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e5 Abstract: Pediatric vascular access and centeses are core skills in pediatric critical care Intraosseous access is essential when intravenous access is not readily attainable, but central venous access remains an essential skill Arterial access is important for both hemodynamic monitoring and blood sampling All intravascular devices carry the risk of complications, including infection Pericardiocentesis is important for both diagnostic and therapeutic purposes, and for treating cardiac tamponade Thoracentesis and tube thoracostomy are needed for both the treatment of pneumothorax and pleural effusions or hemothorax Abdominal paracentesis and catheterization is an important technique for managing abdominal compartment syndrome Key words: central venous catheterization, intraosseous infusion, arterial cannulation, paracentesis, pulmonary artery catheterization, thoracentesis, tube thoracostomy, pericardiocentesis 15 Ultrasonography in the Pediatric Intensive Care Unit ERIK SU, AKIRA NISHISAKI, AND THOMAS CONLON Critical care ultrasound has been a topic of exploding interest and impact in the pediatric intensive care unit (PICU), with thousands of citations published yearly on use of ultrasound for diagnostic and procedural applications in the critically ill Since the 1980s technologic advancements in imaging quality and digital storage have resulted in bedside ultrasound equipment that is both portable and powerful Its use in the ICU results from recognition of its value for procedure guidance, interaction with cardiac intensivists in specialty ICUs, and shared interest from other clinical specialties, such as emergency medicine Ultrasound is emerging as standard of care for many ICU procedures and is increasingly recognized as a valuable diagnostic tool Nonprocedural bedside ultrasound applications performed by intensivists contrasts with traditional diagnostic imaging It is interpreted at the bedside, occurs synchronously with management decisions, and is repeated as necessary by the bedside clinician for serial assessment Intensivistoperated examination is more focused on assisting the direct and immediate delivery of care, whereas diagnostic imaging by radiologists and cardiologists emphasizes optimizing study detail The ability of the intensivist to repeat diagnostic point-of-care studies to evaluate the effect of targeted therapies as a form of monitoring and further guide management decisions emphasizes an additional clinical benefit of this unique technology Ultrasound Physics and Basics of Image Optimization The physical principles that make ultrasound possible are well described Ultrasound energy is transmitted in sound waves through substances at a frequency greater than 20 kHz, beyond the range of human hearing Probes used for bedside ultrasound 114 • Intensivist-operated ultrasound is an important adjunct for vascular procedures at the pediatric intensive care unit bedside and is emerging as an important procedural aid across all percutaneous vascular access applications Accuracy of intensivist-operated ultrasound examining the heart, lungs, abdomen, and cerebral circulation has also been explored Cardiac ultrasound examinations by intensivists • • • PEARLS demonstrate high sensitivity for pathology similar to studies performed by imaging specialists Practice and familiarity with ultrasound’s capabilities, advantages, and disadvantages contribute to responsible and effective practice Successful translation of ultrasound from training to clinical practice requires thoughtfully designed processes with regard to protocols, recordkeeping, and quality improvement activities typically use frequencies in the range of to 20 MHz or more Ultrasound impulses are transmitted in a coordinated manner from an array of piezoelectric elements in a transducer aimed at a target of interest Reflected sound from tissue and interfaces between substances of differing density is received by the same piezoelectric array Based on known speeds of ultrasound in tissue as well as image processing by the ultrasound device, a two-dimensional image is generated and cycled over time to compose a continuous video or cine Variability in tissue sound velocity and attenuation properties—including refraction, reflection, absorption, or scattering of returning ultrasound energy—may introduce artifact in images (Fig 15.1).1 A portion of image quality is naturally machine dependent; a variety of parameters are adjustable for image optimization to reduce diagnostic error Image size modification tools include depth, zoom, and scan area Though a wide and deep scan area permits capture of a large field of view, short depth or high zoom factor facilitates imaging of small or superficial structures (Fig 15.2) Gain is also an important setting for image optimization and can be adjusted in several ways Gain affects the sensitivity of the machine to returning ultrasound energy Returning ultrasound energy is attenuated as it passes through tissue Time to return of reflected ultrasound energy determines depth of visualized structures rendered on the screen Time-gain compensation settings can be adjusted to emphasize gain at different depths based on when the return signal reaches the transducer On various machines, these may appear as sliding controls or knobs that control near and far gain At the bedside, gain is adjusted relative to ambient room lighting so that relevant pathology can be visualized (Fig 15.3) Frequency is an adjustable control that also affects image quality Higher ultrasound frequencies allow for higher-resolution imaging as a function of improved spatial discrimination of structures CHAPTER 15  Ultrasonography in the Pediatric Intensive Care Unit 115 * A A * B B • Fig 15.1  ​Ultrasound artifacts (A) Reflection artifact across the pleural line (asterisk) (B) Mirror artifact across a diaphragm (asterisk) Higher frequencies also result in poorer imaging of deep structures owing to signal attenuation Conversely, lower-frequency imaging can identify deeper structures at the expense of image resolution (Fig 15.4) Doppler ultrasound is used in a variety of procedural and diagnostic applications to derive the speed and direction of moving structures in the body, blood flow being a common target Using color flow Doppler in vascular imaging, ultrasound machines color code areas on the two-dimensional image where tissue and fluid are moving Returning reflected ultrasound frequency is either Doppler shifted higher (movement toward the transducer) or lower (movement away from the transducer) In the human body, this can correspond not only to blood but also to the motion of effusion fluid and other potential spaces as well Pulsed wave Doppler samples a small area of the screen for Doppler data that are then plotted along a velocity/time