Ultrasonography for Novices

Transcription

1 module 03 Ultrasonography for Novices Stephanie J. Doniger, MD, RDMS, FAAP, FACEP Lei Chen, MD, FAAP Objectives 1Understand the basic principles of ultrasound physics. 2Be familiar with basic controls common to most medical ultrasound equipment. 3 Understand the adjustments needed to obtain the ideal image for the purpose at hand. 4Understand how to incorporate bedside ultrasonography into the ABCD assessment. 5 Understand 6 Use how to perform ultrasound assessments of the airway, breathing, circulation, and trauma. ultrasonography in the initial resuscitation and stabilization of acutely injured or ill children. Module Outline Introduction Principles of Ultrasound Physics and Equipment Airway/Breathing Cardiovascular: Cardiac Assessment Cardiovascular: Vascular Access Disability

2 Introduction Ultrasound technology was originally introduced in the 1950s, and portable scanners made their way into general emergency departments (EDs) in the 1980s. 1,2 Bedside ultrasonography in the ED emerged as a skill performed and interpreted by emergency physicians. This information was incorporated into their clinical decision-making process. In contrast to traditional ultrasound imaging, the goals of bedside ultrasonography are to answer specific yes/no questions and to assist in resuscitative procedures. It is only recently that the use of bedside ultrasonography has begun to emerge in the field of pediatric emergency medicine. It is an especially attractive modality for children because it involves no ionizing radiation and has the potential to decrease the use of radiography and computed tomography (CT). Bedside ultrasonography or point-of-care ultrasonography can be used as an adjunct to the assessment of the critically ill, injured patient. Specifically, bedside ultrasonography can be used as part of the ABCD assessment and resuscitation. 3 The airway assessment includes the assessment for patency, spontaneous ventilation, and trachea displacement and as an adjunct for endotracheal tube placement confirmation. For breathing, one can assess the lungs, pleura, and diaphragm, to determine the presence of a pneumothorax or a pleural effusion, and to assist in needle aspiration and tube thoracostomy. For circulation, one can determine cardiac and hemodynamic status, assess for asystole, the presence of a pericardial effusion, and hydration status via the evaluation of the inferior vena cava (IVC). Ultrasonography can be further used for procedural guidance for central and peripheral venous access and pericardiocentesis. Lastly, bedside ultrasonography can be used in determining disability as with the focused assessment sonography for trauma (FAST). Although extremely useful in the assessment and stabilization of an acutely ill or injured patient, bedside ultrasonography has some limitations. With certain applications, the sensitivity of bedside ultrasonography might be lower than one would prefer. Therefore, it is best to avoid using it to rule out diagnoses; rather, it should be used to answer specific yes/no questions. Furthermore, to standardize training and quality assurance of this modality, the American College of Emergency Physicians published recommendations in 2008 for bedside ultrasound indications and guidelines for training, credentialing, and developing continuing quality assurance. 4 Principles of Ultrasound Physics and Equipment Physics Sound is a form of pressure wave. The basic characteristics of the wave include speed, frequency, and amplitude. Ultrasound is defined as sound of frequencies that are above the upper range of human hearing (20 khz). Medical ultrasonography used for imaging usually uses sound waves of frequencies between 1 and 10 MHz. When a sound wave encounters a barrier, three effects can result: reflection, refraction, and absorption. An ultrasound instrument uses these properties to present a visual representation of the structures of interest. The frequency of the ultrasound determines a variety of properties of the images formed. Simplistically, higher frequency sounds are associated with less penetration but greater spatial resolution (greater image detail). Lower frequency sound waves are conversely associated with greater penetration at the cost of poorer image quality. For example, in imaging superficial structures, such as the internal jugular vein or the pleura, high-frequency probes (8 12 MHz) are generally used. To visualize intra-abdominal structures, lower-frequency probes (1 5 MHz) are more suitable. Most modern probes allow manipulations of the base frequency to optimize the imaging. Transducers Transducers (Figure 1), broadly speaking, are instruments that transform one form of energy to another. In medical ultrasonography, the transducers, or probes, change electrical signals to acoustic signals and perform the reverse. The probe contains between 64 and 256 piezoelectric crystal elements, neatly arranged and tightly Principles of Ultrasound Physics and Equipment 3

3 packaged into the footprint. These elements vibrate and emit extremely short bursts of sound waves of certain frequencies. The sound waves are emitted, reach the object of interest, and return to the transducer. The transducer then spends most of the time listening to the return of the emitted sound waves. The piezoelectric elements in the probes convert the sound signals to electrical signals, using the strength to determine the brightness of the object when displayed on the screen. Figure 1 Types of transducers, from left to right: phased array, curvilinear, and linear. The notch on the side of the curvilinear transducer is an indicator, or orientation marker. Most modern ultrasound probes are of the broadband variety, allowing them to operate at a broad range of frequencies and allowing the user to choose the optimal frequency for a variety of applications. In essence, each element within the transducer senses only two factors: the strength and the timing of the return echo. These factors are affected by the properties and the location of the tissue under investigation. These signals are in turn used by microprocessors to form a two-dimensional representation of the tissue. The strength, or loudness, of the echo in acoustics is determined by the relative differences of impedances of materials on both sides of an interface. The impedance can be calculated by the product of the density of the material and the speed of sound in the material. For example, bones and metallic objects have higher impedances than blood and muscle and generate louder echoes. Therefore, they look bright or hyperechoic on ultrasounds of biological tissues. Plastic and wood, on the other hand, have lower densities than blood. They also appear hyperechoic (look bright) in biological tissues; however, because their impedances are still higher than that of blood, the speeds of sound in these materials are higher. Therefore, ultrasonography can be used to image foreign bodies, such as wood, that are not visible on conventional radiographs. The location of the reflector in the tissue is calculated from the timing of the return echo. Each transducer element collects the information on the strength and timing of the echo. The microprocessor synthesizes the one-dimensional information from each element into a two-dimensional display on screen. Linear transducers have flat footprints where they contact patients. They are usually designed to study superficial structures. They use relatively higher frequencies (8 12 MHz) and yield highresolution images of superficial structures. The beams are parallel to each other and perpendicular to the probe footprint. Therefore, the imaging window is in a rectangular shape (Figure 2). The limitation inherent in the use of these transducers is poor penetration. These transducers are best used to investigate superficial structures, such as central and peripheral blood vessels, testes, and superficial abscesses. Another potential use is the facilitation of invasive procedures involving these structures (eg, vessel cannulation and arthrocentesis) because they provide excellent spatial resolutions, offering fine details of the structures under investigation. Figure 2 Image of the pleura with adjacent rib performed with a linear high-frequency probe. Shadowing is seen deep to the rib. Curvilinear transducers are general-purpose probes, usually with a frequency range of 2 to 7 MHz. They are designed to investigate 4 Ultrasonography for Novices

