Chapter 92 Ultrasound and Telemedicine in the Wilderness

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In a remarkable departure from historic norms, modern wilderness travelers sometimes have ready access to global communications systems. In the last 20 years, communicating to and from any point on the globe with lightweight devices has become reality. Fueled by recent advances in technology, wilderness travelers may utilize robust and portable medical equipment with advanced imaging and diagnostic abilities while in the field. When joined together, these technologies offer the ability to communicate voice, video, and data to medical experts thousands of miles away.

When the ice of the Weddell Sea of Antarctica crushed and sank Ernest Shackleton’s expedition ship in 1915, leaving his group with minimal supplies in a hostile environment hundreds of miles from human settlements, their fate was not different from that of wilderness travelers for preceding millennia; they depended on skills and traits already within their group. Meaningful options for outside communication and anatomic imaging did not exist. Extraordinary advances in communication and portable diagnostic equipment over the last century have altered the inevitability of risk and nature of isolation.

Within 20 years following Shackleton’s heroic journey, radio-based communication became commonplace. Less than a century later, satellite-based communications and advanced medical technology have the potential to become ubiquitous. Telephone and data transmission from the summit of Mt Everest, the South Pole, or the earth’s orbit are quotidian occurrences. With the possible exception of deep cave explorers, a wilderness sojourner anywhere on Earth can arrange to readily communicate. Although the combination of communication with advanced medical technology offers an illusion of comprehensive risk mitigation for wilderness experiences, we are sobered in remembrance of a world-class Mt Everest mountaineer who successfully communicated with his pregnant wife thousands of miles away, even as he lay hypoxic, dying and beyond hope of rescue. Despite technology, austere environments present circumstances and conditions severe enough to kill.

Wilderness travelers now have the ability to practice advanced medical interventions and wilderness imaging that might save a limb or life. Before this time, the only hope for immediate medical expertise in the wilderness was to include a medical professional as part of the expedition; now, rapid communication capabilities allow expert diagnosis and treatment to be administered from a great distance with assistance of telemedical technology. In this chapter, we explain how current communication and medical devices offer an array of powerful diagnostic techniques for medical care in the field. We discuss the role of ultrasound as the one form of human anatomic imaging with obvious and direct application in wilderness clinical care and research.

It is beyond the scope of this chapter to provide detailed descriptions of all ultrasound techniques or of the huge range of telemedicine devices. Excellent texts are dedicated purely to telemedicine or to clinical ultrasound.^59,^63,⁹⁰ Our goal is to provide sufficient detail so that a provider, even with little or no formal training in ultrasound, can acquire and begin to apply potentially lifesaving techniques while in the field. Experimental studies and routine clinical practice have established that these skills can rapidly, safely, and effectively be learned for use in austere environments, both by medical experts and nonphysicians. A growing body of evidence reveals that ultrasound images of diagnostic quality can be acquired by untrained nonphysicians under the real-time guidance of ultrasound experts thousands of miles away.⁶⁵ With these examples in mind, we provide simple instructions and representative images for various clinical ultrasound techniques that are likely to prove critical in austere environments. We also discuss multiple field-based research techniques for which ultrasonographic imaging is especially well suited and for which there is great promise.

Given the ready availability and advanced capabilities generated by combining modern diagnostic medical devices with global communication systems, we discuss concepts underlying modern telemedicine. Examples using current techniques (both on land and in space) are presented to illustrate the relevance of telemedicine in austere environments. Finally, because of inherent limitations of electric power sources in austere environments, we discuss systems that can reliably power electrical devices.

Ultrasound

Ultrasonographic imaging devices promise to revolutionize wilderness medical care. Wilderness travelers now have access to machines that are lightweight, compact, and sufficiently robust to be carried in a daypack, and outputs that can be interpreted in the field (Figure 92-1, online). Commercially available ultrasound machines readily provide diagnostic imaging of organs under austere conditions.

FIGURE 92-1 The SonoSite 180 is a simple, portable ultrasound device well suited to austere environments.

(Copyright N. Stuart Harris.)

