Clinical Use of Ultrasound in Thoracoabdominal Trauma
September 1, 2000
Clinical Use of Ultrasound in Thoracoabdominal Trauma
Author: Robert Jones, DO, FACEP, Emergency Ultrasound Coordinator OUCOM/Doctor’s Hospital Emergency Medicine Residency Program, Columbus, OH; Attending Physician, MetroHealth Medical Center, Cleveland, OH.
Peer Reviewer: Dennis Hanlon, MD, Director, Emergency Medicine Residency Program, Assistant Professor of Emergency Medicine, Allegheny General Hospital, Pittsburgh, PA.
Sonographic technology has made significant advances in recent years. In the past, ultrasound (US) machines were prohibitively large, difficult to use, and therefore, not practical for bedside use by nonradiologists. Current machines are more compact, mobile, and easy to use. A new generation of hand-held machines capable of producing excellent images has been introduced in the market recently and will allow more flexibility in clinical applications.
Despite these advances, US is still operator dependent, and a basic understanding of its physical principles is essential for accurate image production and interpretation. The use of US in the emergency department (ED), particularly for trauma patients, is gaining popularity. This article provides a review of the physics of US, and the advantages, disadvantages, and limitations of this new technology for use in the ED for trauma patients.
— The Editor
Physical Principles
Sound waves require a medium for transport and cannot travel through a vacuum. As sound waves are transmitted through a medium, they form a series of compressions and decompressions. The number of these compressions and decompressions per second is referred to as frequency and is measured in hertz (Hz). US is defined as any sound wave with a frequency greater than 20,000 Hz, which is the upper limit of human hearing. In diagnostic medical sonography, the frequencies utilized are in the 2-20 megahertz (mHz) range. US is defined by the same characteristics that define all forms of waves, notably frequency, period, wave length, and amplitude.
Image Production
Generating an US wave impulse is achieved through the piezoelectric effect, which literally means "pressure electricity." When the piezoelectric crystals in the transducer receive an alternating current from the machine, they begin to vibrate. This expansion and contraction of the crystals results in the generation of the US wave. The crystals act as both signal generator and receiver. On average, the crystals send out signals less than 1% of the time and receive signals greater than 99% of the time.
The speed of US waves through human tissue is dependent on the compliance and density of the reflecting tissue. The average speed of US waves in human tissues is 1540 meters per second. As the US wave is propagated through the tissue, it will lose power or attenuate. This occurs as a result of scattering, reflection, and absorption. Mediums, such as air or bone, do not allow transmission of ultrasound waves and will reflect the sound waves back at the interface. Higher frequency probes will result in greater rates of attenuation. This is an important concept to understand, since the clinician has the responsibility of choosing the probe for each study. In the average adult, intra-thoracic and intra-abdominal studies require the use of a probe in the 2.0-3.5 mHz range.
The ability to discriminate between two small objects is defined as resolution. Resolution can be broken down into two types: axial and lateral. Axial resolution refers to the ability to differentiate between two points that are parallel to the wave, while lateral resolution refers to the ability to differentiate between two points that are perpendicular to the wave. Higher frequency probes have better resolution, but less penetration.
An echo is produced when the US wave is reflected back from a tissue interface. When the US wave hits the tissue interface, the amount of the US wave that is reflected back to the source (vs transmitted through the interface) depends on the physical differences of the two interfacing tissues. This physical characteristic is called acoustic impedance.
Structures are described in regard to their relative echogenicity. A structure that is without echoes is referred to as anechoic. A structure that produces echoes of a lower amplitude than surrounding tissues is referred to as hypoechoic, while a structure that produces echoes of a greater amplitude than surrounding tissues is referred to as hyperechoic. A structure that produces echoes of the same amplitude as the surrounding tissues is referred to as isoechoic. US is imaged on a gray scale with anechoic structures appearing black and structures with echoes of the strongest amplitude appearing white. The remaining echoes will fall in somewhere on the gray scale.
As the US wave returns to the transducer, the piezoelectric crystals convert the US waves into electrical energy. Given that the speed of US transmission through a given medium is constant, it should be apparent that the distance from the transducer to the structure is directly related to the time required for the reflected wave to return to the transducer. The machine is able to convert this into graphic form based on the time each reflected wave took to return to the transducer.
Instrumentation
The US machine used for bedside trauma scanning will need the following features: portability, realtime B-mode scanning capability, ease of use, 2.0-3.5 mHz probe capable of intercostal scanning, and ability to print images for documentation purposes. Doppler physics, as it relates to diagnostic ultrasonography, concerns the behavior of high-frequency sound waves as they are reflected off moving fluid and is not required to perform the current state-of-the-art trauma exam. A-mode is a graphic display of amplitude strength of the returning echo signals along the vertical axis and is useful for determining whether a structure is cystic or solid, but it is not useful in performing trauma scanning. M-mode displays B-mode dots on a moving time base and allows the motion of mobile structures to be observed. This mode of scanning is still used in echocardiography but is not required for trauma scanning.
Transducers can be electronically or mechanically steered and come in various shapes and sizes. The mechanical transducers have one or more transducer elements that create sound waves through their physical movement. These transducers produce a sector (pie-shaped) image and are less expensive than electronic transducers. The electronic transducers have no moving parts and consist of multiple piezoelectric crystals that are arranged in different patterns and fired in different sequences to improve image production. These transducers are more expensive than mechanical transducers. The piezoelectric crystals can be arranged in a linear fashion (linear array), a curved fashion (curvilinear array), or a circular fashion (annular array). For trauma scanning, the transducer can be either electronically or mechanically steered, but the footprint (part of transducer in contact with patient) must be small enough to allow intercostal scanning.
