ASHOT E. SARGSYAN, MICHAEL BLAIVAS, PHILIP LUMB and DIMITRIOS KARAKITSOS

“There is geometry in the humming of the strings . . . there is music in the spacing of the spheres”

Pythagoras of Samos, 520 bc

Fundamentals: Principles, terms, and concepts

Ultrasound is a mechanical wave that requires a medium to travel (i.e., human tissue), with a frequency above the audible range ceiling of 20 kHz. Ultrasound systems are tomographic devices that transmit short pulses of ultrasound into the body and measure the round-trip time and intensity of each of the numerous echoes returning after the pulse. The time of arrival of an echo determines the distance from the transducer, that is, the location of its source in the body. The intensity of the echo is converted to brightness of a given point in the image. In other words, each pixel (element of the image) on the display device corresponds to a point inside the body, and its brightness depends on the strength of the echo that came from that location. Together, all pixels form a grayscale tomographic image. Parts of the image with mostly bright pixels (a brighter overall appearance) are termed hyperechoic, as opposed to hypoechoic (darker) areas. The relative ability of an organ or tissue to produce echoes is called echogenicity, that is, tissues or structures producing hyperechoic image are considered more echogenic.1–7 Parts of the image with only black pixels are called anechoic or echo-free and mostly correspond to homogenous liquids (e.g., blood, urine, effusion, cystic fluid).

Frequency (measured in cycles per second [hertz, Hz]) is the number of wave cycles in 1 second. Frequency is determined solely by the sound source and not by the medium. Frequencies used by general-purpose ultrasound machines range between 2 and 15 megahertz (MHz). Higher frequencies, up to 40 MHz, are used for intravascular and other catheter-based applications and in specialized ophthalmologic and dermatologic techniques. Propagation speed is the velocity of sound in a given medium and is determined solely by the characteristics of the medium, such as density and stiffness (does not depend on the source of sound or its frequency). Ultrasound travels through soft tissues at a speed of approximately 1.54 mm/μsec, or 1540 m/sec). The stiffer the tissue, the greater the propagation speed (Figure 1-1). Ultrasound waves are generated by piezoelectric crystals (e.g., lead zirconate titanate, or PZT) that convert electrical energy into mechanical energy and vice versa (see Figure 1-1). Electrical pulses or short bursts of alternating voltage stimulate crystals to produce ultrasound pulses in the medium, causing displacement and oscillation of its molecules. Pressure change–Velocity of such oscillations in response to sound pressure determines the acoustic impedance (lower velocities correspond to higher impedance). As ultrasound passes from one medium to another (e.g., from gas to liquid), an impedance gradient at the tissue boundary causes a part of the energy to form a reflected wave (echo) while the remainder of the energy proceeds into the second medium.^1–7 Reflection occurs every time the ultrasound pulse encounters a new boundary (reflector). Specular (mirror-like) reflectors are smooth and flat boundaries larger than the pulse dimensions (e.g., diaphragm, walls of a major vessels). The echo reflection angle equals the angle of incidence; when the beam strikes a specular reflector at 90 degrees (normal incidence, Figure 1-2), a very strong echo travels back toward the source. Nonspecular reflection, or scattering, occurs when the incident beam strikes boundaries that have irregular surface or are smaller than the beam’s dimensions, resulting in the beam’s energy scattering in multiple different directions (see Figure 1-2). The beam travels around even smaller obstacles without scattering (diffraction). Because a higher frequency results in smaller beam dimensions, obstacles diffracting at lower frequencies act as scatterers at higher frequencies. This explains both higher imaging resolution and higher beam attenuation at higher frequencies. Refraction is the redirection of a beam when striking obliquely at a boundary between two media with different propagation speeds. Unlike reflection, refraction does not contribute to the image formation process but contributes to attenuation (see Figure 1-2). Part of the ultrasound beam’s energy is transferred to the medium in the form of heat. This is absorption, which also increases proportionally to frequency in soft tissues. The bones absorb ultrasound more intensely, together with other energy loss mechanisms, producing acoustic shadows behind them. Finally, part of the original beam is converted by tissues to waves with double or higher-order frequency (harmonic waves). The total propagation losses from the combined effects of scattering, refraction, and absorption are called attenuation, which is directly proportional to frequency. Body compartments with low attenuation that allow imaging deeper structures through them are good acoustic windows (e.g., liquid cavities), while those with high attenuation are acoustic barriers (e.g., bones). The near-total loss of ultrasound at boundaries between tissues and gas makes gas the strongest barrier; nevertheless, important lung ultrasound techniques rely on the abundant artifacts that the aerated lung creates.