Sound frequencies employed for ultrasound imaging are well in excess of those perceived by humans (20-20,000 Hz). Frequencies of 2 to 20 megahertz (MHz) may be used. The technology underlying ultrasound imaging predates Wilhelm Konrad Röntgen discovery of x-rays (1895) by more than a decade.^17,⁷⁵ In 1942, Dussik²¹ described the utility of ultrasound for human medical imaging. Early machines were massive, cumbersome, and produced images limited in detail when compared with the crisp images of early radiographs. During the 1960s and early 1970s, ultrasound technology evolved to the point that it began being used for clinical care. Obstetrics and cardiology were early adopters. Clinical use of ultrasound by European emergency care providers grew rapidly throughout the late 1970s, and by their American counterparts in the late 1980s. Education in ultrasound imaging is now a mandated skill for many nonradiology physician-training programs. A component of sonographic imaging, Focused Assessment With Sonography for Trauma (FAST examination), is now recommended for early use by the Advanced Trauma Life Support (ATLS) guidelines developed by the American College of Surgeons.

Sonographic imaging is based on the principle of the piezoelectric effect, a property defined by Pierre and Jacques Curie in 1880. They described the piezoelectric effect as a property of quartz crystals that creates an electric potential when stimulated by a mechanical force. In corollary, these same crystals produce a mechanical potential when an electric force is applied.¹⁷ Using these properties, a modern ultrasound machine functions by applying an electric charge to a piezoelectric crystal (the transducer or probe) that then converts the electric signal to a sound wave. Structures within the body reflect these sound waves back toward the piezoelectric crystal of the probe. The crystal turns this mechanical force into an electric current, which is quantified and interpreted to produce an image.

Sound waves (as a mechanical force) require a medium. Fluid-filled structures provide an excellent medium. Solids and air provide very different qualities of conduction. The sometimes subtle, sometimes profound, differences in wave propagation, refraction, and reflection allow internal structures to be imaged.

Advances in ultrasound technology have dramatically decreased cost and increased portability of machines. These advances have allowed what once was a ponderously heavy, hospital-based imaging technique to be adopted in many wilderness settings. Current clinical and research practices reveal that ultrasound is at the forefront of wilderness imaging devices.

Ultrasound for Wilderness Clinical Care And Research

Advantages

Portability

Extraordinarily powerful ultrasound equipment can be readily carried by hand. As compared with other diagnostic imaging devices that might be considered for wilderness use (such as conventional radiography, computed tomography [CT] or magnetic resonance imaging [MRI]), ultrasound machines are much more compact, lighter, and more portable (Table 92-1). On rare occasions, when a full-sized ultrasound machine may be required, it can be transported to a road-accessible field site or hypobaric research chamber, which is not the case with CT or MRI. The revolution in digital and ultrasound technology and manufacturing has allowed small laptop-sized machines to overtake the capabilities of older, larger machines for virtually every application. Because portable machines have become small (often weighing less than 8 to 12 lb), their utility has increased for wilderness settings. In austere locations with no power source (e.g., remote alpine environment or postdisaster area), ultrasound devices can be powered using batteries, including portable photovoltaic arrays. A SonoSite MicroMaxx ultrasound machine used successfully on the 2007 Caudwell Xtreme Everest expedition at Camp 4 on the South Col (8016 m [26,299 feet]) of Mt Everest was powered by standard MicroMaxx batteries charged at base camp and Camp 2 using a combination of generators or solar power.⁸ We have conducted research using a SonoSite 180 machine carried to the summit of Mt Kilimanjaro (5895 m [19,341 feet]) powered only by hand-carried solar arrays. A portable ultrasound setup with batteries, power source, laptop computer for image storage and backup, and enough ultrasound gel for clinical use or research can reasonably be expected to weigh approximately 13.6 kg (30 lb) and be carried to remote locations by a single physician or investigator. Some machines (Terason Ultrasound) are essentially modular attachments to laptop computers, further reducing the need for separate pieces of equipment. Prototypes exist that can fit in the palm of the hand; it is inevitable that miniaturization and power system improvements will continue and become increasingly affordable.