4 deeper structures. These transducers generally have fewer elements than linear probes. The sound waves are closer together near the transducer and fan out as they travel deeper into the tissue, offering a wider field of view. Images are fan-shaped. Near field resolution, where the beams are closer together, is better than far field, where the beams are further apart. In other words, although the sound can penetrate deeper into tissues, images nonetheless degrade as the distance to the transducer increases. Other special purpose probes are less commonly used in bedside ultrasonography. One such example is the phased-array probe, also called a sector probe, usually used for cardiac indications. Sound beams are electronically steered in a fanned manner. These beams have smaller flat footprints and are designed to visualize structures through small acoustic windows, such as the intercostal spaces. Another specialized transducer is the endocavitary probe, sometimes specifically referred to as the transvaginal probe. These probes are designed to be inserted into body cavities to get close to the tissues, such as peritonsillar tissues, ovaries, and the prostate. Basic Controls When confronted with a medical ultrasound unit, the clinician is often intimidated by the bewildering number of controls. It is useful to keep in mind that each element only senses two factors: how loud the return echo of a particular tissue is and how far the tissue is in relation to the element. The various adjustments offer the means to balance competing factors to best visualize the structure under investigation. The most important balance is that among frequency, depth, and resolution. B-Mode and M-Mode The two major imaging modes used in bedside ultrasonography are termed the B (brightness) mode and M (motion) mode. In the B-mode ultrasonogram, a two-dimensional representation of the structure under question is displayed using all elements in the transducer. Most studies in bedside ultrasonography are performed using B-mode ultrasonography. In the M-mode, only structures in the line of sight of one or a few elements are displayed. These one-dimensional signals are displayed on the screen, with the x- axis representing time (Figure 3). Figure 3 M-mode image of a fetus showing rhythmic activity of the heart (arrows). Frequency and Depth Generally speaking, higher-frequency probes offer better resolution for superficial structures. Broadband probes offer several operating frequencies to optimize either penetration (lower frequencies) or resolutions (higher frequencies). For detailed studies involving a limited and superficial area, such as a suspected skin abscess, a linear high-frequency probe would offer the best option. For structures deeper in the body, such as the abdominal organs in the FAST scan, deeper penetration is needed. Therefore, a curvilinear probe of lower frequencies is used. Gain and Time Gain Control The gain control determines how much the return signals are amplified as displayed on the screen. This control affects the signals globally and is otherwise known as the overall brightness of the image. On the other hand, the time gain control, often likened to a graphic equalizer on an audio system, offers a way to separately adjust the gain at various depths. Time equals depth in medical ultrasonography. Although termed time gain control, depth gain control is probably a more logical term. These controls are used to optimize the image by adjusting the dynamic range (gain) and contrast (time gain control), respectively, of the structures under investigation. Artifacts Artifacts are important concepts for a sonographer to recognize. They are defined as echoes appearing on the image that do not correspond Principles of Ultrasound Physics and Equipment 5

5 in location, intensity, or to actual structures in the patient. Although artifacts can cause false images, they can also allow the sonographer to more readily identify particular structures. Attenuation describes how quickly the signal fades. This concept is related to the degree of penetration or how far into the biological tissue the ultrasonogram can see. Water and blood have the lowest attenuation. Conversely, structures such as bones and metal have high attenuation. An acoustic window is a structure that is a good transmitter of ultrasound. Structures such as the bladder and heart chambers are good acoustic windows. In essence, acoustic windows allow visualization of deeper structures because of their low attenuation of the sound signal. By scanning through these acoustic windows, deeper structures can be imaged. A good example is the transabdominal view of the uterus. A full urinary bladder acts as an acoustic window, affording improved views of the uterus and other pelvic structures. Shadowing is an easy concept to understand. A strong reflector of sound casts an acoustic shadow; there is little energy left to visualize the tissue behind the objects, such as bones and gallstones. Figure 2 shows a shadow behind the rib. Mirror-image artifacts are created when strong reflectors of sound, such as the diaphragm, act as acoustic mirrors. In the example, the skull is creating a false image of structures deep to the skull, whereas, in fact, the visualized structure is actually a mirror image of the scalp hematoma (Figure 4). Figure 4 Image of a scalp hematoma in an infant. The immature skull acts as a reflector, with the mirror-image artifact visualized seemingly beneath the skull. Landmarks, Orientation, and Standard Views Unlike other imaging modalities, ultrasound images can be performed in multiple imaging planes. To facilitate communication, recordkeeping, and quality assurance, it is important when acquiring bedside ultrasound images to adhere to a standard orientation and a set of landmarks as reference points, when available. Consistent orientation (right vs. left, caudal vs. rostral) should be ensured by making use of the transducer marker or indicator, which is usually a dot on the screen that corresponds to the mark on the transducer (see Figure 1). The transducer indicator is usually oriented towards the patient s right side or head. The exception is the cardiac examination (see Cardiovascular: Cardiac Assessment). Text labels should be provided since they can be helpful in providing orientation. Conclusions The principles of ultrasound physics and basic controls of medical ultrasonography are understood by most pediatric emergency medicine physicians. Many of the existing and developing applications can be useful in the initial resuscitation and stabilization of critically ill or injured children. It is important to understand the strengths and limitations of these imaging modalities. Airway/Breathing As part of the assessment of the airway and breathing, one must determine whether the patient is spontaneously breathing and whether the patient s airway is maintainable. Part of the assessment of the airway and breathing includes the early determination of whether the patient has a pneumothorax or a pleural effusion. Lung ultrasonography, although relatively new, has developed into a modality that can easily and rapidly detect pneumothoraces and pleural effusions. More recently, bedside ultrasonography has been shown to be a useful adjunct to traditional methods of the confirmation of endotracheal placement. 6 Ultrasonography for Novices