Knobology
Adjusting the "knobs" is necessary to produce the best images and is an important part of the art of scanning. The power "knob" controls the strength or intensity of the sound wave. Only the minimum amount of power needed to obtain the desired image should be used. The gain "knob" measures the strength of the US signal throughout the image and regulates the degree of echo amplification. The time gain compensation curve "knob" is used to compensate for the attenuation of the sound wave as it passes through tissue and allows sonographically similar tissue to appear the same in the near-field and far-field. The focal zone "knob" enhances the resolution of an area in the image by electronic focusing.
History
The first case report of sonography performed for the evaluation of blunt abdominal trauma (BAT) was published in 1971 by Kristensen and colleagues in Germany.1 In 1976, Asher and associates published a study evaluating the use of US to detect splenic injury and defined sonographic criteria for diagnosing splenic injury that are still used today.2 The following decade, Tiling and colleagues published their work, which emphasized the cost-effectiveness and repeatability of US to evaluate both the thorax and abdomen.3 They found that US exceeded the capabilities of diagnostic peritoneal lavage (DPL) for detecting intra-abdominal injuries. This led to a study in 1988 from Gruessner and coworkers in Germany where they evaluated these claims.4 In their study, DPL had a greater sensitivity and specificity for detecting intra-abdominal injuries than did US, and they concluded that US should complement DPL and not replace it. Numerous studies from Europe and Japan support US as a rapid, reliable, cost-effective diagnostic modality for the evaluation of BAT.5-10 In 1988, the German Association of Surgery included mastery of US in its guidelines for surgical resident education.11 Today, US has virtually replaced DPL in Europe and Japan.
The first American report on the use of US in the evaluation of BAT was published in 1992 by Tso and colleagues.12 The sonographic exams were performed by trauma fellows who had one hour of didactic and one hour of practical training. After exclusion of patients who had sustained hypotension (longer than 5 minutes), suspected closed head injury, or indications for immediate laparotomy, 163 patients with blunt trauma were included. The overall sensitivity of US for detection of parenchymal injuries was 69%, with a specificity of 99%. All false-negative studies occurred in patients with parenchymal injury who had minimal or no hemoperitoneum that was detected by DPL or computed tomography (CT). The sensitivity for detection of hemoperitoneum alone was 91%. The authors concluded that additional sonography experience would have decreased the number of missed parenchymal and retroperitoneal injuries, and that a negative US study did not rule out parenchymal injury.
Since that first study in 1992, numerous studies have been published in this country favoring the use of US in the evaluation of the trauma patient with BAT.13-20 In 1997, the American College of Surgeons included the use of US in the Advanced Trauma Life Support (ATLS) secondary survey.21 That same year, an international panel of experts met to discuss key issues related to performing the FAST exam in order to allow broader recognition of the procedure and its goals.22.
The FAST Exam
Background. The acronym FAST (Focused Abdominal Sonography for Trauma) first appeared in the literature in 1996. It was felt that this did not accurately describe the exam since the exam was not limited to the detection of hemoperitoneum. In 1997, an international panel of experts met to discuss key issues related to the use of US in trauma, including standardization of terminology.22 It was felt that use of a distinct name would allow broad recognition of the exam and an understanding of its goals. The FAST Consensus Conference Committee concluded that the acronym should stand for "Focused Assessment with Sonography for Trauma."
The current FAST exam is a screening, bedside exam focused on the detection of hemoperitoneum and hemopericardium. Its success and popularity are due to the exam being noninvasive, accurate, and easily performed with limited training. The FAST exam is not meant to be a formal sonographic study for the identification of all sonographically detectable pathology.
Anatomic Considerations
The FAST exam’s use is based on the assumption that all clinically important abdominal and cardiac injuries will be associated with free fluid. The detection of hemopericardium is relatively straightforward since the fluid is confined between the parietal and visceral layers of the pericardium. However, the detection of hemoperitoneum is dependent on factors such as body habitus, patient position, injury location, presence of clotted blood, and the amount of fluid present. Free intraperitoneal fluid continually circulates throughout the peritoneal cavity and will preferentially collect in dependent intraperitoneal compartments formed by peritoneal reflections and mesenteric attachments.23 A basic understanding of the fluid movement from various locations within the peritoneal cavity will help improve the sensitivity of the FAST exam. (See Figure 1.)
The peritoneal cavity consists of the greater and lesser sacs. The lesser sac is a diverticulum of the greater sac, which is confined to the left side of the upper abdomen and communicates on the right with the greater sac through the epiploic foramen. The greater sac is the main compartment of the peritoneal cavity and is divided into the supramesocolic and inframesocolic compartments, with the transverse colon and its mesocolon being used as the boundary. The paracolic gutters connect the supramesocolic and inframesocolic compartments. The right paracolic gutter is deeper and wider than the left and connects the pelvis with Morison’s pouch. The left paracolic gutter, in comparison, is shallow, and its course to the splenorenal fossa is blocked by the phrenicocolic ligament in most people.24 Fluid preferentially will travel via the right paracolic gutter because it meets less resistance. In the supine patient, the most dependent supramesocolic location in both male and female patients is Morison’s pouch (hepatorenal fossa). The rectovesicular pouch is the most dependent location overall in the supine male, while the pouch of Douglas is the most dependent location overall in the supine female.