TABLE 92-1 Advantages and Limitations of Ultrasound in the Wilderness

Advantages of Ultrasound	Limitations of Ultrasound
Portable, lightweight, field-ready	Electronic equipment—sensitive to cold, dust, trauma
Safe, nonionizing (allows multiple assays) Relatively inexpensive Provides immediate data	Power system (or numerous batteries) required for extended backcountry use
	Requires operator to learn techniques
Easy to learn techniques, or to be guided from a distance using telemedicine
Allows multiple organ systems to be investigated
Excellent flexibility for both research/clinical care
Allows novel research investigations

Safety/Noninvasiveness

Diagnostic ultrasound has few known risks.³¹ To highlight this fact, ultrasound is the clinical imaging modality of choice for many delicate high-risk patient populations, including pregnant women and their fetuses. Ultrasound is routinely used at the bedside for rapid assessment of critically ill trauma and medical patients. A physician with a bedside ultrasound can quickly and in real time assay for a range of common, acute life threats, including intraperitoneal free fluid or pericardial effusion, obviating the risks associated with delays due to patient transport or image processing. No other clinical imaging system allows this combination of speed, safety, portability, and immediacy of result.

In many research applications, such as replacement of invasive pulmonary arterial catheterization by transthoracic echocardiography, ultrasound provides an essentially risk-free, noninvasive option to take the place of a potentially risky, invasive technique.³ The safe, noninvasive, and painless nature of ultrasound compared with other diagnostic and research techniques encourages patient compliance and aids recruitment for research protocols by making participation more attractive to potential subjects. Importantly, for research purposes, the U.S. Department of Health and Human Services’ Office for Human Research Protections specifically identifies ultrasound, Doppler measurements, and echocardiography as research methodologies eligible for expedited investigational review board review, significantly easing administrative burdens and potentially shortening the time between conception and execution of studies. For research purposes, because ultrasound itself does not significantly impact subjects or other experimental manipulations, sonographic monitoring can easily be added to other experimental protocols for purposes of collecting additional relevant data (such as monitoring additional parameters of potential interest during a drug trial) or conducting a separate parallel study. For example, we joined research conducted by U.S. Army Research Institute of Environmental Medicine collaborators and were able to assay sonographically measurable parameters (e.g., optic nerve sheath diameter [ONSD] and pulmonary arterial pressure) during a study originally designed to examine the effects of moderate altitude prepositioning on combat readiness of military recruits during exposure to high altitude. Ultrasound does not employ ionizing radiation, so there is no known additive risk of repeated ultrasound exposures.

Versatility

Ultrasound machines provide immediate access to advanced diagnostic imaging. It provides imaging of soft tissue, abdominal and thoracic organs, bone, muscle, and skin processes that can immediately and directly inform medical decision making.

For both routine clinical and specialized research purposes, a single ultrasound machine and ultrasonographer (with appropriate training) can monitor a wide range of organ systems and potential research parameters, as described later. For example, in a research project using ultrasound, multiple parameters (e.g., eyes, chest, and heart) can be studied at no more cost to the clinician/researcher or risk to the patient/subject than if a single measurement were taken. For example, researchers studying intracranial pressure (ICP) and ONSD in trekkers with severe acute mountain sickness could incorporate the use of chest ultrasound to investigate a separate hypothesis in the same cohort—while all the time having the ultrasound available to help diagnose common clinical conditions. Learning and employing ultrasound techniques is an intellectual annuity; gaining facility with current techniques prepares ultrasonographers for rapid adoption of new techniques.

Cost

The cost of ultrasound machines varies, depending on factors such as size, weight, image quality, and special imaging capabilities (e.g., Doppler or color-flow imaging). Selecting a machine appropriate for an austere setting requires forethought. To ensure reliability, especially under the stresses of harsh conditions, a preference for solid-state, nondynamic systems (e.g., fixed versus phased array probes) must be considered. These systems often offer the additional benefit of being more reasonably priced. It is critical for wilderness practitioners to define exactly what capabilities they will require from the device in their unique work environment, so that the correct ultrasound unit can be chosen. The device need not be burdened with unnecessary features (discussed later) that drive up cost, increase power requirements, and promote the likelihood of unit failure under austere conditions.

Limitations

There are some general drawbacks of ultrasound for wilderness clinical and high-altitude research use.⁹² Although portable and relatively durable for in-hospital clinical use, ultrasound machines can be fragile and require careful packing and handling during transport and field use. Wilderness use can place strains on ultrasound machines far in excess of what the machines ordinarily encounter in routine clinical practice. Mechanical failure has jeopardized or terminated wilderness clinical/research expeditions.^9,^28,^40,⁸⁹

Ultrasound machines are not easily serviced in the field. By picking a machine with the fewest movable mechanical parts (i.e., choosing fixed array rather than phased array probes and solid state rather than spinning hard discs for memory), the risk for mechanical failure can be somewhat mitigated.