Equipment and imaging modes

Image quality and optimization

Resolution is a general term denoting the ability of the imaging method to discriminate the structural detail. The better (higher) the resolution, the greater the clarity and detail of the image. Spatial resolution (axial and lateral) refers to the ability of the B-mode to identify and display echoes from closely spaced echo-producing structures as distinct and separate objects. Axial resolution is the ability to discriminate individual echoes along the direction of the ultrasound beam (beam axis) and is approximately 0.5 to 1 mm at the operating frequency of 3.5 MHz. Higher frequencies produce better axial resolution at the expense of penetration. Lateral resolution is the ability to discriminate echoes located side by side at the same depth, and is approximately 1 to 2 mm at 3.5 MHz. Besides choosing the highest possible frequency that still penetrates to the depth of ROI, spatial resolution is improved by “focal zone” placement at the depth of the ROI (focus control) and by avoiding excessive gain settings. Contrast resolution, also known as grayscale resolution, is the ability to discriminate returning echoes of different amplitudes and assign different grayscale values to the respective pixels. Most ultrasound systems permit assignment of 256 shades of gray, which resolves the subtle differences among various structures. Increasing the contrast (less shades of gray) results in an image that is more pleasing to the human eye but likely contains less diagnostic information.1–5 Temporal resolution corresponds to the image frame rate (refresh rate), which ranges from 15 to 100 frames per second in different imaging modes and decreases when the depth or the number of focal zones is increased.

Modern portable ultrasound systems are significantly automated and similar in their user-adjustable functionality; however, controls (knobs) of the machine are still important to know. Depth (a knob or toggle switch) controls the depth of view and should be used to keep ROI in the central area of the screen (Figure 1-5). Depth is displayed along the edge of the image on a centimeter scale. Depth function alters the manner of acquisition of imaging data (preprocessing). An image at a shallower depth takes less time to form because only earlier-arriving echoes are processed, hence higher frame rates (better temporal resolution). The focus control allows moving the focal zone(s) to the ROI depth to ensure a narrower beam and therefore a better lateral resolution. The focal zone may be indicated as an arrowhead at the side of the image (usually on the depth scale). Most machines allow setting multiple focal zones; multifocusing degrades temporal resolution but improves spatial (both axial and lateral) resolution. Zoom control magnifies the selected image section without adding new information or changing the data acquisition (postprocessing). Some systems have an additional “high-definition zoom” option, whereas the machine’s beam-forming and data-processing capabilities are mobilized from other areas to optimize the image of the ROI.

Gain adjusts overall image brightness by amplifying electronic echo signals; thus it works only on the receiving side and has no impact on transmitted power or bioeffects. Gain must be adjusted to such a level that anechoic structures (e.g., fluids) appear black on screen. Using too much gain can degrade the image and create artifacts, and using too little gain can negate real echo data (see Figure 1-5). In addition, most machines also have time gain compensation (TGC) controls (usually a group of slider rheostats) to adjust the gain selectively at various depths. To compensate for attenuation, echoes are electronically amplified proportional to the depth of their origin (i.e., time of their return to the transducer). TGC controls need readjustment when, for example, a large fluid-filled window is used; otherwise, the ROI behind the window will be too bright (overamplified). Further improvement of B-mode image quality can sometimes be achieved by THI.^1–5

In the color Doppler mode, a color box is placed over the grayscale image to cover the part of the image that requires Doppler information; an excessively large color box may compromise temporal resolution. To properly assign colors to the magnitudes of Doppler shift, the pulse repetition frequency (PRF), or scale control, is adjusted. The proper color assignments to show direction of flow may fail if the Doppler shifts exceed the scale determined by the PRF. For this and other reasons, color Doppler is used for identifying and visually assessing the flow, but not for measuring actual velocities. Precise measurements of flow velocities are conducted with spectral Doppler (pulsed wave and continuous wave).

