly detects a traumatic effusion or hemothorax is worthwhile because it dramatically shortens the interval from the admission of the patient with hemothorax to the insertion of a thoracostomy tube.
The technique for this examination is similar to that used to interrogate the upper quadrants of the abdomen in the FAST and also uses the same type and frequency transducer. In point of fact, it is performed one to two rib spaces higher than the RUQ and LUQ FAST views using the same probe. Ultrasound transmission gel is applied to the right and left lower thoracic areas in the midaxillary to posterior axillary line between the 9th and 10th intercostal spaces (Fig. 16-10). The transducer is slowly advanced cephalad to identify the hyperechoic diaphragm and to interrogate the supradiaphragmatic space for the presence or absence of fluid (Fig. 16-11A and B) that appears anechoic. In the positive thoracic ultrasound examination, the hypoechoic lung can be seen “floating” amidst the fluid. The same technique can be used to evaluate a critically ill patient for a pleural effusion as discussed earlier.
Transducer positions for thoracic ultrasound examination (detection of hemothorax).
(A) Sagittal view of liver, kidney, and diaphragm. Note supradiaphragmatic (lung) area but absence of pleural effusion. (B) Sagittal view of right supradiaphragmatic space. The right hemithorax contains fluid (blood) that appears anechoic.
One of the earliest reports on the use of ultrasound for the evaluation of fluid collections in the pleural space was described by Joyner et al. in 1967.41 Later, Gryminski et al. documented the superiority of ultrasound over standard radiography for the detection of pleural fluid.42 In that study, they reported that ultrasound detected even small amounts of pleural fluid in 74 (93%) of 80 patients, whereas plain radiography detected pleural fluid in only 66 (83%) of these patients.
Surgeons at Emory University have also examined the accuracy of this examination in 360 patients with blunt and penetrating torso injuries.9 They compared the time and accuracy of ultrasound with that of the supine portable chest x-ray and found both to be very similar with 97.4% sensitivity and 99.7% specificity observed for thoracic ultrasound versus 92.5% sensitivity and 99.7% specificity for the portable chest x-ray. Performance times, however, for the thoracic ultrasound examinations were statistically much faster (P< .0001) than those for the portable chest x-ray. Although it is not recommended that the thoracic ultrasound examination replace the chest x-ray, its use can expedite treatment in many patients and decrease the number of chest radiographs obtained.
The use of ultrasound for the detection of a traumatic pneumothorax is not a new diagnostic test, having been reported by numerous acute care surgeons dating back to the early 1990s.43–47 This examination is useful to the surgeon to evaluate a patient for a potential pneumothorax in the following circumstances: (1) bulky radiology equipment is not readily available; (2) inordinate delays for obtaining a chest x-ray are anticipated; or (3) numerous injured patients (mass casualty situation) must be rapidly assessed and triaged.48,49 Although useful in the trauma resuscitation area, surgeons may also find this examination helpful to detect a pneumothorax in a critically ill patient who is on a ventilator, after a thoracentesis procedure, or, potentially, after discontinuing the suction on an underwater seal device.
A 5.0- to 7.5-MHz linear array transducer is used to evaluate a patient for the presence of a pneumothorax. The examination may be performed while the patient is in the erect or the supine position. Ultrasound transmission gel is applied to the right and left upper thoracic areas at about the third to fourth intercostal space in the midclavicular line and the presumed unaffected thoracic cavity is examined first. The transducer is oriented for longitudinal imaging, is placed perpendicular to the ribs, and is slowly advanced medially toward the sternum and then laterally toward the anterior axillary line. The normal examination of the thoracic cavity identifies the rib (seen as black on the ultrasound image because it is a refraction artifact), pleural sliding, and a comet tail artifact (Table 16-1). Pleural sliding is the identification of the visceral and parietal layers of the lung seen as hyperechoic superimposed pleural lines. When a pneumothorax is present, air becomes trapped between the visceral and parietal pleura and does not allow for the transmission of the ultrasound waves. Therefore, the visceral pleura is not imaged and pleural sliding is not observed. The comet tail artifact is generated because of the interaction of two highly reflective opposing interfaces, that is, air and pleura (Fig. 16-12). When air separates the visceral and parietal pleurae, the comet tail artifact is not visualized. If desired, the examination may be repeated with the transducer oriented for transverse views, with images obtained with the probe parallel to the ribs.