6 Pneumothorax Conventional methods of identifying a pneumothorax include auscultation, portable chest radiographs, and tomography. Compared with portable radiographs, bedside ultrasonography might actually be more sensitive. 5 Chest radiographs are especially inaccurate in a supine patient because air layering anteriorly is difficult to detect. Blaivas et al 6 showed ultrasonography to be superior to chest radiography in identifying traumatic pneumothoraces, with ultrasonography having a sensitivity of 98.1% and a specificity of 99.2% compared with chest radiographs, which had a sensitivity of 75.5% and specificity of 100%. When performing a more comprehensive lung assessment, one can further differentiate small, medium, and large pneumothoraces. 6 Pleural Effusions When compared with chest radiographs for a traumatic hemothorax, ultrasonography is comparable with a sensitivity of 96.2%. 7 Lung consolidations and effusions are most readily identified at the dependent and dorsal regions of the chest. Ultrasonography may also serve as an adjunct to performing the drainage of pleural effusions, or thoracentesis. Endotracheal Intubation Confirmation Endotracheal tube misplacement in emergent pediatric intubation ranges from 17% to 40%. The reliance of physical examination alone is not sufficient for confirmation, and the other traditional methods of primary confirmation are often unreliable. These methods of primary confirmation of endotracheal tube placement include the direct visualization of the tube passing through the vocal cords, auscultation, end-tidal carbon dioxide detection, and esophageal detection devices. End-tidal carbon dioxide in particular has a sensitivity of almost 100%, but it is notably unreliable in low flow states or cardiac arrest. 8 Chest radiography is the criterion standard for secondary confirmation and determines the position of the endotracheal tube in the airway. Bedside ultrasonography can be an accurate, rapid method to determine the presence of the endotracheal tube within the trachea in pediatric patients. Initially, an indirect method was used, which looks for bilateral lung sliding and diaphragmatic movement. More recently, a more direct ultrasound evaluation performed at the cricoid region evaluating the trachea and esophagus has been suggested. It can be a useful adjunct, especially when visualized in two views (transverse and longitudinal) 9 and when a dynamic rather than static assessment is performed. 10 With dynamic or real-time ultrasound visualization as endotracheal intubation is performed, both sensitivity and specificity approach 100%. 11 Kerrey et al 12 showed overall agreement of ultrasonography with chest radiography for endotracheal tube placement in pediatric patients. Although ultrasonography was not equivalent to chest radiography, ultrasonography was timelier and detected more misplacements when compared with standard primary confirmation methods alone. Although there is insufficient research to support the widespread implementation of its use, the American College of Emergency Physicians policy statement endorses the use of ultrasound imaging as a helpful adjunct to detect and monitor the proper location of endotracheal tubes. 13 Technique Lung: Pleural Interface Although a comprehensive lung assessment involves six regions on each hemithorax, 14 the rapid assessment as part of the extended FAST (E-FAST) examination assesses two regions at the anterior chest. If a pneumothorax is identified at the anterior aspect of the chest, one can extend the examination to the lateral regions of the chest wall to localize the point where the normal lung pattern replaces the pneumothorax pattern ( lung point ). 15 In a supine patient, a 5- to 10-MHz linear transducer is placed in a longitudinal orientation on the chest wall, with the probe indicator toward the patient s head, at the third to fifth intercostal spaces at the midclavicular line (Figure 5). Alternatively, a low-frequency (3 5 MHz) transducer can be used. Although the image detail will be less with a low-frequency transducer, the pleural line with lung sliding or its absence is readily discernible. It is Airway/Breathing 7

7 important to perform ultrasonography on the patient s right and left anterior aspects of the chest for comparison. A B Figure 5 Transducer placement for screening lung ultrasonography. In order to identify the pleural line, one may look for the pattern known as the bat sign. The two ribs with their posterior shadows are the bat s wings, while the hyperechoic pleural line represents the bat s body. This is shown in Figure 6A (linear probe) and 6B (curvilinear probe). A normal lung ultrasound screening result includes lung sliding at the pleural interface and comet-tail artifacts. Conversely, an ultrasound screening result is positive for a pneumothorax when both lung sliding and comet-tails are absent. The absence of lung sliding and absence comet-tail artifacts yields a sensitivity of 100% and a specificity of 96.5%. 16 Physiologically with respirations, the visceral and parietal pleura slide against each other. This to and fro movement is called lung sliding. Air or fluid within the pleural space hinders the propagation of sound waves, thereby preventing lung sliding from occurring. This lung sliding can be evaluated by gross visual inspection or, more objectively, using M-mode with the cursor oriented over the pleural line. Normal lung sliding creates a characteristic appearance called the seashore sign (Figure 7A), whereas absent lung sliding will create a characteristic appearance called the stratosphere sign (Figure 7B). A comet-tail artifact occurs when sound waves bounce between two closely spaced interfaces and create hyperechoic reverberation artifacts that originate from the pleural line and extend vertically. 17 Figure 6 A. Bat sign with the high-frequency linear transducer. B. Bat sign with the low-frequency transducer. Pleural Effusions Ultrasonography of the pleural space is performed with a low-frequency 2.5 to 5.0 MHz transducer. Pleural effusions typically appear as hypoechoic or anechoic fluid collections superior to the diaphragm (Figure 8A). These effusions also create a loss of the typical mirror image artifact that one sees of the liver or spleen superior to the diaphragm on the right upper quadrant or left upper quadrant views (see FAST) (Figure 8B). In addition, when assessing the anterior aspect of the chest, lung sliding will be absent because the fluid in the interpleural space interferes with normal lung sliding. Endotracheal Tube Placement Confirmation The indirect evaluation for endotracheal tube placement examines the lungs for lung bilateral sliding and for diaphragm movement. Unilateral absence of lung sliding or diaphragm movement suggests a mainstem intubation, while bilateral loss of sliding or diaphragm movement suggests an esophageal intubation. More recently, a direct approach has been suggested. A high- 8 Ultrasonography for Novices