Hypobaric cold (high altitude), blowing dust (alpine or desert environments), water damage (riparian or rain forest environments), salt exposure (marine environments), rough handling during transport, and other factors can quickly disable a machine (sometimes permanently), but most difficulties can be anticipated and mitigated. For example, machines should be protected in strong, well-padded rigid cases and ideally carried by a responsible person at all times. Such cases can be readily and inexpensively fashioned from rigid tool cases retrofitted with hand-cut foam padding. Pelican and other strong, waterproof cases can be customized for ultrasound devices and probes.

To enhance effectiveness and protect the ultrasound device, it may be necessary to sleep with batteries next to the body, prepare warm water in which to soak a probe to achieve reliable functioning in the cold, or to operate the machine within a plastic bag to protect against dust. The risk for theft (particularly in chaotic, urban, or postdisaster settings) is real. Traditional (spinning) hard drives have been implicated in machine failure at high altitude, probably due to cold and the influence of marked decreased barometric pressures on internal air-filled cushioning components.¹³ Solid-state memory devices (i.e., flash cards) without moving parts are readily available for primary data storage on laptop computers and USB drives. These solid-state units should prove to be significantly more reliable than their mechanical counterparts. A plan for rapid servicing or exchange of machines in case of malfunction should be made in advance. Bringing a backup machine is an effective hedge against malfunction. Thorough testing before field use, careful machine handling, device security, and thoughtful attention to potential site-specific problems enable a clinician or researcher to increase the likelihood of maintaining the ultrasound unit during a wilderness experience.

Before departure, sufficient ultrasound training must be completed to ensure that diagnostic-quality images and accurate measurements can be reliably obtained. Although some imaging techniques, such as echocardiography, require months of intensive training, other applications, such as thoracic, long-bone, and abdominal ultrasound imaging, are straightforward and have been taught successfully in brief sessions, or even through remote expert guidance at the time of image acquisition. A growing body of evidence, both on Earth and in space, confirms that these techniques can be effectively taught to nonphysicians with minimal prior training when guided by audiovisual linkage to expert ultrasonographers.⁶⁵

Digital ultrasound images can readily be saved on typical computer-based media. Multiple factors influence the size requirements for image storage. For purely clinical use, when no independent review of imaging for quality control is required, there exist only limited data storage needs. A means to save and review images obtained in the field is typical and prudent. When ultrasound assays are being used primarily for research purposes, great attention should be paid to the quantity and robustness of data storage options. Data storage needs are influenced by the types of images that are saved (e.g., still images versus video clips), definition of the individual images (e.g., low-definition black-and-white images from an older machine versus color and velocity data–embedded images from a state-of-the-art echocardiography device), and the total number of individual images. It is good practice to routinely back up all data, even if the ultrasound machine is capable of storing all necessary images in its memory. Most machines are accompanied by software packages that allow image downloading and handling on a personal computer. In planning data storage, it is important to be aware of the capabilities of the software. If saved images will be analyzed at a later time by a blinded observer, it is important to be sure that the imaging software is capable of allowing the necessary analysis, and that image labeling at the time of acquisition does not compromise blinding.

Research-Specific Concerns

When employed as part of a research protocol, ultrasound often requires an investigator (sometimes using an observer-dependent technique) to have direct contact with research subjects. When this is the case, impeccable care in study design and execution is required to ensure that the ultrasonographer remains blinded to the variables of interest. Given these concerns, researchers need to consider whether reliable blinding of results requires the presence of multiple ultrasonographers in the field. Although having two or more researchers acquiring images may be the best way to ensure inter-rater reliability, the vast majority of ultrasound machines are capable of recording image clips and stills that can later be over-read by a researcher separated in time and place blinded from the original data collection.