Comet tail artifact (arrow).
Several studies have documented excellent sensitivity and specificity of ultrasound for the detection of a pneumothorax in the resuscitation area.43,45,46,50 Dulchavsky et al. from Detroit Receiving Hospital, Wayne State University, showed that ultrasound can be successfully used by surgeons to detect a pneumothorax in injured patients.51 Of the 382 patients (364 trauma; 18 spontaneous) evaluated with ultrasound, 39 had pneumothoraces and ultrasound successfully detected 37 of them, yielding a 95% sensitivity. Not unexpectedly, pneumothoraces in two patients could not be detected because of the presence of significant subcutaneous emphysema. The authors recommended that when a portable chest x-ray cannot be readily obtained, the use of this bedside ultrasound examination for the identification of a pneumothorax can expedite the patient’s management.
One study, published in 2006, however, documents significant loss of accuracy of an ultrasound examination starting about 24 hours after chest tube insertion.52 This study documents the hospital course of 14 patients with tube thoracostomies undergoing 126 ultrasound examinations over their hospital course. While ultrasound detection of pleural sliding in uninstrumented pleural cavities remained 100% accurate over time, the accuracy of ultrasound examination after chest instrumentation fell to 65% after 24 hours.52 It was theorized that adhesion formation led to false-positive examinations, in that normal pleural sliding was unable to be appreciated in patients with lung adhesions. This point illustrates the subtle difference in the usefulness of the ultrasound examination for pneumothorax: a “positive” examination is related to the absence of a normal finding (pleural sliding) rather than the presence of abnormal finding (i.e., fluid within the peritoneal cavity after blunt abdominal trauma). Thus, other causes of the loss of sliding (i.e., adhesion formation) can cause a false-positive and misleading examination. Indeed, a review of the literature revealed studies in animal models in which significant adhesion formation occurred as early 24 hours after thoracotomy.53 This same study also noted that the rapidity and degree of adhesion formation not only was variable based on the type and degree of injury but also varied within animals with similar injuries.53 Thus, one should be cautious when interpreting ultrasound examinations in patients with acute or chronic evidence of chest manipulation as false-positive studies may occur.52
Fractures of the sternum are visualized on a lateral x-ray view of the chest, but this film may be difficult to obtain in a patient with multiple injuries. For this reason, an ultrasound examination of the sternum can rapidly detect a fracture while the patient is still in the supine position and, therefore, avoid the need to obtain a lateral x-ray.54
The ultrasound examination of the sternum is performed using a high-frequency (>5.0 MHz) linear array transducer that is oriented for sagittal (longitudinal) views. Ultrasound transmission gel is applied over the sternal area while the patient is in the supine position. Beginning at the suprasternal notch, the transducer is slowly advanced in a caudad direction to interrogate the bone for a fracture. The examination is then repeated with the transducer oriented for transverse views. The examination of the intact sternum is shown in Fig. 16-13. A sternal fracture is identified on the ultrasound examination as a disruption of the cortical reflex (Fig. 16-14). Investigators have found that the use of ultrasound for this diagnosis is more accurate (and much more rapid) than a lateral x-ray view of the chest.54
Sagittal view of sternum. Normal findings.
Sagittal view of sternum illustrating fracture (interruption of hyperechoic line).