Physiologic factors, such as intraperitoneal pressure gradients and gravity, will play a role in fluid distribution. Using an animal model, Overholt demonstated that the hydrostatic pressure in the upper abdomen is less than that of the lower abdomen and varies with respiration.25 The outward movement of the ribs during inspiration enlarges the subdiaphragmatic (subphrenic) space and decreases the upper abdomen pressures in relationship to the lower abdomen, and can result in fluid being drawn into the subphrenic space.26 This pressure gradient is so great that it can even result in fluid traveling up the paracolic gutters with the patient standing.27
The introduction of fluid into the right upper quadrant of a supine patient will result in preferential fluid flow into Morison’s pouch, with overflow fluid going to the right subphrenic space and down the right paracolic gutter to the pelvis.24 Less commonly, fluid will travel to the splenorenal fossa via the epiploic foramen. The falciform ligament will prevent fluid from traveling directly between the right and left subphrenic spaces.
The introduction of fluid into the left upper quadrant of a supine patient will result in fluid flow preferentially into the subphrenic space.24 Overflow fluid will travel to the splenorenal fossa, across the epiploic foramen to Morison’s pouch, and ultimately, to the pelvis via the right paracolic gutter. It is important to remember that the splenorenal fossa is not the most common location of fluid accumulation in the left upper quadrant. Therefore, in a patient with a suspected splenic injury, the diaphragm and subphrenic space must be visualized.
Fluid introduced into the pelvis preferentially will go to the rectovesicular pouch in supine males and the pouch of Douglas in supine females.24 Overflow fluid will ascend up the paracolic gutters, with a majority going up the right side to Morison’s pouch. As previously mentioned, fluid meets more resistance in the left paracolic gutter and will preferentially travel up the right.
Trauma scanning may be performed using as many as six and as few as one acoustic windows. The fact that Morison’s pouch is a common site of fluid accumulation has made it a popular scanning site for hemoperitoneum. Ma and colleagues compared the sensitivities, specificities, and accuracies of the single-view technique and multiple-view technique for detecting hemoperitoneum in a supine patient.18 The single-view technique had a sensitivity of 51%, specificity of 100%, and accuracy of 93%, while the multiple-view technique had a sensitivity of 87%, specificity of 100%, and accuracy of 98%. The single-view technique was performed in one minute while the multiple-view technique was performed in four minutes. They concluded that the multiple-view technique was much more sensitive, but the addition of the paracolic windows did not improve the sensitivity.
The use of five degrees of Trendelenburg positioning has been shown to improve the sensitivity of fluid detection in the single-view (perihepatic) exam.28 One study by Abrams and colleagues used a DPL model that assumed an inframesocolic source of bleeding. They found that using five degrees of Trendelenburg positioning decreased the threshold for fluid detection from 700 cc to 400 cc. While this study shows that five degrees of Trendelenburg positioning improves sensitivity for fluid detection in patients with an inframesocolic source of bleeding when the single-exam technique is used, its results cannot be applied to supramesocolic souces of bleeding, such as the liver and spleen. It does not appear, based on preferential fluid pathways in the supramesocolic compartment, that Trendelenburg positioning would be helpful in improving sensitivity of fluid detection. Placing a patient with a splenic injury in the Trendelenburg position would not improve detection in the right upper quadrant. Another disadvantage of using a single-view technique is that no information about pericardial fluid will be obtained.
The FAST Exam: Terminology and Goals.
The current FAST exam, as recommended by the FAST Consensus Conference Committee, consists of four acoustic windows with a supine patient: pericardial, perihepatic, perisplenic, and pelvic.22 (See Figure 2.) Consistency of terminology has been advocated, and terms such as right upper quadrant and left upper quadrant are no longer recommended. The addition of paracolic windows, which do not appear to improve sensitivity, was not recommended by the committee for inclusion in the standardized FAST exam.The goals of the FAST exam are straightforward and include the detection of hemoperitoneum and hemopericardium. An exam is called positive if there is fluid seen on the pericardial window or on any of the three abdominal windows. An exam is called negative if there is no fluid seen on the pericardial window and all three abdominal windows. An exam is called indeterminate if any of the windows are not adequately visualized. An indeterminate exam is not a negative exam.
Sonographic Technique
The pericardial view is obtained using a subcostal or transthoracic window. The subcostal window is easier to perform and will provide information about hemopericardium, gross chamber enlargement, and gross wall motion abnormalities. The probe is placed in the subxiphoid region, with the beam directed toward the patient’s left shoulder and the probe indicator toward the patient’s right. The liver is used as a sonographic window so the transducer may need to be moved slightly to the right if there is shadowing from air in the stomach or duodenum. The probe can be slightly angled or rotated to obtain the desired four-chamber view. If the desired view cannot be obtained through slight movements of the probe, then have the patient take in a deep breath and hold it. This will flatten out the diaphragm and bring the heart closer to the transducer.
The subcostal window also can be obtained in the sagittal orientation. This orientation frequently is described in the trauma surgery literature.29 To obtain this view, the probe is placed in the subxiphoid region with the beam angled slightly cephalad with the probe indicator directed toward the patient’s head. This orientation will not give you a four-chamber view of the heart and will not provide information about gross wall motion abnormalities or gross chamber enlargement.
In morbidly obese patients or patients with a midepigastric injury, the subcostal approach may be difficult to perform. The ability to get the probe under the rib cage will be compromised in the morbidly obese patient and will be uncomfortable in the patient with a midepigastric injury, since some pressure will need to be applied. In these patients, a transthoracic window should be attempted, although it, too, will be difficult to perform in the morbidly obese patient.