Introduction to Clinical Imaging

Several excellent resources discuss the physics and theoretic underpinning of ultrasound.^58,^59,⁶³ We now address the practical topics of “gain” and “depth” controls (Figure 92-2, online). Clinicians may think of gain as analogous to volume on a radio. By turning up gain, the “brightness” of signals displayed on the screen is increased. This can be useful to adequately view an image on a screen under sunny conditions. Different machines have different methods for adjusting depth. Depth controls the distance under the probe that is displayed on the screen. Depth adjustments allow the user to maximize the size of the anatomic structure of interest within the viewing area. For example, to locate a subcutaneous foreign body, depth typically should be very limited (2 to 3 cm [0.8 to 1.2 inches]) so that fine detail may be appreciated in this superficial structure. In the case of deeper tissue imaging (e.g., a subxiphoid view of the heart or examination of the aorta in an obese adult), depth might need to be markedly increased (16 to 20 cm [6.3 to 7.8 inches]). B-mode stands for brightness and is the typical display mode with which clinicians and patients are familiar. B-mode produces a two-dimensional image. M-mode stands for motion and displays a representation of motion within a single anatomic plane over a linear axis of time. It can be effectively used to highlight and quantify the movement of structures (e.g., physiologic lung sliding or fetal heart rate). Probes (transducers) can vary in two critical manners: frequency (high or low) and shape (linear or curved). The probe qualities preferred for each of the techniques we describe are shown in Table 92-2.

TABLE 92-2 Types and Ideal Uses of Different Ultrasound Probes

Probe Type	Ideal Uses
Low-frequency (2-5 MHz) curved array	FAST examination Thoracic evaluation (for pulmonary edema)
(Excellent depth of imaging)	FAST examination Thoracic evaluation (for pulmonary edema)
(Excellent depth of imaging)	Cardiac views
High-frequency (8-10 Mhz) linear array	Foreign body locationThoracic assessment (for pneumothorax)
(Excellent resolution, limited depth)	Foreign body locationThoracic assessment (for pneumothorax)
	Eye evaluation, optic nerve sheath assays
	Fractures

FAST, Focused Assessment With Sonography for Trauma.

FIGURE 92-2 Ultrasound device displays are modulated by adjusting “near” and “far” field and “gain” controls to maximize image quality.

(Courtesy N. Stuart Harris.)

Common Clinical Imaging Techniques

Focused Assessment with Sonography for Trauma (fast)

The FAST examination is a standard component of initial assessment of the trauma patient. The FAST examination consists of a rapid, limited, bedside ultrasound examination of the abdomen and heart using a curvilinear 2.5- or 3.5-MHz probe. The FAST examination allows quick examination of four specific anatomic locations to assess for the presence of free fluid: the right upper quadrant (Morison’s pouch), the left upper quadrant (perisplenic space), the suprapubic pelvis (rectovesical pouch in males, the pouch of Douglas in females), and the pericardium.

The FAST examination is indicated in a patient who has sustained abdominal trauma. FAST examinations are invaluable to reveal free intraperitoneal fluid or pericardial fluid (and therefore potential impending cardiac tamponade). Due in no small part to application of ultrasound early within the trauma evaluation, this study has an appreciable degree of false-negative results. This should encourage clinicians to frequently repeat this study over time if clinical concerns warrant. The FAST examination is an insensitive indicator of solid organ injury, bowel injury, or retroperitoneal bleeding.

The FAST examination consists of four standard views:

1 Right upper quadrant: Morison’s pouch is the most dependent portion of the upper peritoneal cavity. Thus it is where early evidence of intraperitoneal free fluid may be discovered. To obtain this view, place the probe in the right posterior axillary line at the level of the 11th and 12th ribs (Figure 92-3). Move the probe anteriorly with sweeping, angular adjustments until a clear view of the anterior fascia of the kidney and the posterior portion of the liver capsule is obtained. A “negative” image shows these two structures clearly and directly apposed (Figure 92-4). A “positive” image for free fluid reveals a hypoechoic, dark stripe between the liver and kidney (Figure 92-5).

2 Left upper quadrant: The left upper quadrant examination visualizes the spleen, left kidney, and perisplenic space. To obtain this view, place the probe in the left posterior axillary line between the 10th and 11th ribs (Figure 92-6). A “negative” image will reveal the spleen and kidney in close apposition (Figure 92-7). A “positive” image will reveal a dark strip (free fluid) between these two organs (Figure 92-8).

3 Pelvis: The suprapubic view visualizes the most dependent section of the lower abdomen and pelvis. The anatomy of the cul-de-sac is gender specific. In males, the rectovesical pouch is visualized; in females, the pouch of Douglas is seen. These respective spaces are in the most dependent portion of the lower abdomen and pelvis; hence they are where fluid is likely to collect.