PRF is the rate of pulses used to analyze the Doppler shift. For PW Doppler measurements of arterial flow, PRF is generally set at 3000 to 4000 pulses/second or Hz, which allows a wide enough Doppler range to fit spectra with most arterial velocities. If the actual shifts exceed the scale, the peak part of the spectrum in excess of the scale appears in the wrong place (PW Doppler) or in the wrong color (color Doppler). This phenomenon is called aliasing, and the Doppler shift limit at which it occurs is called the Nyquist limit and equals 1⁄2 PRF. In color Doppler, aliasing is avoided by increasing the scale (PRF) and/or using the baseline control to dedicate a larger portion of the scale to the flow in the dominant direction (toward or away from the probe), and/or by increasing the angle between the ultrasound beam and the flow vector (e.g., from 45 to 60 degrees) to reduce the actual shifts. In spectral Doppler, similar controls are available. In veins with much lower flow velocities, a PRF setting of 1000 Hz is a typical starting frequency. Modern machines have built-in “presets” for arterial and venous examinations, with PRF set to appropriate values. Older machines may have to be adjusted manually, including frequency filters. In PW Doppler, sample volume (also known as gate) size and placement are essential for correct measurements. A smaller sample volume of 1 to 2 mm is used when detailed investigation of flow within the vessel is required (e.g., when the degree of flow turbulence is to be assessed). The sample volume is placed in the center of the vessel or at the point of peak velocity indicated by the color image. In cases where blood flow is reduced (e.g., venous circuits) a larger sample volume may be appropriate. The Doppler spectral waveforms are produced by spectral analysis of the frequencies contained in the echoes returning from the sample volume area, using real-time fast Fourier transform or similar algorithms.1–5

The spectral displays in most machines automatically calculate and display flow velocities, rather than the frequency shifts that they measure. The calculated velocities are correct only when the operator adjusts the angle cursor line manually, aligning it with the flow direction of the vessel (i.e., “informs” the machine about the direction of actual flow). Otherwise, the machine will likely assume an angle of 60 degrees, which may not be correct, and the displayed velocities will be overestimated or underestimated.

Artifacts

B-mode images are expected to accurately represent the cross-sectional anatomy under the probe. However, some artifacts are commonly generated; those should be readily recognized by the ultrasound operator. Artifacts are image features that are formed by mechanisms other than standard placement of pixels with grayscale assignment based on reflection and backscattering. Artifacts “are determined by real anatomy, but are not real anatomy,” and may be a source of misinterpretation. Usually, they have regular vertical or horizontal shapes and differ from anatomic structures. The most common types of artifacts are detailed below.

Acoustic shadowing (Figures 1-6 and 1-7) appears as an echo-free void (shadow) in anatomy image when the beam is unable to pass through a strongly attenuating structure (e.g., a strong absorber or reflector). The orientation of the shadow is always in the direction of beam propagation (away from the probe—vertical with linear probes or radial in sector or convex probes). Shadowing because of large stones, calcifications, and bones is caused mainly by sound absorption, refraction and reflection and the associated shadow tends to be more anechoic (“clean”). In a tissue-air interface, shadowing is caused by complete reflection, whereas secondary reflections created at the interface are displayed as false low-level echoes within the shadow (“dirty shadowing”). Edge shadowing appears as shadowing from the edge of circular structures mainly because of refraction and beam spreading. It is a useful criterion for diagnosing cysts but can mimic stones, especially in the gallbladder fundus and cystic duct. Absence of shadowing from an echogenic (bright) object does not rule out a stone or calcification if it is very small (<3 mm in the most common imaging circumstances). Furthermore, some ultrasound machines use additional beams steered at angles that may bypass the small stone and create elements of true anatomic image behind it, thus suppressing the shadow. This technique is commonly known as real-time image compounding.

Posterior acoustic enhancement appears as a hyperechoic (bright, overamplified) area because of reduced attenuation by the area above it. It usually indicates the fluid nature of the weakly attenuating structure, although some low-echogenicity solid masses may cause similar enhancement patterns (see Figure 1-6).