8 A B Figure 7 A. Seashore sign - the normal lung pattern on M-mode. B. Stratosphere sign - the pneumothorax pattern on M-mode. A B Figure 8 A. Pleural effusion as visualized in the view of the right upper quadrant. Note the absence of a normal mirror-image artifact of the liver; alternatively, one may visualize the collection of anechoic fluid above the diaphragm. B. Normal right upper quadrant view; above the diaphragm there is a normal mirror-image artifact of the liver. frequency (5 to 10 MHz) linear probe is used and oriented transversely at the anterior neck. Scanning can begin superior to the suprasternal notch, then moving cranially to visualize the glottis. 18 The true vocal cords can be identified as paired hyperechoic linear structures with mobility with respiration and swallowing (Figure 9). Once the endotracheal tube is placed, it can be difficult to visualize the tube itself. Ultrasonography cannot easily see the tube within the trachea; rather, confirmation relies on the absence of a tube within the esophagus, which appears as a shadow within the esophagus (Figure 10). Rotating the transducer 90 and obtaining a longitudinal image improves visualization of the endotracheal tube in the larynx. Figure 9 Lower neck anatomy. Note that the esophagus is to one side of the trachea. It can be to the left or the right of the trachea rather than directly behind it. Courtesy of Beatrice Hoffmann, MD, PhD, RDMS. Reprinted with permission. If the endotracheal tube is placed in the esophagus (Figure 11), ventilation will not result in lung expansion, and lung sliding will be Airway/Breathing 9

9 absent. Similarly, an endotracheal tube placed in the right mainstem bronchus will result in the absence of lung sliding on the left side 19 (Figure 12). Figure 12 Right mainstem intubation. Note normal sliding (the seashore appearance) on the right lung. Note the absence of sliding (the stratosphere appearance) on the left lung, suggesting a pneumothorax or, in this case, no ventilation due to a right mainstem intubation. Courtesy of Beatrice Hoffmann, MD, PhD, RDMS. Reprinted with permission. Figure 10 Tracheal intubation confirmed. The yellow curve shows the anterior aspect of the airway. The yellow oval shows the soft tissue of the esophagus. Tracheal intubation is confirmed because the soft tissue appearance of the esophagus is confirmed to lack a tube. Courtesy of Beatrice Hoffmann, MD, PhD, RDMS. Reprinted with permission. 3. Observation of the widening of the glottis as the tracheal tube passes. 4. Observation of enhanced posterior shadowing of the trachea. 5. Absence of a tube in the esophagus Demonstration of bilateral lung sliding to confirm placement above the carina. 18 Cardiovascular: Cardiac Assessment Figure 11 Esophageal intubation. The yellow curve shows the anterior aspect of the airway. The black space with the yellow asterisk is a shadow cast by a tube in the esophagus. Note that the soft tissue appearance of the esophagus is absent, indicating that the esophagus is intubated. Courtesy of Beatrice Hoffmann, MD, PhD, RDMS. Reprinted with permission. Suggested elements as sonographic criteria for pediatric tracheal intubation are as follows: 1. Identification of the trachea and tracheal rings. 2. Visualization of the vocal cords (linear hyperechoic structures that move during spontaneous respirations). The incorporation of bedside echocardiography has been shown to have a significant impact on medical decision-making and be a useful adjunct to the clinical examination when radiologic and laboratory studies are often unreliable and nonspecific. 21,22 In cardiac assessment in the ED, bedside ultrasonography is not meant to replace formal, comprehensive echocardiography. Rather, it is best seen as an extension of the physical examination and is meant to answer specific binary yes/no questions, such as Is there a pericardial effusion? Is cardiac activity present? 23 Emergency physicians can rapidly assess for the following: pericardial effusion, tamponade, cardiac activity, contractility, and central volume status. 24 Asystole vs. Pulseless Electrical Activity Pulseless cardiac arrest is the cessation of cardiac activity and the absence of palpable central pulses. In children, it is especially important to determine whether a pulse is present to initiate early basic and advanced life support to improve survival Ultrasonography for Novices