FIGURE 92-3 Right upper quadrant of FAST examination. Place the probe in the right posterior axillary line at the level of the 11th and 12th ribs. This allows the liver, the right kidney, and Morison’s pouch to be imaged.

(Courtesy Ian Barbash.)

FIGURE 92-4 Right upper quadrant of FAST examination. Normal anatomy reveals the anterior fascia of the kidney (right midscreen) and the posterior portion of the liver capsule (left midscreen). A “negative” image shows these two structures clearly and directly apposed. The brightly reflective, curvilinear line of the diaphragm is seen at the far left, lower portion of the screen.

(Courtesy Ultrasound Division, Department of Emergency Medicine, Massachusetts General Hospital.)

FIGURE 92-5 Right upper quadrant of FAST examination. A “positive” image for abdominal free fluid reveals a lenticular, hypoechoic, dark stripe between the liver (middle of screen) and kidney (to right). Morison’s pouch is the most dependent portion of the upper peritoneal cavity and so is where early evidence of intraperitoneal free fluid may be discovered.

(Courtesy Ultrasound Division, Department of Emergency Medicine, Massachusetts General Hospital.)

FIGURE 92-6 Left upper quadrant of FAST examination. To obtain this view, place the probe in the left posterior axillary line between the 10th and 11th ribs. This allows the spleen, the left kidney, and the perisplenic space to be imaged.

(Courtesy Ian Barbash.)

FIGURE 92-7 Left upper quadrant of FAST examination. A “negative” image reveals the spleen (left side of screen) and kidney (right midscreen) in close apposition.

(Courtesy Ultrasound Division, Department of Emergency Medicine, Massachusetts General Hospital.)

FIGURE 92-8 Left upper quadrant of FAST examination. A “positive” image for abdominal free fluid reveals a hypoechoic, dark stripe, here most notable at the inferior tip of the spleen (center of screen), or between the spleen and kidney. The kidney is seen on the far right.

(Courtesy Ultrasound Division, Department of Emergency Medicine, Massachusetts General Hospital.)

To obtain this view, place the probe in the midline just superior to the symphysis pubis (Figure 92-9). Angle the probe toward the rectum. Then with probe held in place, slide and rotate up the spine until the pouch of Douglas or rectovesical space is visualized. A negative image shows each gender-specific space without evidence of hypoechoic free fluid (Figure 92-10). A positive image reveals perivesicular free fluid (Figure 92-11).

4 Cardiac: The cardiac examination allows rapid assessment for pericardiac traumatic processes. It screens for fluid between the fibrous pericardium and the heart and hence evaluates for possible impending cardiac tamponade. To obtain this view, place the probe just inferior to the xiphoid process, with the angle upward under the costal margin toward the left shoulder (Figure 92-12). A “negative” image displays the echogenic, bright pericardial sac immediately adjacent to the active cardiac surface (Figure 92-13). A “positive” image for free fluid reveals a hypoechoic, dark stripe between the pericardial sac and the cardiac surface (Figure 92-14). An additional view of the heart may be obtained using the “parasternal long” cardiac view. This view can be helpful in patients who are uncooperative or obese. To achieve this view, position the probe to the left of the sternum in the 4th to 5th intercostal space with the probe indicator pointing toward 4 o’clock (Figure 92-15). Representative negative and positive images are provided in Figures 92-16 and 92-17.

FIGURE 92-9 Suprapubic view of FAST examination. To obtain this view, place the probe in the midline just superior to the symphysis pubis. Angle the probe toward the rectum, then with probe held in place, rotate up the spine until the pouch of Douglas or rectovesical space is visualized.

(Courtesy N. Stuart Harris.)

FIGURE 92-10 Suprapubic view of FAST examination. A “negative” image shows each gender-specific space without evidence of hypoechoic free fluid outside of the bladder. Here, the full bladder (globular, dark structure in middle screen) is visualized. There is no free fluid outside of the bladder. Sagittal view.

(Courtesy Ultrasound Division, Department of Emergency Medicine, Massachusetts General Hospital.)