Mirror images appear as two reflectors (true and spurious) with the spurious reflector located deeper than the true reflector and disappearing with transducer’s position change (see Figures 1-6 and 1-7). Refraction artifacts appear as copies of true reflectors whenever the beam strikes a boundary; this is different from the mirror image because it is visualized side by side with the true anatomic structure at the same depth (see Figure 1-7). Reverberations appear as multiple echoes between reflectors. They appear often at the anterior aspect of the distended urinary bladder (see Figure 1-6). Special forms of reverberations include ring-down and comet-tail artifacts (see Figures 1-6 and 1-7). Ring-down artifacts occur from a large mismatch in the acoustic impedance of media (e.g., when an air bubble is encountered) and are usually displayed as a vertical line that goes all the way or almost all the way down the image. Comet-tail is another type of reverberation artifact that appears as hyperechoic trail of reverberations arising from an echogenic structure (e.g., irregularity on the lung surface, some foreign bodies, cholesterol deposits in the gallbladder wall) that fade and taper down distally. Thus the main difference between these two reverberation artifacts is in their length and character (ring-down artifact continues all the way down the image, whereas comet-tails taper fairly close to the origination point). Side-lobe artifacts appear as areas of faded duplicate image side by side with the true anatomic structure and therefore could be mistaken for sediment or septa, usually within a fluid compartment (e.g., ascites), or could artificially enlarge the image of the anatomic structure (e.g., the prostate imaged through the urinary bladder). Fortunately, small probe movements usually eliminate this artifact. Section thickness artifacts appear as a fill-in of an anechoic structure (e.g., a cyst) if the beam has a greater width than the structure in question and could mimic debris, sludge, or clotted blood. As with B-mode, imaging artifacts can arise in color Doppler imaging as well. Color flow artifacts may appear as bright black and white structures within the vessel lumen. Also, if color gain is set too high, then color may appear as pouring out of the vessel, or anechoic areas may be filled with speckled color; however, these artifacts may be also produced by tissue bruits near a vessel stenosis or subtle tissue movements resulting from respiration (color noise). Whenever vessels overlie boundaries (e.g., subclavian artery overlying lung and pleura) mirror-color artifacts may be produced because of multiple reflections. Finally, aliasing and changes in the angle of insonation also produce artifacts.^5–7

Although artifacts in general degrade the image and often replace anatomic information, some important ultrasound techniques take advantage of certain types of artifacts or even completely depend on the presence or absence of certain artifacts for accurate diagnostic determination. For example, many pleural and lung ultrasound techniques are based on recognition and assessment of specific artifacts (see Chapters 19 and 20).

Ultrasound technique and safety issues

Selecting the appropriate transducer depends mainly on the depth and spatial resolution requirements of the ROI; what is gained in depth is lost in image quality or detail, and vice versa. In general, the highest ultrasound frequency allowing penetration to the depth of interest should be selected. For superficial structures, transmit frequencies of 7 to 15 MHz are usually used (e.g., vascular and small parts imaging). For deeper structures (e.g., abdominal organs), lower frequencies of 2 to 5 MHz are necessary. Current technology offers broadband probes that permit selecting a central frequency from several choices, or multiple frequencies can be used at the same time to achieve the best possible image resolution/beam penetration balance for ROI in question (broadband imaging).

A liquid material (gel) is used to ensure a good acoustical contact between transducer surface and patient’s skin by eliminating the interposed air. The transducer must be held lightly in the hand but firmly, with the thumb pointing toward its marker side. By placing the edge of his or her hand (or the tips of the fourth and fifth digits) against patient’s skin, the operator ensures stability and fine control of the probe position. All transducers have an orientation marker that corresponds to the marker on the screen. Manipulation of the transducer (pressure, translation, rotation, panning, tilting) allows finding the target, optimizing the view, and reviewing the entire volume of an organ, lesion, or area of interest. Applying the correct pressure evenly can significantly improve image quality (or confirm compressibility of a vein); on occasion, more pressure can be applied on one side of the transducer (panning, or heel-toe maneuver) to create a necessary angle for Doppler modes. Rotating is usually used to transition between sagittal or coronal and axial scanning of the ROI (in whole-body anatomy terms) or between long-axis and short-axis views (in reference to an organ, vessel, or lesion). Tilting the transducer aids in “guiding” the scanning plane and/or direction. Panning sustains the current imaging plane but extends the view in one of the directions within the same plane. The transducer, and consequently the beam, can thus be oriented freely in any anatomic plane of the body (sagittal, axial, coronal, and any intermediate or oblique variations of these planes). For some structures, the operator may depart from the references to the anatomic body planes and use references to the structure itself (long-axis or short-axis planes). This is usually the approach to scanning vessels, kidneys, pancreas, or spleen. Real-time, multiplanar imaging capability is a unique characteristic of ultrasound that allows rapid determination of spatial relationships of examined structures. As a general rule, the transducer’s marker should be directed toward patient’s right side in axial planes or toward the patient’s head in sagittal and coronal planes.