The perihepatic view is obtained using an intercostal window. The probe is placed in the midaxillary line somewhere between the eighth and 11th ribs, with the probe indicator directed toward the patient’s axilla. Rotating or angling the probe slightly can be done if the desired view is not obtained on the initial attempt. Since fluid frequently accumulates in the subphrenic space, it is important that the diaphragm be visualized. The probe can be moved caudally and oriented in a coronal plane to visualize the lower pole of the liver and kidney and identify any fluid that has not yet reached Morison’s pouch. In the study comparing single-view vs. multiple-view examinations by Ma and colleagues, the right intercostal coronal view was positive in 11 of the 32 cases, while the standard perihepatic view was positive in 19 of the 32 cases.18 In only one case was the coronal view positive when the perihepatic view wasn’t. In this patient, the remainder of the FAST exam was negative, so it may be worthwhile to scan this area if the FAST exam is negative. The right paracolic gutter is clearly one of the preferential fluid pathways in the peritoneal cavity and may harbor fluid that has not yet made it to one of the standard viewing locations.
The perisplenic view also is obtained using an intercostal approach. The probe is placed in the posterior axillary line somewhere between the eighth and 11th ribs, with the probe indicator directed toward the patient’s axilla. Rotating or angling the probe slightly can be done if the desired view is not obtained on the initial attempt. As with the perihepatic view, the subphrenic space must be visualized since this is a common location of fluid accumulation. A coronal view also can be obtained to visualize the lower pole of the spleen and kidney by moving the probe caudally and rotating it from the oblique orientation to a coronal orientation. In the study comparing single-view vs. multiple-view examinations by Ma and colleagues, the left intercostal coronal view was positive in 11 out of 32 cases, compared with the standard perisplenic view, which was positive in 20 out of 32 cases.18 In only one of these cases was the coronal view positive when the perisplenic view wasn’t. However, in this case the perihepatic and pelvic views were positive, so the addition of this view doesn’t appear to add anything based on the current literature.
The pelvic view is obtained by placing the probe in the suprapubic region in either a longitudinal or transverse plane. Although Rozycki recommends the transverse view, others recommend both the transverse and longitudinal views as being necessary for optimal sensitivity.29 Since the bladder is used for a sonographic window on this view, the pelvic view is best accomplished when the bladder is full. In patients who are undergoing foley catheter insertion, clamp the catheter to prevent bladder drainage. In patients who have an empty bladder on arrival, the foley catheter can be used for retrograde filling.
Sonographic Findings
The subcostal window will provide a four-chamber view of the heart. (See Figure 3a.) The hyperechoic pericardium will be seen surrounding the heart. Some fluid may be present in normal individuals, but if there is fluid present in a nondependent area, it is definitely abnormal. The presence of hemopericardium will be demonstrated by separation of the visceral and parietal pericardial layers. (See Figure 3b.) Blood will sonographically appear black (anechoic) during the acute phase, but echoes may be found if clotting has occurred. Isoechoic fluid collections have been reported in the literature as a cause of false-negative studies.30 Normally, there is only one hyperechoic line seen surrounding the heart, which represents both layers of the pericardium. If two hyperechoic lines are seen surrounding the heart without the presence of anechoic fluid between the lines, the presence of an isoechoic fluid collection should be suspected. A pericardial tamponade can be diagnosed by the presence of a circumferential fluid collection with diastolic collapse of the right atrium or ventricle seen on real-time scanning.
Although the peritoneal and pleural windows are limited on this view, a large hemothorax or large subphrenic fluid collection can be mistaken for a hemopericardium.31 The key to preventing this mistake is to always visualize the hyperechoic pericardium and assess the fluid in its relationship to the pericardium. Subphrenic fluid or a hemothorax will not be located within the layers of the pericardium. If a large hemothorax is present, rescan the patient after chest tube drainage has occurred, since a large hemothorax potentially can obscure a small hemopericardium.
The perihepatic view will provide fractional views of the liver and right kidney and will allow visualization of fluid in Morison’s pouch, the subphrenic space, the right pleural space, and the retroperitoneal space. (See Figure 4a.) Hemoperitoneum will appear as an anechoic area in Morison’s pouch or in the subphrenic space, and this is the only goal of the standardized FAST exam. (See Figure 4b.) Fluid in adjacent structures, such as the gallbladder, hepatic flexure of the colon, and duodenum, can be mistaken for hemoperitoneum.32 Careful inspection for the presence of peristalsis during real-time scanning and demonstration of an echogenic border surrounding the fluid is essential to prevent making this error. The coronal view, although not a part of the standardized FAST exam, can detect fluid around the tip of the liver that has not yet reached Morison’s pouch. The perihepatic view should not be considered negative unless the subphrenic space is seen.
The perisplenic view will provide fractional views of the spleen and left kidney and will allow visualization of fluid in the subphrenic space, the splenorenal fossa, the left pleural space, and the retroperitoneal space. (See Figure 5a.) The detection of hemoperitoneum, the sole goal of the FAST exam on this view, will be made by visualizing anechoic fluid in the subphrenic space or in the splenorenal fossa. (See Figure 5b.) Fluid in this area preferentially will go to the subphrenic space, so the diaphragm and subphrenic space must be visualized. Fluid in adjacent structures, such as the stomach and splenic flexure of the colon, can be mistaken for hemoperitoneum.32 Careful inspection for the presence of peristalsis during real-time scanning and demonstration of an echogenic border surrounding the fluid in the stomach or splenic flexure of the colon is essential to prevent this error.
