Critical care ultrasonography (CCUS, or point-of-care ultrasound) has become an increasingly valuable skill in the diagnosis and management of patients who are critically ill presenting in a variety of clinical scenarios (Table 3.1). Widespread adoption has been facilitated by advances in ultrasound technology because modern devices have become increasingly portable, less expensive, and yet capable of high-quality imaging. Conceptually, CCUS functions as an extension of the physical examination. It is immediately available at the bedside, is noninvasive, and in skilled hands provides a wealth of information that can be used to rapidly narrow differential diagnoses and to initiate or modify treatment. CCUS is not intended to replace other more comprehensive forms of diagnostic imaging. Rather, the real-time acquisition and interpretation of images that are then interpreted in the context of the physiology of a patient who is acutely unstable is the hallmark of CCUS. Of the very broad range of techniques that comprise CCUS, this chapter focuses primarily on the use of ultrasound to evaluate cardiorespiratory status.
Ultrasound Basics- An ultrasound image is produced from the emission of sound waves and the interaction of these sound waves with various tissue interfaces that are then reflected and detected by a transducer. The wavelength of a sound wave is the distance between two repetitive points along a single cycle, whereas the frequency of a wave is the number of repetitions per second. The wavelength and frequency of the sound wave determine the spatial resolution. The shorter the wavelength, and thus the higher the frequency, the better the spatial resolution. High-frequency waves provide excellent near-field resolution but poor tissue penetration because they are subject to attenuation with increasing tissue depth. Conversely, low-frequency waves offer improved tissue penetration, allowing visualization of deeper structures, but poor spatial resolution. Thus, higher frequency probes (~5-10 MHz) are used for visualization of relatively superficial vascular structures and lower frequency probes (~2-5 MHz) for imaging deeper tissues such as the heart and abdomen. The three main probe types used are the linear probe, the curvilinear probe, and the phased-array probe.
- Spatial resolution differs from temporal resolution. Spatial resolution relates to the ability to determine the location of a structure in space. Temporal resolution relates to the ability to determine the position of a structure at a particular instant in time. Temporal resolution is determined by the pulse repetition frequency (PRF). The higher the PRF, the higher the temporal resolution. A higher PRF increases the number of frames per second. This becomes important when assessing a fast-moving structure such as the heart. The temporal resolution can be improved by decreasing the sector width or imaging depth.
- Doppler ultrasonography is of great value in the assessment and quantification of blood flow through the heart. Doppler techniques are divided into pulsed-wave Doppler (PWD), continuous-wave Doppler (CWD), and color Doppler. PWD is used to measure flow velocities at a single defined location, such as at a point (defined by the sampling volume) in the left ventricular outflow tract (LVOT). However, the maximum velocities that it can accurately measure is limited, beyond which PWD is susceptible to a form of sampling error called aliasing. CWD, on the other hand, is not limited by aliasing and can be used to measure very high flow velocities (such as those from a stenotic aortic valve). However, the trade-off is range ambiguity, that is, CWD cannot identify the precise location of the maximum velocity—it simply measures the maximum velocity anywhere in the path of the ultrasound beam. Color Doppler is a form of pulsed Doppler (and thus subject to aliasing) that measures flow at multiple sampling volumes on a two-dimensional (2D) grid rather than along a single line. One important caveat to the use of these techniques is that they are all angle dependent. In other words, they are accurate when the angle of the transducer beam is less than 20° to the direction of blood flow. Consequently, Doppler measurements may underestimate true flows or provide inconsistent values on serial examination.
- M-mode or motion mode is another useful ultrasonography tool that displays a view of all structures along a single vertical line over time. Because M-mode is confined to a single scan line, it provides the highest temporal resolution. This is useful for accurately measuring distances, such as the diameter of the inferior vena cava (IVC) or the size of a pericardial effusion, determining presence and absence of lung sliding, and for tracking the path of structures, such as valve opening (see Figures 3.1F and 3.2D).
- Cardiac ultrasound, or echocardiography, can be performed by a transthoracic echocardiographic (TTE) or transesophageal echocardiographic (TEE) approach, depending on patient factors and the specific clinical question. TTE offers the advantages of being noninvasive, presenting no risk to the patient, and can be rapidly deployed. TTE imaging may be limited in situations such as increased patient body habitus, recent sternotomy, or the presence of dressings or drains. TEE overcomes many of these limitations related to image acquisition and can offer better image resolution, making it the preferred technique for detailed examination of valvular morphology such as for diagnosing endocarditis or assessing the position and function of implanted valves, for determining the presence of left atrial thrombus, for guiding placement of cannulas such as for extracorporeal membrane oxygenation (ECMO), and guiding resuscitation during cardiac arrest. However, TEE is invasive, often requires sedation, and, although rare, poses risk of damage to the oropharynx or esophagus during probe placement and manipulation. Because of its accessibility, faster deployment, and absent risk profile, TTE should be the initial approach for patients in the intensive care unit (ICU), whereas TEE is reserved for specific indications or in patients in whom TTE images proved inadequate by a skilled operator. The remainder of this chapter focuses on the use of TTE.