FIGURE 92-11 Suprapubic view of FAST examination. A “positive” image reveals perivesicular abdominal free fluid (irregularly shaped, hypoechoic, dark stripe) in the center of screen. This stripe is to the left (cephalad) of the partially decompressed bladder (round, hypoechoic, dark stripe) on right side of screen. Sagittal view.

(Courtesy Ultrasound Division, Department of Emergency Medicine, Massachusetts General Hospital.)

FIGURE 92-12 Subxiphoid view of FAST examination. To obtain this view, place the probe just inferior to the xiphoid process, with the angle upwards under the costal margin toward the left shoulder.

(Courtesy Ian Barbash.)

FIGURE 92-13 Subxiphoid view of FAST examination. The pericardial examination screens for fluid between the fibrous pericardium and the heart. In this “normal” study, there is no pericardial effusion. The atria are seen on the left side of the screen, the ventricles to the right. The right atrium and ventricle are at the top of the screen, the left atrium and ventricle at the bottom. The right atrium is notably prominent in this particular image.

(Courtesy Ultrasound Division, Department of Emergency Medicine, Massachusetts General Hospital.)

FIGURE 92-14 Subxiphoid view of FAST examination. A very large pericardial effusion (dark, anechoic layer) envelops the anterior and posterior heart.

(Courtesy Ultrasound Division, Department of Emergency Medicine, Massachusetts General Hospital.)

FIGURE 92-15 Parasternal long view of FAST examination. To achieve this view, position the probe to the left of the sternum in the 4th to 5th intercostal space with the probe indicator pointing toward 4 o’clock. This view can be helpful in patients who are uncooperative or obese.

(Courtesy N. Stuart Harris.)

FIGURE 92-16 Parasternal long view of Focused Assessment With Sonography for Trauma (FAST) examination. This normal anatomy reveals the left ventricle and outflow track lying horizontally in the middle of the screen (at 6 cm [2.4 inches] depth). The aortic valve is seen in the right midfield of this image. The left atrium is a dark, anechoic space at the bottom right side of screen.

(Courtesy Ultrasound Division, Department of Emergency Medicine, Massachusetts General Hospital.)

FIGURE 92-17 Parasternal long view of FAST examination. A large pericardial effusion (darker, hypoechoic layer) envelops the anterior and posterior heart, best seen at top left and bottom center of this image.

(Courtesy Ultrasound Division, Department of Emergency Medicine, Massachusetts General Hospital.)

Thoracic Ultrasound for Pneumothorax and Pulmonary Edema

Ultrasound of the thorax can be used to effectively screen for pneumothorax, pulmonary edema, and rib fracture.

Unfortunately, current nomenclature for thoracic ultrasound findings is not universal and can lead to confusion. Thankfully, an ongoing international consensus conference is trying to standardize terminology. A primary point of confusion arises from differing definitions of the term comet tails. As described earler, comet tails are findings obtained with a low-frequency (2 to 5 MHz), curved probe with depth set at 18 to 22 cm (7.1 to 8.7 inches). These comet tails are coherent, vertical artifacts that start at the pleura and continue in a wedge-shaped pattern to the bottom of the screen. They are quantifiable and indicate the presence of excessive interstitial fluid (pulmonary edema). This same finding is increasingly coming to be known by the term B-lines. “Comet tail signs” can also describe findings with a high-frequency (8 to 10 MHz), linear probe. Until an international standard is agreed upon, for this chapter, comet tails discovered with a low-frequency, curved probe indicate pulmonary edema; with a high-frequency, linear probe, the absence of a pneumothorax.

To assay for pneumothorax, a linear-array, high-frequency (8 to 10 MHz) probe is used. The probe is placed on the chest in the 2nd to 3rd intercostal space in the anterior axillary line in a sagittal position and slowly moved toward the lateral sternal margin. In real time, normal lung anatomy is revealed through two characteristic findings: the presence of “lung sliding” and of comet tail signs within the lung parenchyma. Lung sliding describes the readily visualizable movement of visceral against the stationary parietal pleura in normal tissue. As noted above, comet tails are seen only with high-frequency linear probes when the two pleura are apposed, and they are wedge-shaped structures that originate from the bright pleural line and extend into the lung tissue. They appear as bright straight lines and indicate the absence of pneumothorax. In patients with a pneumothorax, neither lung sliding nor comet-tails will be visualized. Typically, lung sliding is much easier to identify and so is more often used to rule out pneumothorax. Discussion on the significance of comet tails visualized using a low-frequency, curved probe, indicating pulmonary edema, is found later.