The pelvic view provides only an intraperitoneal window and utilizes the bladder as a sonographic window. (See Figure 6a.) Hemoperitoneum on the pelvic view in a male patient will appear as an anechoic area in the rectovesicular pouch or cephalad to the bladder. (See Figure 6b.) In a female patient, fluid will appear as an anechoic area in the pouch of Douglas just posterior to the uterus. In their study, Nyberg and colleagues found that subtle pelvic fluid collections may occur over the uterine fundus creating a fundal "cap," and that small amounts of fluid in the pouch of Douglas can be masked by an overdistended bladder.33 Having the patient partially void or partially empty the bladder after foley catheterization if the bladder is overdistended may unmask small fluid collections in the pouch of Douglas. It is important to note that US will show you there is the presence of an abnormal fluid collection, but it will not tell you what it is. Premenopausal females frequently will have small amounts of fluid in the pouch of Douglas, so clinical correlation is definitely required. Localized extraperitoneal fluid collections (pelvic hemoatomas) associated with pelvic fractures can be mistaken for hemoperitoneum.32
In male patients, the seminal vesicles can be mistaken for hemoperitoneum on the pelvic view.32 The seminal vesicles are paired structures that appear hypoechoic and lie posterior to the bladder and can easily be confused with hemoperitoneum. They can be distinguished from hemoperitoneum based on their appearance between the bladder and the prostate and by the fact that on the longitudinal view, the seminal vesicles taper off in the cephalad direction and do not extend beyond the bladder like hemoperitoneum does.
Clinical Applications
In patients with suspected cardiac injury, US can be a very useful test to guide patient management.31,34 US is very sensitive in detecting even small amounts of pericardial fluid. In their study of 261 patients with possible penetrating cardiac wounds, Rozycki and colleagues found that US was 100% sensitive, 96.9% specific, and 97.3% accurate in the detection of hemopericardium.31 The patients with false-positive studies were all found to have benign pericardial effusions, and it was felt that surgery was appropriate in these cases based on their presentation and documentation of pericardial fluid on US. An earlier study reported similar numbers except for two false-negative studies due to isodense hemopericardium.30
Rozycki et al have concluded, based on their study, that cardiac US is highly accurate and should be the initial modality for the evaluation of patients with suspected penetrating cardiac wounds.31 Due to its high sensitivity and specificity, important clinical decision-making, such as operative intervention, can be based on the results of the cardiac US. Rozycki et al recommend immediate operative intervention in patients with a positive cardiac US, regardless of stability.31 Patients with a negative cardiac US warrant a minimum of six hours of observation with repeat US scanning later in the hospital course. Patients with equivocal or indeterminate studies warrant formal echocardiography or supxiphoid pericardial window imaging, based on patient stability.
A retrospective chart review of a 22-year experience of penetrating cardiac trauma at an urban Level 1 trauma center concluded that more rapid transport from the field, more use of surgeon-performed US in modestly hypotensive and normotensive patients, and earlier operative intervention are the only current approaches likely to lower mortality.35 A comparison of morbidity and mortality was made between the two 11-year periods. Since surgeon-performed cardiac US began in 1994, hemopericardium has been diagnosed correctly in 12 patients, with a resultant 100% survival rate. The overall mortality for all patients during the 22-year study interval was 25% and did not significantly differ in the two 11-year periods. However, in patients who are normotensive or mildly hypotensive, surgeon-performed cardiac US may lead to a more rapid diagnosis of hemopericardium and a potential improvement in patient survival.
US in BAT has been well-studied, and reported sensitivities and negative predictive values for US in detecting hemoperitoneum vary from 78-99% to 93-99%, respectively.36 The detection of intra-abdominal injuries with FAST is based on the assumption that clinically important injuries will be associated with hemoperitoneum. However, Shanmuganathan and colleagues recently reported that in their study of 466 patients with visceral injuries, 34% of patients had no associated hemoperitoneum on US or CT.36 In their study, 27% of splenic injuries, 71% of hepatic injuries, 48% of renal injuries, 11% of mesenteric injuries, and 29% of pancreatic injuries were without associated hemoperitoneum. Therefore, it is apparent from this study that reliance on the presence of hemoperitoneum as the sole indicator of abdominal visceral injury limits the value of FAST as a screening diagnostic modality in patients with BAT, but doesn’t completely negate its usefulness.
Recent studies have shed light on the issue of how FAST can be incorporated into the clinical decision-making process. (See Figure 7.) In their study of 1540 patients, Rozycki et al found that US was the most sensitive and specific for the evaluation of patients with precordial/transthoracic wounds (sensitivity, 100%; specificity, 99.3%) and hypotensive patients with BAT (sensitivity, 100%; specificity, 100%).37 Wherrett and colleagues retrospectively evaluated the use of FAST in BAT in 400 patients and found that of the 22 patients who were hypotensive, 19 required an emergent laparotomy.38 They concluded that since 86.4% of the hypotensive patients in their study required laparotomy, US is an accurate indicator of the need for urgent laparotomy in the hypotensive patient with BAT. The FAST Consensus Conference Committee unanimously agreed that the hypotensive BAT patient with a positive FAST exam requires emergent laparotomy and created a consensus recommendation for this.22
The hemodynamically stable patient with a positive FAST requires CT scanning to better define the nature of the injuries. With the exception of the hemodynamically unstable patient with a positive FAST, this is the only other clinical area in which the FAST Consensus Conference Committee came to a unanimous agreement.22 Taking every hemodynamically stable patient with a positive FAST to the operating room for an exploratory laparotomy would result in a very high nontherapeutic laparotomy rate. Knowing the amount of free intraperitoneal blood present may affect the decision for operative vs. nonoperative management, but we are currently limited by a general classification, such as minimal, mild, moderate, and large. An early study by Tiling and colleagues concluded that a small anechoic stripe in Morison’s pouch represented about 250 mL of fluid, while a 0.5 cm and 1.0 cm stripe corresponded, respectively, to fluid volumes of 500 mL and 1,000 mL.3 Branney and colleagues found a wide variation in measured depths, suggesting that the brief FAST exam is not a reliable method for estimating intraperitoneal fluid volume.39 Detecting fluid depends on many factors and it seems unlikely that such a simple rule would reliably quantify the amount of free fluid in the peritoneal cavity.