10 Unfortunately, pulse palpation is extremely unreliable in diagnosing pediatric cardiac arrest, with an accuracy of only 78%. 26 However, ultrasonography can reliably distinguish cardiac activity from standstill and can be performed in 10 seconds, the time recommended to perform a pulse check. One can rapidly correlate the absence of left ventricular (LV) motion and the presence or absence of a pulse. 27 Once the physician determines that the child is pulseless, it is crucial to identify potentially correctable causes of pulseless electrical activity (PEA), which include hypovolemia, hypothermia, hypokalemia or hyperkalemia, acidosis, hypoxia, tamponade, tension pneumothorax, toxins, and thromboembolism. 28 Of the causes of PEA, bedside ultrasonography has the potential to rapidly identify tamponade, tension pneumothorax, thromboembolism, and hypovolemia. Pericardial Effusions and Tamponade In adult patients with PEA or near PEA, emergency physicians can detect pericardial effusions with correctable causes and distinguish them from true PEA with ventricular standstill. 29 Of particular clinical importance is the development of tamponade, which can lead to cardiovascular collapse and ultimately death. Echocardiography is the most sensitive and specific means of detecting tamponade 30 and has the potential to detect early signs of tamponade before the patient becomes unstable. 23 The treatment of tamponade includes emergent pericardiocentesis or drainage of the pericardial fluid. Few medical situations exist in which a simple, quickly performed medical procedure can result in immediate life-saving results. 31 The routine use of ultrasonography is strongly encouraged because it is the most effective means of reducing the risk of complications associated with pericardiocentesis. 32 A study of ultrasoundguided pericardiocentesis in pediatric patients revealed a 99% success rate, with 93% on the first attempt, and 1% major complication and 3% minor complication rates. 33 Shock A common presenting problem in the ED is dehydration. As dehydration progresses, shock can develop, which is defined as the failure of the circulatory system to provide adequate oxygen delivery to vital organs. The sonographic assessment of LV function or ejection fraction can be useful in patients presenting with unexplained hypotension or shock. This assessment can expeditiously narrow the differential diagnosis (ie, distinguishing cardiogenic from hypovolemic shock) to begin early aggressive and appropriate resuscitation. 21 Bedside ultrasonography can also assist in monitoring the effectiveness of inotropic support or vasopressor infusion and aid in the optimization of therapy. 34 Another means by which ultrasonography can aid in evaluating shock and hydration status is the estimation of central venous pressure by performing measurements of the inferior vena cava (IVC). Echocardiograph measurement of the IVC was first introduced as an estimation of right-sided cardiac function in dialysis patients. 35 However, because the IVC diameter correlates with body height and body surface area, it is especially challenging and potentially unreliable in children. Recent studies have suggested using measurements in relation to the respiratory cycle and as a ratio to the diameter of the aorta During normal respiration, negative intrathoracic pressure draws blood from the highly compliant IVC to the right atrium, causing the IVC to collapse. Although the absolute size varies among patients, one can grossly estimate whether the central venous pressure is very high or very low by measuring the maximum and minimum diameters during respiration. 21 With dehydration states, the IVC will show almost complete collapse with respirations. It has been recently suggested that because the aorta does not change with hydration status and is also correlated with body surface area, age, and sex, the IVC:aorta ratio can be a reliable, easy method of assessing hydration status. 38 Chen et al 37 showed that this ratio was lower in clinically dehydrated children and increased with the administration of intravenous (IV) fluid boluses. Technique Low-frequency transducers (2 to 5 MHz) should be used for cardiac assessment with either the phased array or the curvilinear probe with a small footprint to allow for imaging between ribs Cardiovascular: Cardiac Assessment 11

11 (Figure 13). Although there is much controversy surrounding probe orientation, 39 emergency ultrasonographers often suggest using the abdominal setting on the machine to avoid confusion. The cardiac setting on the machines flips the image on the screen. Therefore, to create a screen image that is consistent with echocardiographs, the probe marker needs to be rotated 180 in relation to those transducer positions traditionally used by cardiologists. the patient s body, in contrast to most other positions, which are perpendicular to the patient s body (Figure 14). Figure 14 The subxiphoid view. The transducer is oriented with the probe marker towards the patient s right and at a 15-degree angle to the patient. Figure 13 Curvilinear transducers 2 to 5 MHz. Note the small footprint, which can easily scan in between rib spaces, and which is ideal for bedside cardiac ultrasonography. Standard views for bedside echocardiography include subxiphoid, parasternal long-axis, parasternal short-axis, and apical four-chamber views. Angling and tilting of the transducer might be necessary to obtain the proper images. All abnormal findings should be confirmed with multiple views. 4,40 Additional assessments involve IVC measurements for hydration status. Subxiphoid View The subxiphoid view is the preferred view in the FAST examination and is the most useful during resuscitation because it does not typically interfere with resuscitation and procedures. The probe marker should be oriented to the patient s right side and is placed subcostally, just below the xiphoid process, directing the probe in the direction of the patient s left shoulder. The probe itself is flat and should be held at a 15 angle to The subxiphoid view uses the liver as an acoustic window and readily visualizes all four chambers of the heart (Figure 15). It is the most accurate in identifying pericardial effusions, especially at the posterior pericardium, where effusions begin to develop. It can be difficult to obtain in obese patients. An example of an abnormal subxiphoid view is of a child with dilated cardiomyopathy and a circumferential pericardial effusion (Figure 16). Parasternal Long-Axis View The parasternal long-axis view is the preferred view for the assessment of LV contractility and the identification of pericardial effusions when adequate subxiphoid images cannot be obtained. The probe is placed perpendicular to the chest wall, immediately to the left of the sternum, between the third and fourth intercostal spaces, above the level of the nipple line. The probe marker should be directed toward the patient s left hip or at the four o clock position (Figure 17). A distinguishing feature is that the aortic outflow tract is stacked on top of the left atrium (Figure 18). The parasternal long-axis view is best used to measure contractility. In review of methods of sonographically determining LV function, McGowan et al 41 performed a visual estimation 12 Ultrasonography for Novices