An additional technique to screen for the presence of pneumothorax can be accomplished using M-mode scanning. Two characteristic findings may help determine presence or absence of a pneumothorax. To visualize these findings, the ultrasound machine must have an M-mode setting (this typically is available on even rudimentary devices). M-mode imaging produces a linear representation of movement in a single plane over a period of time. By aligning that plane through the structure of interest, changes within that plane (e.g., lung sliding or fetal heart rate) can be readily appreciated. To obtain thoracic images, the linear, high-frequency probe is placed on the anterior chest, in the 2nd to 3rd intercostal space, starting at the anterior axillary line and moving toward the costal margin. Using M-mode imaging, presence or absence of lung sliding results in characteristic images. Normal sliding movement between parietal and visceral pleura will produce the “waves on the beach” sign (Figure 92-18). Normal skin and intercostal muscles have minimal movement during breathing and appear as flat, stacked lines using M-mode. Below the bright white line of the pleural surface, normal lung tissue moves, which results in this structure having a granular appearance. This image has been described as reminiscent of waves (the superficial/superior, stacked, linear portion of the image) against a beach (the deeper/lower granular portion of the image). In patients with a pneumothorax, the lung tissue appears stationary and so appears in M-mode as a series of stacked lines, just like the superficial muscle above it. This results in the “bar code” sign (Figure 92-19).

FIGURE 92-18 “Waves on the beach” sign indicates normal anatomy with no pneumothorax. Images obtained of thorax employing B-mode imaging (top) and M-mode imaging (below). Normal skin/intracostal muscles (at top of lower image) have minimal movement during breathing and so appear as flat, stacked lines using M-mode. Below the bright white line of the pleural surface, normal lung tissue does move, and this results in this structure having a granular appearance (at bottom of image). This image has been described as reminiscent of waves (the superficial/superior, stacked, linear portion of the image) against a beach (the deeper/lower granular portion of the image). Views obtained using high-frequency linear probe.

(Courtesy Ultrasound Division, Department of Emergency Medicine, Massachusetts General Hospital.)

FIGURE 92-19 Barcode sign indicates the presence of a pneumothorax. Images obtained of thorax employing B-mode imaging (top) and M-mode imaging (below). In a patient with a pneumothorax, the lung tissue appears stationary and so appears in M-mode as a series of stacked lines no different than the superficial muscle above it. Views obtained using high-frequency linear probe.

(Courtesy Ultrasound Division, Department of Emergency Medicine, Massachusetts General Hospital.)

Ultrasound may also be used to assay for pulmonary edema, using the comet-tail technique. (Figure 92-20). Monitoring of pulmonary edema is of interest both for typical congestive heart failure with related pulmonary edema and for high-altitude pulmonary edema (HAPE). Thoracic ultrasound assays allow visualization of severity of edema in overt HAPE and allow monitoring for subclinical forms that may affect oxygenation at high altitude. Conventional radiography has proven insensitive to subclinical pulmonary edema and early HAPE and does not allow easy tracking of accumulation or resolution of pulmonary edema.^37,⁸⁸ Techniques that overcome these shortcomings, such as CT or MRI, are not available in the field. A series of papers describes use of ultrasound for identification and monitoring of pulmonary edema in hospitalized patients.^1,⁴⁶ We described its use for diagnosis and monitoring of HAPE.²³^–²⁶ Use of this technique for high-altitude clinical use and research, particularly for the monitoring of subclinical edema, requires further study; ease of performance and early results are encouraging. This technique requires a low-frequency (2.5 to 5 MHz), curved “cardiac” probe. The chest is scanned in the 2nd to 5th intercostal spaces on the right and the 2nd to 4th intercostal spaces on the left in the midaxillary, anterior axillary, midclavicular, and parasternal lines (28 total fields) (Figure 92-21). The total number of comet tail artifacts—defined as echogenic, coherent, wedge-shaped signals with a narrow origin in the near field of the image, arising from the pleural line and extending to the edge of the screen (Figure 92-22)—are tallied to yield a total comet-tail score that correlates with the degree of pulmonary edema. These findings can also be accurately referred to as B-lines. This examination can be reliably completed in less than 5 minutes.⁴⁶