The use of a scoring system to estimate fluid volume has been proposed in the literature. McKenney and colleagues developed a simple method of fluid quantification.40 Using their method, the depth of the largest collection of fluid is measured from anterior to posterior in centimeters and each additional site positive for fluid is given one point. The patient’s total hemoperitoneum score is equal to the depth of the largest collection of fluid in centimeters plus the score for additional areas positive for fluid. They found that nine out of 10 patients (90%) with a score of 2 or less were managed nonoperatively, while 35 of 46 patients with a score greater than 2 required a laparotomy. Huang and colleagues also have also developed a scoring system to determine the need for laparotomy.41 Their system was the first scoring system and assigns two points for significant fluid collections greater than or equal to 2 mm and one point for minimal fluid less than 2 mm in the pelvic, perihepatic, and perisplenic windows. They found that 24 of 25 patients (96%) with a score greater than or equal to 3 required a therapeutic operation, while only nine of 24 patients (38%) with a score less than 3 required a therapeutic operation. The FAST Consensus Conference Committee has concluded that the current US scoring systems for abdominal trauma were designed and tested on a limited number of subjects, and their ability to identify patients requiring operative therapy have not been established and will require further study.22
The hemodynamically stable patient with a negative FAST requires a minimum of six hours of observation and a follow-up FAST. There is no definitive literature to support the application of the FAST exam to this class of patient and the final decision to discharge or perform further testing must be based on the clinician’s sound clinical judgment. The FAST exam will miss patients with visceral injuries without hemoperitoneum, hollow viscus injuries and also will miss patients with hemoperitoneum below the minimum threshold for detection with US. Currently, the minimum threshold for the detection of hemoperitoneum is not known. Some have suggested that less than 100 cc of localized bleeding can be detected with US. Kawaguchi and colleagues found that 70 cc could be detected,42 while Tiling and colleagues found that 30 cc is the minimum threshold for detection of localized bleeding.3 Both have reported that fluid volumes greater than 200-250 cc can be reliably detected with multiple-view studies. A recent study by Paajanen and colleagues concluded that small volumes of free intraperitoneal fluid (10-50 mL) can be detected with current US scanners, but only near the site of injury.43 This is in contrast to other authors who, using a DPL model, have found the minimum threshold for detection using a single, perihepatic view to be at least 600 mL.39,44 The most likely reason for the high threshold in the DPL model is that it simulates a pelvic source of bleeding, with detection occurring in the perihepatic space.
Patients in the hemodynamically stable category (with a negative FAST) frequently have CT scanning performed to detect intraperitoneal injuries early in their observation course. Commonly, these patients have experienced significant mechanisms of injury or have limited physical exams due to drug intoxication or significant head injuries. The use of FAST in these patients can result in significant cost-savings. Boulanger and colleagues found that the mean diagnostic cost for patients in their FAST algorithm was $156, compared with $540 for patients in their no-FAST algorithm.45 McKenney and colleagues found that by instituting a FAST protocol, the average cost per patient evaluation with BAT decreased from $406 to $236, which is significant in this cost-conscious era.46 The hemodynamically stable patient with a negative initial FAST and no other injuries requiring hospitalization can be observed safely and discharged in six-hours if he or she remains hemodynamically stable and has a nontender, reliable abdominal exam. Patients with persistent abdominal pain or transient hypotension need to receive further diagnostic testing, such as CT scanning.
Hemodynamically stable patients with a negative FAST but with other significant injuries necessitating hospitalization, require, at a minimum, repeat FAST scanning later in their hospital course to improve sensitivity, but there should be strong consideration for CT scanning. Ballard and colleagues have recently reported that patients with pelvic ring-type fractures and spinal injury have a higher incidence of occult intra-abdominal injuries, and patients with pelvic ring-type fractures should have a CT scan of the abdomen because of the higher incidence of occult intra-abdominal injuries.47 In their study, a more specific algorithm was studied in an effort to decrease the number of missed injuries in patients with false-negative FAST exams. The algorithm prospectively was applied to patients with a blunt mechanism of trauma, a negative FAST exam, a spinal fracture (with or without spinal cord injury), or a pelvic fracture. Of the 102 patients entered in the study, there were 32 patients with spinal injury and 70 patients with pelvic fractures. Only one of the patients with a spinal injury had a false-negative FAST exam, but 13 of the patients with pelvic fractures had false-negative FAST exams. Ring-type fractures accounted for 11 of the 13 false-negative studies.
Hemodynamically unstable patients with a negative FAST need to be evaluated for other sources of hemorrhage. A negative FAST in this setting can be very helpful to the clinician by guiding them to look for extra-abdominal sources of hemorrhage. The hemodynamically unstable trauma patient who has sustained multiple injuries is a diagnostic challenge for the trauma surgeon and/or emergency physician. The sensitivity of physical exam in these patients for detecting life-threatening injuries is very low. Even assuming that the minimum threshold for the detection of intraperitoneal hemorrhage with the FAST exam is 250 cc, it would very unlikely that hemorrhage below this amount would be responsible for hypotension. Repeat scanning in these high-risk patients later in the resuscitation is necessary to detect any ongoing hemorrhage.