12 Figure 15 The subxiphoid view, with the liver shown anteriorly. (RV=right ventricle, LV= left ventricle, RA= right atrium, LA= left atrium.) Figure 17 The parasternal long-axis view. The transducer is placed just lateral to the sternum and is oriented with the probe marker towards the patient s left hip, or 4:00 perpendicular to the patient. Figure 16 Subxiphoid view of a 2-year-old child with dilated cardiomyopathy and a circumferential pericardial effusion. Figure 18 The parasternal long-axis view. (LV= left ventricle, RV= right ventricle, Ao= aortic outflow tract, LA= left atrium.) of cardiac function and classified it into normal, mild, moderate, or severe dysfunction. This visual estimation was shown to be as good or better than other more complex, calculated methods. Parasternal Short-Axis View The parasternal short-axis view is used to evaluate contractility and valvular function. Once the parasternal long-axis view is obtained, the transducer should be rotated 90 to obtain the parasternal short-axis view. The probe marker should be directed toward the patient s right hip or at the seven o clock position (Figure 19). One can sweep the transducer from the base of the heart to the apex to visualize various levels of the heart in cross-section, from the level of the mitral valve, papillary muscles, and tricuspid valve. The distinguishing feature of the parasternal short-axis view is that the left ventricle is circular with a valve in the middle, resembling a fish-mouth or doughnut (Figure 20). Apical Four-Chamber View The apical four-chamber view is the preferred view for evaluating the relative dimensions of the right and left side of the heart. The transducer is placed at the cardiac apex, which can be located by palpating the point of maximal impulse. This apex is typically located at the thoracic spinal nerve 4 to thoracic spinal nerve Cardiovascular: Cardiac Assessment 13

13 Figure 21 The apical four-chamber view. The transducer is placed over the PMI, oriented with the probe marker towards the patient s right. Figure 19 The parasternal short-axis view. The transducer is oriented with the probe marker towards the patient s right hip, or 7:00. This is a 90-degree rotation from the parasternal long-axis view. Figure 22 The apical four-chamber view. (RV= right ventricle, LV= left ventricle, RA= right atrium, LA= left atrium.) Figure 20 The parasternal short-axis view, also known as the doughnut or fish-mouth view of the left ventricle (LV). 5 (T4 T5) level, at the fifth intercostal space, just lateral to the nipple. The probe marker is typically oriented toward the patient s right side (Figure 21). Patients often need to be rotated to their left side to bring the heart more anterior to the chest wall and to decrease artifact associated with the left lung (Figure 22). Inferior Vena Cava The transducer is placed in the subxiphoid position perpendicular to the patient s body (Figure 23), and the IVC can be traced as it travels Figure 23 Subxiphoid probe position to view the IVC. 14 Ultrasonography for Novices

14 behind the liver. 23 In adults, the mean IVC size is 9.2 ± 2.4 mm/m 2 of body surface area. In mild hypovolemia, the IVC collapses by 50% during quiet inspiration. 42 In severe hypovolemia, the IVC collapses completely during inspiration. Absolute measurements of the IVC have not shown to be as effective as the percentage of collapse during respiratory variation. 43 In adult patients, this measurement is made within 3 to 4 cm of the entrance into the right atrium in the longitudinal view or 2 cm distal to the hepatic vein confluence. At this location in the sagittal view, the anterior and posterior walls are parallel (Figure 24). One can measure the maximal and minimal diameters of the IVC with respiration. This is best obtained in M-mode ultrasonography, with a cine-loop to evaluate several respiratory cycles (Figure 25). An additional method is to compare the ratio of IVC to aorta diameters. The aorta lies adjacent to the IVC (to the right), just anterior to the vertebral body (Figure 26). A proposed IVC/aorta index value has been suggested: 1.2 ± M-Mode Ultrasonography: Asystole M-mode ultrasonography allows for a one-dimensional tracing of structure movement over Figure 25 IVC measurements using M-mode cine loop. Figure 26 Transverse or cross-sectional view of the inferior vena cava (IVC) in relation to the aorta. Figure 24 The sagittal, or longitudinal view of the IVC. time. In the parasternal long-axis view, the cursor can be placed over the walls of the left ventricle to assess for contractility (Figure 27). This is the most effective method for evaluating for asystole (Figure 28). When performing M-mode ultrasonography to assess for cardiac contractions, compressions and artificial respirations must be held to avoid motion from external sources other than from the ventricle. Tamponade and Pericardiocentesis On sonography, a pericardial effusion appears as an anechoic or dark stripe surrounding the Cardiovascular: Cardiac Assessment 15

15 needle. This approach is associated with fewer episodes of cardiac lacerations, pneumothoraces, pneumoperitoneum, and liver lacerations associated with pericardiocentesis. 44 Figure 27 M-mode of normal cardiac activity. When the cursor is placed over the left ventricular walls, cardiac contractions are plotted on the 1-D representation of motion. Figure 28 M-mode of asystole, with absence of cardiac contractions. heart. Early effusions appear first at the posterior pericardium and eventually develop anteriorly, ultimately creating a circumferential effusion. Features of sonographic tamponade include a circumferential pericardial effusion, with a hyperdynamic heart that might exhibit right atrial compression during late diastole and right ventricular collapse during early diastole also known as scalloping. Other sonographic findings include abnormal mitral valve motion, a dilated IVC with lack of inspiratory collapse, and a swinging heart. 33 A swinging heart is the counterclockwise rotational movement of the heart. Tamponade is an indication for pericardiocentesis. The parasternal long-axis approach provides a more direct anatomical approach (Figure 18) and direct visualization of the Cardiovascular: Vascular Access An important aspect of the cardiovascular assessment and intervention is vascular access. Pediatric patients in particular can present challenges with IV access, which can be especially difficult in smaller or medically complex patients. Moreover, IV catheter placement in this group requires considerable time and expertise. Failure might necessitate alternative routes that include intraosseous (IO) needle placement, central venous cannulation, or a venous cut down. Such alternatives are more time-consuming 45 and involve patient discomfort and the risk of complications. 46 Bedside ultrasonography has been shown to facilitate central venous access in adult 47 and pediatric patients. 48 Compared with traditional landmark-guided approaches, ultrasound-guided central venous cannulation results in faster access with fewer complications, mean insertion attempts, and placement failures. 49 In 1999, the Institute of Medicine report 50 recommended ultrasound guidance as the standard of care in the placement of all central catheters as a means to improve patient safety. Ultrasound guidance is especially beneficial for the subset of patients with difficult venous access. 51 More recently, bedside ultrasonography has been applied to vein localization and the placement of peripheral IV catheters in adults and children. 55 When compared with traditional cannulation techniques, ultrasoundguided peripheral IV catheter placement has a greater success rate, with fewer skin punctures, decreased time for IV catheter placement, and fewer complications. Although this idea seems attractive, it may be challenging, especially for very small veins. As alternatives to peripheral and central catheter placement, IO catheters can be placed manually and by automated devices. Although 16 Ultrasonography for Novices