Ultrasound in the Pregnant Trauma Patient
Ultrasound would seem to be an ideal method of evaluating a pregnant patient with suspected blunt abdominal trauma as it is portable, noninvasive, and free of ionizing radiation. The Advanced Trauma Life Support course teaches that unrecognized abdominal trauma is a major problem in the pregnant trauma patient.19 Concerns over changes in abdominal anatomy leading to difficulty in obtaining images have not borne out in objective evaluations. Goodwin et al.55 reported on their 8-year experience with the FAST exam in 127 pregnant patients, including 5 of 6 patients with hemoperitoneum who were found to have fluid on the FAST exam (sensitivity 83%). Of the 120 without abdominal injury, 117 had a true-negative FAST (specificity 98%), with three false-positive exams due to serous intraperitoneal fluid. Furthermore, Brown et al.56 reported on their experience with a more extensive ultrasound exam in 101 stable, pregnant patients with suspected blunt abdominal trauma. Median gestational age was just over 24 weeks, and these patients underwent an official abdominal ultrasound by a certified technician to include images of the fetus and placenta. The sensitivity was 80% (four of five patients had correct identification of injuries) with one missed placental hematoma that required an emergent cesarean delivery for fetal distress. Injuries identified included one placental abruption, two splenic lacerations, one hepatic laceration, and one renal injury. None of the 96 patients with a negative ultrasound had injuries discovered later in their hospital course (specificity 100%). Thus, it would seem that ultrasound remains a good screening tool for the pregnant patient with blunt abdominal trauma and has the advantages of repeatability and a lack of radiation exposure.
Ultrasound in Penetrating Trauma
Ultrasound for the diagnosis of injuries after penetrating trauma has been studied much less extensively than ultrasound used after blunt trauma. Several of the larger, well-known series4,9,10 have included patients with penetrating trauma and, as stated previously, ultrasound of the pericardium has been shown to be accurate for diagnosis of injury in patients with penetrating injury to the “cardiac box.”9 In a recent study of 32 patients with penetrating anterior chest trauma, ultrasound was used to diagnose 8 pericardial effusions with a reported 100% accuracy (8 true-positive and 24 true-negative exams).57 Eight other patients were noted to have intraperitoneal fluid and underwent therapeutic exploration including repair of five diaphragmatic injuries, three hepatic lacerations, three splenic lacerations, three gastric injuries, two injuries to the small bowel, and one injury to the adrenal gland. No false-positive or false-negative examinations of the peritoneum were reported. Other studies have shown that the accuracy of FAST after penetrating trauma is somewhat less with one study reporting a sensitivity for abdominal injury after penetrating trauma as low as 67%.58
A recent report by Murphy et al. looked at the utility of ultrasound to diagnose fascial penetration after anterior abdominal stab wounds.59 In this study, 35 patients underwent ultrasonic evaluation of their anterior abdominal fascia with an 8.0-MHz linear array probe followed by a local wound exploration. While ultrasound had only a 59% sensitivity (13/22 patients), it did have a 100% specificity with no false-positive studies. Thus, if fascial penetration is noted on ultrasound, a more invasive wound exploration is probably not needed; however, a negative ultrasound evaluation is clearly less helpful and does not preclude peritoneal penetration.
Ultrasound in Pediatric Trauma Patients
Ultrasound as a modality would seem to be attractive for use in a pediatric population for many of the same reasons that have already been elaborated upon, including the lack of radiation exposure and the noninvasive nature of the examination. Many of the early studies in the pediatric literature regarding the use of ultrasound after trauma involved studies performed by radiologists or sonographers.27,60,61 However, there are now several studies of surgeon-performed FAST examinations that show similar sensitivities, specificities, and accuracies as in the adult population.62–64 For example, in a series by Soundappan et al.,64 FAST examination had a sensitivity of 81%, a specificity of 100%, and an accuracy of 97% in a group of 85 pediatric patients who were victims of blunt abdominal trauma. Thus, while the literature is not as robust as in the adult population, the use of surgeon-performed ultrasound in the pediatric trauma bay is becoming much more widespread.