Basic ultrasound scanning orientation terms are shown in Figure 1-8. Coronal refers to the longitudinal scan performed from the patient’s side, and the plane separates the anterior from the posterior. Transverse or axial refers to a plane that separates the cephalad from the caudad. Sagittal refers to the longitudinal anteroposterior plane that divides right from left. Cranial (cephalad) indicates the direction toward the head and caudad the direction toward the feet. Anterior (ventral) and posterior (dorsal) refer to structures lying toward the front or the back of the subject, respectively. Medial means toward the midline and lateral away from it, whereas proximal means toward the origin and distal away from it.^5–7

The American Institute of Ultrasound in Medicine (AIUM) and the U.S. Food and Drug Administration (FDA) agree that ultrasound is safe if used when medically indicated and with the output power and exposure times not exceeding the necessary levels (As Low As Reasonably Achievable—the ALARA principle).

High-intensity focused ultrasound (HIFU) is a therapeutic modality used for ablation of breast tumors, prostates, uterine fibroids, and so on, by producing intensities exceeding 1000 W/cm² and raising tissue temperatures by up to 25° C. Diagnostic ultrasound devices use orders of magnitude lower intensities and very small duty factors (proportion of transmitting time relative to the total examination time); its thermal effects (the first recognized adverse bioeffect—tissue heating) are expressed as the thermal index (TI), the value of which equals the predicted rise of tissue temperature in degrees C with unlimited exposure. Temperature elevations less than 1° C are considered safe even for ophthalmic imaging.1–3

The second notable adverse bioeffect of ultrasound is cavitation—explosive formation of microscopic bubbles in tissues caused by abrupt pressure fluctuations. This phenomenon is highly unlikely at diagnostic ultrasound intensities. However, experimental studies suggest that contrast agents and agitated saline may, under certain circumstances, promote cavitation even at moderate energies. For example, when investigating a patient with probable right-to-left shunt, agitated saline or a special contrast agent is administered to perform transcranial Doppler (TCD) for bubble detection in the middle cerebral artery (MCA) or the ophthalmic artery (OA). TCD operates at high acoustic power to penetrate the skull through the temporal window; if the same power level is applied through the orbital window, the energy passing through the low-attenuation ocular media may create cavitation within retinal arterioles containing bubbles and result in a hemorrhage. The ability of the given ultrasound mode to cause cavitation is best characterized by the mechanical index (MI), which is required to be displayed along with the thermal index on the screen of all modern ultrasound machines. Minding the vulnerability of the eye, several guidelines require lowering the energy output to limit the ocular scanning energies to levels corresponding to MI less than or equal to 0.23 and TI less than or equal to 1.0. Notwithstanding the cautious approach, all the evidence and theoretic considerations attest to a very high safety margin of diagnostic ultrasound in the clinical context, making it the safest tomographic modality, with no electromagnetic or particle radiation and very low overall energy delivery.5–8

Scope and evolution of ultrasound imaging

Unlike most other nontomographic and tomographic imaging modalities that have a standardized data acquisition process with preprogrammed and predictable data sets, ultrasound is a hands-on patient examination method with real-time continuous display of anatomic information. This feature, along with its excellent safety profile, could make ultrasound a highly informative component of the physical examination in most medical disciplines. However, from early stages of its clinical implementation, the use of ultrasound has been adapted to the routines of radiology departments, and most medical systems do not take full advantage of the real-time nature, universality, and versatility of ultrasound imaging. Similar to other technician-performed modalities, the “radiologic,” or “referred,” ultrasound is mostly performed by specialized technologists trained to follow standardized protocols. Limited sets of still images are obtained for subsequent interpretation by radiologists or other appropriately trained physicians who, with rare exceptions, do not see the patient or the clinical situation at hand. Although the analysis of these data sets is comprehensive and extremely valuable for establishing a diagnosis, the data sets themselves carry only a subset of potentially useful information. Furthermore, referred ultrasound results are reported with a delay, further reducing its contribution to real-time patient management in the prehospital environment, emergency rooms, intensive care units, operating rooms, and other settings when the value of information diminishes very quickly with time. In some settings, it is not the diagnosis that is unknown but the physiology and its trends and response to therapy; the “radiologic” ultrasound cannot assist at all in most of those situations.