16 IO catheters can be expeditiously placed, there are potential complications of extravasation of fluid, thereby creating tissue necrosis and compartment syndrome. Traditional methods of placement confirmation, including aspiration of bone marrow, blood visualized at the needle tip, the needle standing firmly upright, or the infusion of fluid without soft tissue infiltration, are unreliable. It has recently been suggested that Doppler ultrasonography can further assist in determining whether an IO needle is placed correctly in the IO space rather than soft tissue. 56 Technique For procedural guidance and vascular access, the high-frequency ( 7.5 MHz) broadband linear array transducer is used, which offers high resolution at superficial depths. This technique can be applied to central or peripheral catheter placement. Skin preparation should be performed, and the probe must be properly prepared to maintain a sterile field, which is especially important for central catheter placement. Sterile sheaths are commercially sold. Alternatively, a sterile glove with sterile surgical lubrication gel can be used. Because ultrasound must travel through a liquid medium to provide an image on the screen, one must apply gel on the transducer, then place a sterile sheath over this, followed by sterile gel external to the sheath before placing on the patient for imaging (Figure 29). It has been suggested that probe preparation with Tegaderm tm and povidone-iodine gel as the conductance medium is a cost-effective and reproducible technique for maintaining sterility. 57 Whenever possible, long needles or echo-tip needles, which can offer improved visualization and success, can be used. Tourniquets should be used for peripheral catheter placement as with traditional placement techniques. Different techniques for obtaining vascular access can be used. The static approach is one in which the operator identifies the vein by ultrasonography, marks the skin, then places the IV catheter without further use of ultrasonography. Real-time guidance has been shown to be superior in success rates and allows the operator to visualize in real time as the needle enters the vessel. Single- and dual-operator methods Figure 29 Probe preparation. have also been described. In the single-operator method, one person holds the transducer in the nondominant hand while placing the central or peripheral catheter with the dominant hand (Figure 30A). A dual-operator technique has one person as the probe person and another as the IV catheter person (Figure 30B). Vessels can be visualized in the short axis, which is a cross-section of the vessel (Figure 31), or the long axis, which visualizes the length of the blood vessel (Figure 32). Studies have shown the short-axis approach to be quicker and more accurate, especially with novice sonographers. In the short-axis approach, the indicator or probe marker should be placed to the IV catheter person s left (the person who is holding the IV catheter), so that any movement in the horizontal plane will parallel the visualization on the ultrasound monitor. The orientation of the indicator for the long axis is according to the sonographer s preference. If the indicator is oriented distally, the needle should come from the upper left to the lower right on the ultrasound monitor. Once the vessel is identified, gentle pressure compression is used to distinguish veins from arteries. With gentle pressure with the transducer, Cardiovascular: Vascular Access 17

17 A B veins should collapse while arteries should stay patent (Figure 33). With sufficient pressure, some arteries will collapse. Other means by which one can distinguish artery from vein is color Doppler, which shows constant flow through a vein compared with the pulsations from the artery. Once the vessel is identified, the approximation of needle entry is performed by first determining the vessel depth as measured on the ultrasound screen. Using the right triangle geometry (Figure 34) when the needle is angled at 45 to the skin, it should enter the skin at a distance a from the center of the probe, which is equal to the depth of the vein ( b in Figure 34). Needle entry into the vein can be visualized as tenting (Figure 35) or the ring-down artifact (Figure 36). Needle redirections can be performed under realtime guidance. Limitations are that the needle is not visualized until it actually enters the vessel. Figure 30 A. The single-operator approach. B. The dualoperator approach. Courtesy of J. Christian Fox, MD, RDMS. Reprinted with permission. A Figure 31 Short-axis view, or cross-section of blood vessels. B Figure 32 Long-axis view or longitudinal view of blood vessels as seen on a commercial vascular phantom. Figure 33 A. Pre-compression antecubital region. B. Postcompression antecubital region. Note veins collapse while arteries stay patent. 18 Ultrasonography for Novices

18 Figure 34 Estimation of line placement using right triangle geometry. When using a 45-degree angle, depth equals distance away from the center of the probe. Figure 35 Tenting, or the compression of the blood vessel prior to catheter/needle penetrating the blood vessel. Courtesy of J. Christian Fox, MD, RDMS. Reprinted with permission. Figure 36 Ring-down artifact of the needle within the lumen of the blood vessel. Alternatively, experienced sonographers can use a technique termed following the tip by which the needle is placed adjacent to the center of the probe. The needle is advanced at the same velocity that the probe moves away from the needle. Therefore, the sonographer can visualize the needle tip throughout its entire path until it penetrates the vessel. Confirmation of blood vessel cannulation can be performed by visualizing needle placement in two planes in addition to the visualization of a blood flash in the catheter and the ability to flush saline. Once this occurs, ultrasonography can be discontinued and the catheter secured. Because catheter placement occurs at a 45 angle in the needle-approximation technique, smaller gauge catheters have the potential to kink. To maintain the angle of entry, one can prop the catheter hub with 2 2 gauze before securing the catheter. Regardless of whether ultrasound visualization is used, one might encounter the same difficulties as with traditional catheter placement with valves and vessels collapsing and rolling. The operator should confirm catheter placement in two views and use long catheters whenever possible. With regard to the ultrasound technique, the needle needs to be directly below the transducer to be visualized. Disability E-FAST FAST (Focused Assessment with Sonography in Trauma) is one of the first indications specifically developed for the ED setting. More recently the E-FAST (extended FAST) evolved to include the lung assessment (see Airway/Breathing). The goal of FAST is to rapidly identify intra-abdominal and intrathoracic trauma. The premise of FAST is that blood will collect in the most dependent por- Disability 19