7
The Pediatric Patient
The use of the FAST exam in the evaluation of the pediatric trauma patient has not been widely studied, and there are no definitive guidelines for use of the modality in these patients. A majority of the studies done with US in the pediatric population have utilized a modified FAST exam that includes some formal organ evaluation or a complete abdominal sonographic exam. A study by Thourani and colleagues evaluated the FAST exam in 196 children with both penetrating and blunt injuries.48 There were 56 positive FAST exams in the study. The authors found the FAST exam to have a sensitivity of 80% and noted that the FAST exam was very helpful in detecting hemoperitoneum in hemodynamically unstable patients requiring laparotomies. A major limitation of this study is that patients with a negative FAST were followed clinically, and no further testing was done. Unlike adult patients, pediatric patients with solid-organ injuries may not show clinical evidence of their injury with observation and serial examinations. More importantly, more than 90% of solid-organ injuries in children are managed nonoperatively, so observation is not an adequate control in these studies.
A more recent study by Mutabagani and colleagues at Children’s Hospital of Columbus evaluated the usefulness of the FAST exam as a screening test in children with suspected intra-abdominal injury in an attempt to minimize the number of normal CT scans performed.49 Forty-six hemodynamically stable children were included in the study, and all FAST exams were performed by radiologists. There were four positive FAST exams, whereas there were 13 positive CT scans. The sensitivity and specificity of the FAST exam in this study were 30% and 100%, respectively. Injuries missed by the FAST exams included a liver laceration, an adrenal hematoma, a renal laceration, a small bowel injury, and a splenic laceration. The authors concluded that the FAST exam alone is not a useful screening test in the hemodynamically stable child.
A study by Patel and Tepas found similar sensitivities and specificities for the FAST exam in their study of 94 children with blunt torso trauma.50 Their study goal was to use the radiologist-performed FAST exam to determine the need for laparotomy. Three of the children in their study had injuries requiring laparotomy and the FAST exam was positive in one of the cases (33.3%). The two false-negative studies included a thoracic aortic disruption and an intestinal perforation. They concluded that while the FAST exam is not particularly sensitive (33%), it is specific (95%) in identifying the need for laparotomy and when combined with clinical exam, and can avoid additional abdominal imaging studies in 72% of children.
Both of these studies have significant limitations and cannot be referred to as definitive studies. Keeping in mind the limitations of the FAST exam will help in defining its clinical usefulness. Studies have shown that 37% of children with solid-organ injuries have no hemoperitoneum.51 As with adult patients, the FAST exam is not meant to replace CT scanning. It is designed to be utilized as a bedside screening test, particularly in the hemodynamically unstable patient. Although not a definitive study, the study by Thourani and colleagues showed that the FAST exam is useful in determining the need for laparotomy in the hemodynamically unstable child.48 Further studies will need to be done to evaluate the clinical application of the FAST exam in children, but it is unlikely that it will replace the CT scan in the evaluation of BAT in children.
Training
The issues surrounding credentialing and training are far from resolved. To date, there are no nationally recognized criteria for training in this country. In Germany and Japan, US training is a residency requirement for surgeons, and proficiency is required for board certification. The German Trauma Association’s training requirements include 15 hours in theory and 15 hours in practice, along with completion of 400 examinations under supervision.
Numerous studies have been done in this country regarding the learning curve of performing the FAST exam. The training recommendations vary from 1 to 32 hours. Ma and colleagues recommend a minimum of 10 hours;52 Tso and colleagues recommend two hours;12 Thomas and colleagues recommend eight hours;53 and Tiling and colleagues recommend eight hours.3 A recent study by Shackford and colleagues found that surgeons have a very steep learning curve for the FAST exam and that after a short course of didactic instruction, technical demonstration, and proctored exams, they can be proficient in image production and interpretation, with results similar to that of experienced radiologists.54 They utilized four hours of didactics and four hours of hands-on training in their study and suggested that as few as 10 clinical examinations may be required for nonradiologists to become competent in the FAST exam. However, they conceded that more examinations are required to absolutely establish the competence of any individual clinician and therefore, continued quality assurance monitoring of all studies is essential. The FAST Consensus Conference Committee has recommended that US training be incorporated into residency requirements in order to provide a more consistent method of training and experience.22
Non-FAST Applications
Hemothorax. US has been found to be useful in detecting pleural fluid collections. A supine chest x-ray can accurately detect a minimum of 175 mL of pleural fluid, while an upright chest x-ray can detect a minimum of 50-100 mL. By contrast, US can detect a minimum of 20 mL of pleural fluid.55
The hemothorax can be readily detected during the perihepatic and perisplenic views on the FAST exam. (See Figure 8.) If the diaphragm and pleural space is not seen on the perihepatic and perisplenic views, then the probe can be moved in a cephalad direction until the hyperechoic diaphragm and the pleural space are visualized. A hemothorax will be detected as an anechoic area cephalad to the diaphragm. Clearly determining the relationship of the fluid collection to the diaphragm will prevent the misdiagnosis of subphrenic, intraperitoneal fluid collection as a hemothorax.