To satisfy the unmet need for instantaneous results and repeatable imaging data as part of the patient examination and monitoring by the physician, new branches of ultrasound technology have evolved in recent years: emergency ultrasound (EU) and critical care ultrasound (CCU). These can be considered new modalities that use the same equipment but have a different scope and different effects on patient management. They do not take the place of “radiologic” ultrasound; furthermore, many studies requiring comprehensive analysis are still referred to expert radiologists and cardiologists for thorough consideration, in addition to standardized studies performed by radiologic personnel. In the following sections, we describe the main features of EU and CCU, as well as present the innovative concept of holistic approach (HOLA) to the use of ultrasound in the emergency and critical care environments.

Emergency ultrasound

Emergency ultrasound began strictly out of clinical necessity in the mid-1980s and has expanded based on the notion of a focused examination answering the most relevant, usually binary, clinical question. The initial applications included evaluation for ectopic pregnancy, trauma, and cardiac arrest. EU has since spread widely, and its multiple applications range from pelvic to ocular examinations. The emergency setting does not allow for lengthy examinations, and screening examinations have little place in most cases.

Besides diagnostic applications, EU plays an increasing role in procedure assistance, greatly facilitating procedures previously conducted in a “blind” fashion or rarely even attempted in the emergency department. For example, assessment and drainage of a peritonsillar abscess is facilitated by ultrasound guidance; regional nerve block greatly improves care, saves time, and avoids the dangers and increased workload of sedation. One of the key areas of attention is patient resuscitation, not only in cardiac arrest but also in periarrest and shock states. Ultrasound allows the clinician to accurately assess the patient’s status, as opposed to making critical decisions based on surrogate indicators, such as pulse checks and blood pressure monitors. In addition, lifesaving procedures, such as transvenous pacemaker placement, are significantly easier under ultrasound guidance than by traditional means.

In the emergency environment, many patients are not present long enough for routine rescanning. However, the most critically ill patients, such as trauma, cardiac arrest and shock patients, may be scanned repeatedly to guide resuscitative efforts and assess the effectiveness of interventions made. Patients undergoing diuresis or being watched for expansion of a small pneumothorax may be easily monitored using lung ultrasound techniques with immediate real-time availability of accurate results; this is an important advantage over taking repeated chest radiographs or computed tomography (CT) scans. The confined setting of the emergency department was a primary driver of machine miniaturization in the mid-1990s. A small-footprint multipurpose machine with multiple probe options is ideal. The industry has made enormous progress in creating such machines capable of a wide range of applications, yet rugged enough to withstand intensive use, frequent relocation, and cleaning.

Documentation of ultrasound examinations, including ultrasound-guided procedures, is essential not only for reimbursement purposes but also for communication with other physicians. Use of electronic or permanent medical records is critical, and both image and video loop archiving are very helpful. A comprehensive hospital credentialing plan is mandatory to ensure proper training and quality assurance and to have a productive emergency ultrasound program in place that will aid patient care, not hinder it.⁶

Critical care ultrasound

Critical care ultrasound has many similarities with EU. Both are applied in seriously ill patients and are used to guide procedures. However, there are obvious differences, too. Critical care patients are present for routine rescanning; they are often hemodynamically unstable and have a tenuous respiratory function. Implementing ultrasound in ICU practice greatly augments patient assessment and monitoring, whereas its use in guiding invasive procedures dramatically improves patient safety.

CCU has several limitations. In the ICU, the physical examination is deprived of some basic elements. Patients are usually intubated under sedation and analgesia; they may have difficulty communicating or indicating pain. Mechanically ventilated patients are placed in a supine position, and thus usual ultrasound techniques that are applied in ambulatory patients may not be suitable. Moreover, access to the patient is obstructed by cables, electrodes, catheters, and so on, and, especially in trauma patients, by bandages, splints, burn wounds, and so on, rendering some acoustic windows inaccessible. Acoustic barriers such as bowel gas, subcutaneous emphysema, pneumothorax and pneumoperitoneum may affect clarity of images. Fluid overload is not an absolute barrier, although the presence of diffuse tissue edema in patients with systemic inflammatory response syndrome interferes with image quality.⁹ Still, persistence in CCU is usually rewarded. For example, in patients with limited windows or excessive bowel gas, abdominal imaging can be facilitated by the use of small-footprint (phased array or microconvex) transducers and/or intercostal approaches.