19 tions of the abdomen. In supine patients, these locations are predictably the costophrenic angles in the thorax, pericardium, Morison s Pouch (the potential space between liver and right kidney), splenorenal recess, and rectovesicular space. There is robust evidence that the use of FAST in adult patients with penetrating wounds leads to more rapid definitive operative management. 58 In blunt abdominal trauma, the evidence is less convincing. 59 In pediatric patients, there is a desire to limit the amount of ionizing radiation from CT. However, the evidence for the benefit of FAST over CT in pediatric patients is limited. Part of the limitation is that the sensitivity of FAST is probably lower in pediatric patients, ranging from 60% to 80%. 60,61 Many of the injuries in children are solid organ contusions that are confined in the capsule and do not initially cause intraperitoneal-free fluid. In contrast to adult patients, few pediatric patients undergo exploratory laparotomies, with most surgeons choosing conservative observation strategies, even in patients with high-grade injuries. Despite the limitations and paucity of research in pediatric patients, E-FAST can provide useful information in the assessment of disability in the pediatric patient. Furthermore, it is repeatable and reproducible. If the patient s clinical presentation changes, E-FAST can be repeated. Technique The operator should stand on the supine patient s right side. Ideally, the patient should be placed in a 10 to 20 Trendelenburg position, which increases the sensitivity of detecting free fluid. Generally, either the curvilinear or a phased-array transducer is used. Four views are used in the traditional FAST examination: Morison s pouch (right upper quadrant), splenorenal (left upper quadrant), suprapubic, and cardiac (subxiphoid). The E-FAST examination adds two additional locations at the anterior aspect of the chest to assess for pneumothoraces. Anechoic material in any of these spaces in the context of acute blunt or penetrating trauma constitutes a positive finding. Morison s Pouch View The first view is the Morison s Pouch view. The term refers to the potential space between the liver and the right kidney (Figure 37A). To obtain this view, the operator places the transducer at the anterior axillary line at the fifth intercostal space with the indicator toward the head of the patient (Figure 37B). Splenorenal View The splenorenal, or left upper quadrant view (Figure 38A), is obtained by placing the probe at the fourth or fifth intercostal space at the posterior axillary line, with the indicator pointing toward the head of the patient (Figure 38B). Compared with the right upper quadrant, the splenorenal view is more superior and posterior. This is often the most challenging view to obtain due to rib shadows. Therefore, the operator might need to rotate and shift the probe gently to image between the ribs. Suprapubic Views Additional views in FAST are the suprapubic views. The probe is placed just superior to the pubic symphysis, imaging through the bladder, using it as an acoustic window (Figure 39A). Both A B Figure 37 A. Probe placement for Morison s, or right upper quadrant view. B. The anechoic stripe between the liver and the right kidney represents free fluid in Morison s Pouch. 20 Ultrasonography for Novices

20 A A B B Figure 38 A. Free fluid around the spleen in the left upper quadrant view. B. Probe placement for the left upper quadrant view. transverse (Figure 39B) and sagittal (Figure 39C) views are used to maximize sensitivity. Subxiphoid View The final view is the subxiphoid view of the pericardium (Figure 40A). It is the final view because the operator must change the depth because the heart is deeper than other structures visualized in the FAST examination. For this view, the operator places the probe in the subxiphoid region, with the indicator to the patient s right (Figure 40B). Information on contractility, presence, or absence of pericardial effusion should be readily apparent. It is important to visualize the posterior pericardium because it is the location where pericardial effusions first appear. If unable to obtain this view, an alternative view is the parasternal long-axis view of the heart (see Cardiovascular: Cardiac Assessment). E-FAST (Lung Views) E-FAST adds imaging of the pleural spaces to the standard FAST views. Most commonly, a linear probe is used to look for the lung sliding that is absent in cases of pneumothorax. 62 To obtain the image (Figures 6 and 7), a linear C Figure 39 A. Probe placement for the suprapubic view. B. Normal transverse view of the bladder. C. Normal sagittal view of the bladder. transducer is placed at the third to fifth intercostal space in the midclavicular line with the marker toward the patient s head (Figure 5). The apposition of the parietal and visceral pleura is represented by a line just behind the ribs. Normally, with pleural movement (from breathing or cardiac contractility), there is constant movement, represented by the lung sliding seen on the ultrasound image. This movement is lost in cases of pneumothorax. FAST and E-FAST are some of the most well-established modalities in emergency Disability 21

21 A B Conclusions This module reviews the basic principles behind bedside ultrasonography for the clinician. While it shows basic applications, and how to incorporate ultrasonography into the pediatric assessment, it is not meant to replace formal, comprehensive training. A similar, more comprehensive approach has been proposed for critical care ultrasonography (Figure 41). 63 Bedside ultrasonography is continuously evolving. Further research remains to be performed specifically in pediatric patients to further define these applications and their utility and efficacy. Figure 40 A. Probe placement for the subxiphoid view. B. Subxiphoid view. ultrasonography. Clearly, they are great ways to introduce a new user to the techniques because they incorporate many important concepts in emergency ultrasonography, including orientation, multiplanar imaging, different echo textures, and dynamic vs. static imaging. There is a relative paucity of data on the utility of FAST in place of CT scans in children with significant abdominal and thoracic injuries. 22 Ultrasonography for Novices