Ultrasound in Austere Settings
The portability of ultrasound makes it ideal for use in forward settings. Training courses are in place to teach the use of the FAST exam to military surgeons, and handheld ultrasound is now routinely deployed within the British Defence Medical Services.65 Indeed, in a survey of surgeons reviewing potential preventable casualties in Vietnam, ultrasound was the fourth most commonly mentioned advancement in technology (behind modern ventilators, CT scanners, and modern antibiotics) that may have assisted in better patient salvage.66
Although up to 90% of war wounds are penetrating, ultrasound may allow quicker, more accurate triage decisions as patients with penetrating abdominal trauma with no or minimal hemoperitoneum may be transferred to the next echelon where the study may be repeated or additional diagnostic maneuvers undertaken.67 In a study from the Croatian conflict in 1999, FAST was shown to have a sensitivity of 86%, a specificity of 100%, and an accuracy of 97% when applied to 94 casualties evaluated over a 72-hour period. This was comparable to the accuracy achieved by the authors in their civilian experience with FAST in more than 1,000 patients over the 3 years prior to the conflict. In a somewhat recent small series,67 FAST was used with excellent results in a British military hospital in Iraq. Fifteen casualties were evaluated with serial FAST exams, and 14 had negative exams at admission and again at 6 hours. One patient underwent laparotomy based on trajectory and had no intraperitoneal fluid but two small holes were discovered in the cecum that required repair. The other 13 patients recovered without sequelae. One exam was positive and led to immediate laparotomy in a patient with a grade V liver injury after a motor vehicle collision.
Because ultrasound is portable enough to use in active combat situations, research is ongoing to evaluate the best method to teletransmit images obtained in the field. Several different satellite transmission systems have been evaluated, and high-quality images were able to be obtained in the majority of cases; however, the balance between the weight of the system and the minimum image quality has still not been completely achieved.67 It has been noted that images can be transmitted from up to 1,500 ft from the antennae without significant degradation.68 As technology advances, one would expect imaging systems to continue to become smaller and lighter with improved image quality, making ultrasound even more appealing as a modality for use in the forward setting.
Many of the same qualities that make ultrasound appealing for use in combat make it equally appealing as a diagnostic modality in space, where an injury might mandate abortion of a multimillion dollar mission. Indeed, ultrasound is one of the only feasible diagnostic modalities on space missions, given size and weight restrictions. Also, ultrasound examinations are easily taught and images can be relayed with minimal delay to physicians on the ground. ATLS procedures are also feasible in space,69 and lifesaving procedures could be performed based on ultrasound findings.
Ultrasound has been used in space for several decades. Indeed, it has been ultrasound technology that has taught us much about the physiologic effects of microgravity, especially the fluid shifts associated with space travel.70 As early as 1982, cardiac ultrasound was used to evaluate left ventricular systolic function and cardiac chamber size in cosmonauts. The first American ultrasound system in space was the American flight echograph from Advanced Technology Laboratories (Bothel, WA) that first flew in 1984 and eventually was capable of three-dimensional images using a tilt frame device. Currently, the Human Research Facility aboard the International Space Station is equipped with a state-of-the-art Philips HDI 5000 (Philips Medical, Bothel, WA).70
Because surface tension and capillary action are the principal physical forces in space, scientists questioned whether images obtained on the standard FAST exam would be useful in microgravity. There are now several published studies of ultrasounds performed on parabolic flights in the NASA Microgravity Research Facility, a KC-135 aircraft. This aircraft can generate 25- to 30-second intervals of weightlessness using serial parabolic trajectories. A porcine model of intra-abdominal hemorrhage was created on the ground and studied during parabolic flights.71 Over 2,000 ultrasound segments were recorded with 80% of these considered feasible for diagnosis of the presence or absence of abdominal fluid. The sonographers felt the exam was no more difficult than one done on the ground as long as the sonographer and patient were adequately restrained. For the intraperitoneal portion of the exam, a fourth view (the midline “abdominal sweep”) was added and, with this addition, the FAST exam was able to reliably detect even relatively small amounts of intraperitoneal fluid. The Morison’s pouch view remained the most sensitive window for fluid detection.71 Further study using a similar model revealed that ultrasound can also reliably detect both hemothorax and pneumothorax in microgravity.72
Recently, astronauts aboard the International Space Station performed FAST ultrasounds that were transmitted with a 2-second satellite delay to directors on the ground, who were able to provide them with real-time instructions for probe position and system adjustments. Exams were able to be completed in roughly 5 minutes with adequate images obtained in all views.73 Astronauts have also been able to perform comprehensive ocular ultrasounds with the same real-time feedback.74
In summary, ultrasound fulfills all the necessary criteria for a diagnostic modality in space. It is sufficiently portable, teletransmittable, teachable, and accurate. It will likely continue to be the only feasible technology to assist with medical diagnoses on space missions in the near future.