Several studies have confirmed the usefulness of US as a rapid, bedside test for the detection of hemothorax. A study by Ma and Mateer compared the sensitivity, specificity, and accuracy of US with those of the initial chest x-ray for the detection of hemothorax.55 In their report, 26 of the 240 patients studied had a hemothorax, as confirmed by tube thoracostomy or CT. Both modalities were 96.2% sensitive, 100% specific, and 99.6% accurate. They concluded that US is comparable to the initial chest x-ray for accuracy in the detection of hemothorax and may expedite the diagnosis and treatment of the condition. A study by Sisley and colleagues compared the accuracy of US with a supine chest x-ray and evaluated the surgeon’s ability to detect the hemothorax.56 In their study of 360 patients, 40 patients were found to have a hemothorax, as confirmed by tube thoracostomy or CT. The 97.5% sensitivity and 99.7% specificity of US were similar to the 92.5% sensitivity and 99.7% specificity for supine chest x-ray. The US could be performed much quicker than the chest x-ray, allowing for a more rapid diagnosis. A false-positive study occurred in one patient with a pulmonary contusion without hemothorax. Since this has not been reported previously, they recommended that further evaluation be made into the sonographic qualities of pulmonary contusions in the acute setting. They concluded surgeons can accurately perform and interpret a focused thoracic US exam to detect hemothorax.
Determination of hemothorax is not a primary goal of the FAST exam. The significance of a small hemothorax seen on US and not on chest x-ray is not known and will need further study. The use of focused thoracic US is not meant to replace chest x-ray, since it will not evaluate for mediastinal injuries and pneumothorax. It is meant to expedite the treatment process and decrease the number of chest x-rays obtained.
Solid-Organ Injuries
CT scanning remains the gold standard for the detection of solid-organ injuries. The FAST exam provides only fractional views of solid-organs, such as liver, spleen, and kidney. Therefore, the determination of solid-organ injuries is made indirectly through the detection of hemoperitoneum. The incidence of solid-organ injury without hemoperitoneum has been reported to occur in about one-third of the cases in both adult and pediatric patients, which limits the value of the FAST exam.
Detection of solid-organ injury with US requires formal organ scanning, which requires greater operator skill and takes more time to complete. However, parenchymal injuries can be identified with US and may be beneficial in the case of a hemodynamically unstable patient in whom CT scanning is not feasible. The appearance of an intraparenchymal hemorrhage varies with time. Acute lacerations and hematomas will appear as localized areas of increased echogenicity, while brisk intraparenchymal bleeding without clot formation can initially appear as an anechoic area lacking sharp borders. Over time, hematomas and lacerations will become hypoechoic and lack sharp borders. Asher’s criteria for splenic injury include: splenomegaly more than 12 cm, progressive enlargement on serial exams, double contour (consistent with subcapsular hematoma), free intraperitoneal fluid, irregular splenic border, and change in contour of the spleen from supine to sitting position.2
US can also be useful in evaluating the patient with acute renal trauma.57 Isolated renal injuries are not commonly associated with the presence of free fluid in the abdomen and when present, are associated with more severe renal injuries. In patients with renal injuries, free intraperitoneal fluid is usually due to associated liver or spleen injury. The sonographic appearance of the injured kidney may be normal, despite formal scanning, but is more likely to be abnormal with severe (grade II or greater) renal injuries. Vascular injuries to the kidney may be difficult to diagnose without the use of color or power Doppler imaging. However, to date no study has been published evaluating the role of color or power Doppler US in renal trauma and therefore, contrast-enhanced CT will still be needed to evaluate equivocal cases or to better define the renal injury. A renal sonogram cannot be used to exclude renal injury.
The use of power Doppler appears to be a promising modality in the evaluation of solid-organ injuries. Power Doppler has increased sensitivity over conventional color Doppler US and allows color to be seen even in small vessels throughout both the liver and spleen, producing a "parenchymal blush." An anechoic area will appear where perfusion is lacking. An intravenous echo-enhancing contrast agent can be used when the power Doppler alone produces an inadequate "parenchymal blush." A recent study of 15 patients with BAT were studied using power Doppler.58 There were five parenchymal injuries in the study as determined by CT. All five were detected with power Doppler, resulting in both sensitivity and specificity of 100%. Five of the patients in the study required intravenous echo-enhancing contrast due to lack of "parenchymal blush." A larger, prospective study will need to be done to confirm the results.
Pneumothorax
The use of thoracic US by trauma surgeons and emergency physicians to diagnose pneumothorax is currently being studied.59 US has been used by radiologists after thoracentesis to detect pneumothorax. The presence of lung sliding can be used to rule out complete pneumothorax.60,61 The presence of a comet-tail artifact also can be useful in evaluating pneumothorax, since its presence completely discounts the presence of a complete pneumothorax.61 The comet-tail artifact is generated by a large difference in acoustic impedance between an object and its surroundings. When scanning the thorax, the comet-tail artifacts fan out from the pleural line to the edge of the screen and are assumed to be related to small subpleural water-rich structures surrounded by air, present on the lung surface and throughout the lung tissue, and separated from each other by an average distance of 7 mm. Since the comet-tail artifacts are generated by the visceral pleura and not the parietal pleura, the presence of a complete pneumothorax will prevent visualization of the visceral pleura and the comet-tail artifacts.
While not replacing chest x-ray, thoracic US to detect pneumothorax can be used when chest x-ray is not available to facilitate the diagnosis. During military conflicts and in aerospace medicine, the absence of immediate radiographic capabilities complicates the diagnosis of pneumothorax. Thoracic US appears to be a promising technique for excluding pneumothorax when chest x-ray is delayed or not available.
Summary
Bedside US is a rapid, noninvasive, and accurate test that can be performed successfully by emergency physicians and trauma surgeons with minimal training to diagnose hemopericardium and hemoperitoneum. The FAST exam has gained popularity in this country in recent years and is now a primary screening test in the patient with thoracoabdominal trauma. The use of US by emergency physicians and trauma surgeons to evaluate hemothorax, pneumothorax, and solid-organ injuries is currently being studied to determine its clinical usefulness and may become standard-of-care in these areas in the future.
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