Space is another common CCU issue. ICUs are replete with various devices, such as life-support equipment, ventilators, and hemodialysis units, around patient beds. To allow movement and imaging in the busy ICU, battery-powered laptop-sized equipment with small-footprint transducers is ideal. Early model laptop ultrasound machines exhibited poor resolution and image quality; recent models produce images of excellent quality and offer broader scanning options. We favor small-sized machines with reasonable purchase and maintenance costs and that provide good image quality and full application packages. Intensivists should adequately train and practice to take full advantage of this operator-dependent modality.

Prevention of cross-infections in the ICU is essential. Robust disinfection and procedural guidelines should be implemented in routine practice to avoid transmitting nosocomial pathogens (e.g., multiresistant gram-positive or gram-negative strains) between patients. CCU operators should wear gloves and avoid touching other parts of the device with the hand that holds the transducer. This is done by using one hand for handling the transducer and the other one for making system adjustments; alternatively, two operators may participate in the procedure. Operators should follow universal precautions for infection control. In ultrasound-guided invasive procedures, strict sterile protocols must be followed with the use of sterile transducer covers and gels. Upon completion of the examination, transducers must be cleaned immediately in the direction from the cable to the probe face, and disinfected according to the manufacturer’s recommendations. Care and maintenance of ultrasound machines are critical.¹⁰ Recent reports indicate the possibility of infection transfer through refillable gel bottles. Some medical facilities have decided to use only prefilled bottles and discard them once empty; this trend will likely continue.

So far we have underlined major CCU limitations. Sometimes, a limitation proves to be an advantage. Sedation and analgesia facilitate transducer manipulations in all diagnostic examinations. Ultrasound-guided invasive procedures are facilitated by alleviation of pain or discomfort. Myochalasis results in low muscle resistance, and thus applying transducer pressure to identify ROIs becomes easier (e.g., abdominal examination). Mechanical ventilation, although significantly interfering with surface chest ultrasound (Video 1-4), facilitates visualization of subcostal organs. In general, the advantages of an easy-to-perform bedside examination cannot be overstated. CCU in its full-fledged implementation is seen by us as a companion to physical examinations by clinicians who evaluate every new admission in the unit, and selective routine rescanning is imperative in most cases.^9,11,12 CCU considers likely complications and diagnoses unsuspected abnormalities, besides facilitating critical care monitoring.¹³

The holistic approach ultrasound concept

The HOLA concept of ultrasound imaging defines CCU as part of the patient examination by a clinician, to visualize all or any parts of the body, tissues, organs, and systems in their live, anatomically and functionally interconnected state and in the context of the whole patient’s clinical circumstances. This concept is illustrated in Figure 1-9. Details about the various techniques that are integrated in HOLA-CCU are presented throughout this textbook.

The concept is based on the universality of ultrasound imaging and its real-time visual nature. An ICU that has implemented the HOLA concept and corresponding techniques is able to perform head-to-toe ultrasound imaging as if an imaginary “cocoon” of ultrasound beams is wrapping the entire body. In any given patient, certainly only a part of the available techniques will be clinically indicated, and most techniques and sites of generic scanning are omitted. In each patient, though, quick and simple views of certain organs and anatomic sites are necessary to rule out most common abnormalities, such as pathologic fluid in the potential spaces of pleura, abdomen, pericardium and scrotum; standardized pulmonary sites for interstitial status; and others.

The HOLA concept recognizes the generally accepted division of CCU applications into two categories: basic and advanced (or consultant-level) applications. Basic applications can be seen as either critical, lifesaving applications or focused uses that can significantly expedite care. To this end, ultrasound-guided procedures, focused echocardiography, e-FAST, and abdominal aortic aneurysm examinations may be lifesaving and are considered basic. Focused deep venous thrombosis (DVT) examinations and lung ultrasound as well as dynamic/patient management procedures, such as simple volume status assessment, expedite care and hence also belong in the basic category. Advanced echocardiography, including nonarrest transesophageal applications, is a consultant-level technique. The vast majority of comprehensive ultrasound examinations, such as biliary, renal, vascular, TCD, and open-ended/nonfocused ultrasound examinations are referred for comprehensive radiologic data collection and analysis and also fall into the consultant-level category. Thus practice of HOLA-CCU by a given unit does not mean that all studies are performed by intensivists; the critical care facility is part of the given medical system or hospital and uses radiology and other services as necessary. Although HOLA-CCU could be interpreted globally as the transducer being applicable to all surfaces and tissues, it rather defines the scope of critical care practice, while creating appropriate referrals that require additional ultrasound expertise (see Chapter 57).

In a fictional scenario described later, a sequence of ultrasound-supported physical examination is described to illustrate HOLA-CCU. Examination starts from the head (Figures 1-10 to 1-12) by accessing temporal and ophthalmic windows for TCD; eye and orbit ultrasound is performed using appropriate machine settings (see Chapters 2 to 6). Scanning of maxillary sinuses and other facial structures is also performed (see Chapter 51). Neck and upper limb exploration (Figures 1-13 to 1-19 ) provides information about the trachea, thyroid, soft tissues, and neurovascular and musculoskeletal structures (see Chapters 8 to 16 and Chapters 51 to 54). After both supraclavicular and infraclavicular approaches, scanning reaches the axilla and the shoulder region and is further extended to the upper limbs. The “core” of HOLA ultrasound scanning in the ICU is general chest ultrasound (Figures 1-20 to 1-24), comprising lung, pleural space, and cardiac ultrasound (see Chapters 19 to 34). Lung and pleural ultrasound explores the subpleural lung parenchyma, the diaphragm, and pleural space abnormalities. Echocardiography is essential for cardiac and pericardial pathology and for hemodynamic assessment. Abdominal scanning (Figures 1-25 to 1-28) integrates the e-FAST components for free fluid detection but also targets solids organs (e.g., spleen size), the aorta and abdominal vascular networks, the gastrointestinal tract (peristalsis, small bowel diameter and contents), and the urogenital system, as well as the peritoneum and lower pelvis (see Chapters 8 and Chapters 41 to 46). The inguinal region is often the site for vascular access and corresponding complications, such as hematomas and pseudoaneurysms. Finally, exploration of the inguinal region is protracted to the lower limbs (Figures 1-29 and 1-30). Assessing patency of venous circuits and excluding DVT is essential (see Chapters 9 and 51), whereas gathering information about musculoskeletal abnormalities is often valuable, especially in a patient with appropriate history (see Chapter 51).

Ultrasound-guided procedures and development of complex evaluation protocols¹⁴ (e.g., combination of lung, cardiac ultrasound, and vena cava analysis, linked with clinical and laboratory data to assess volume status) are examples of clinically driven modular applications of the HOLA ultrasound concept. The latter is adjustable to meet the diagnostic and monitoring specificities of individual clinical scenarios (e.g., trauma, sepsis, etc.). HOLA-CCU is easily scaled down to specific application profiles; some of those require expert input to interpret findings that should be processed under the light of clinical judgment or require special expertise.

Although CCU routinely deals with acoustical barriers, there should be no formal or subjective barriers in the way of its implementation in routine intensive care practice. HOLA ultrasound related issues are further discussed in Chapter 57 with an inclusive set of principles to phase in and implement all CCU methods and universal generic scanning of a patient in a critical care facility. We believe that a proper mass of evidence has been reached that shows ultrasound as the “wave” moving in the direction of more vigorous, operationally responsive and efficient patient care. Appropriate application of ultrasound imaging technology can offer crucial information for diagnostic determinations as well as for optimizing management in real time. The HOLA concept is, first and foremost, a means to conceptually embrace the universality of ultrasound imaging and adopt a course toward a balanced system for its use to optimize care and improve patient outcomes, facilitate direct care by intensivists, as well as to inform health care administrators and ensure their support of rapid implementation of new powerful tools for critical care optimization.

Note: The term “holistic” in the HOLA acronym is used in its original meaning in ancient Greek, to emphasize the importance of the whole and the interdependence of its parts. The term and the acronym must not be confused with “holistic medicine,” which has a different patient population, scope, and methodology. The concept of “holistic approach–critical care ultrasound,” the title of the same, and the respective acronym have been suggested by Dimitrios Karakitsos (see Chapter 57). The HOLA ultrasound project has been further refined by Ashot Ernest Sargsyan, Michael Blaivas, and Dimitrios Karakitsos. We define the HOLA concept as an approach to ultrasound imaging in emergency and critical care medicine as follows: ultrasound is part of the patient examination by a clinician, to visualize all or any parts of the body, tissues, organs, and systems in their live, anatomically, and functionally interconnected state and in the context of the whole patient’s clinical circumstances.

Pearls and highlights

References