scale (Fig 15.5) Doppler is most useful when the direction of blood flow is parallel to the ultrasound beam When flow is not parallel, angle of incidence correction is possible in many commercially available systems By convention, angle correction is currently not used in echocardiography because of the difficulty in matching beam angles to nonlaminar flows in the heart C • Fig 15.2  ​Effects of depth on ultrasound image, with (B) demonstrating same image as (A) at greater depth and (C) at shallower depth than (A) Power Doppler displays amplitude of echoes from moving cells and is less angle dependent than traditional Doppler, thereby permitting improved visualization of vessel branching For this reason, power Doppler can be a useful modality for vessel identification in procedural guidance M-mode ultrasound graphs the received ultrasound signal along a one-dimensional beam over time This is useful for tracking objects in motion to quantify and characterize their movement (Fig 15.6) Examples of M-mode applications include assessing respiratory variation of the inferior vena cava (IVC), 116 S E C T I O N I I   Pediatric Critical Care: Tools and Procedures A A B B • Fig 15.3  ​Effects of gain on ultrasound image with (B) demonstrating same image as (A) at lower gain calculating a left ventricular shortening fraction, and evaluating diaphragmatic excursion When using M-mode, it is important that the operator’s scanning hand remain motionless because operator movement will be captured on the M-mode tracing as well Sweep speed is an important machine setting for M-mode and pulsed wave Doppler when data are plotted against time Sweep speed is defined as the speed at which the data tracing is graphed on the screen In some instances a faster sweep speed is necessary to reduce the amount of time displayed on the screen, thereby “stretching” the tracing so that finer details of movement are captured This is helpful in echocardiography with a tachycardic heart so that cardiac motion is more easily discerned Conversely, slowing sweep speed may be helpful in compressing more data into one screen (Fig 15.7) Slowing sweep speed might be necessary for visualizing changes in diaphragmatic motion or changes in arterial flow over a respiratory cycle Transducers Ultrasound transducers appear in a multitude of configurations that serve different specialized functions They are largely divided into higher-frequency linear array probes and lower-frequency sector type probes, with occasional exceptions Linear array transducers are a mainstay of procedural guidance for intensivists and other clinical interventionalists (e.g., interventional radiologists) using ultrasound (Fig 15.8) They are configured C • Fig 15.4  ​Effects of frequency on ultrasound image with (B) demonstrating same image as (A) at higher frequency and (C) at lower frequency than (A) with an array of parallel-oriented piezoelectric elements designed to image a field of tissue below the probe at close proximity to the skin These probes also tend to emit higher frequencies (7 MHz or above) to visualize the region of interest at high resolution, with some small structure probes operating above 20 MHz Linear array transducers are optimal for procedural guidance because needle localization is easier with their wide, shallow, and high-resolution field of view Linear array transducers are also well suited to diagnostic imaging at shallow depths, where higher resolution is preferred Examples include thoracic imaging and vascular imaging CHAPTER 15  Ultrasonography in the Pediatric Intensive Care Unit A 117 B • Fig 15.5  ​Doppler modalities in ultrasound; pulsed wave (A) and color flow Doppler (B) • Fig 15.6  ​M-mode imaging in ultrasound 1, Two-dimensional image for cursor targeting 2, M-mode tracing Linear arrays range in size and shape In addition to conventional arrays, in which the transducer face forms the end of the handpiece, linear transducers also include hockey stick–shaped vascular probes, originally designed with improved ergonomics for carotid imaging but also useful for peripheral vascular procedures in children Sector-type transducers encompass phased array transducers and curvilinear transducers The sector term refers to the image generated by the concave array appearing as a wedge-shaped sector on the ultrasound screen Phased array transducers are lowfrequency devices and have a small footprint on the skin, designed for deeper wide-field imaging from a small surface on the body (Fig 15.9) Because these probes are largely used in cardiac imaging, the ultrasound hardware is optimized for a fast frame rate to capture motion at the expense of image resolution Because of the small footprint and low frequency, these probes are also optimally suited for transcranial imaging for Doppler flow measurements of the cerebral arteries This type of imaging requires a low fundamental frequency of approximately MHz, with most probes ranging between and MHz These probes can also be used for diagnostic imaging in the abdomen, particularly in infants and young children Curvilinear transducers operate at the same to slightly higher frequency (1–12 MHz) than phased array transducers and are engineered for improved image resolution at the expense of a slower frame rate For this reason, they excel at looking at relatively nonmobile structures such as the abdomen and its vasculature Large curvilinear probes are often too large for children; therefore, smaller-radius microconvex curvilinear probes are preferred These probes are also used for cranial ultrasound in infants owing to their higher frequency and small footprint Within the ICU, their higher resolution and footprint that is slightly wider than that of a phased array transducer also make them useful for procedure guidance, particularly for vascular access and regional anesthesia In addition, they may facilitate visualizing the lung and thoracic structures Procedural Guidance Venous Access Long before entering the ICU, ultrasound became a mainstay for procedural guidance in interventional procedures In the hands of intensivists, ultrasound has proved to be a useful adjunct in vascular access, increasing first-pass success and reducing adverse events during central venous catheter (CVC) insertion in a number of large series and meta-analysis of the pediatric literature.2–7 Though the majority of ultrasound trials for vascular access have been conducted with trainees likely less familiar with landmark methods for CVC insertion, the preponderance of the primarily adult evidence supporting ultrasound guidance for internal jugular vein CVC cannulation has added ultrasound guidance to the standard of care in this procedure Although it appears that ultrasound guidance does improve safe vascular access, its use is a skill that requires training and practice Novice providers should practice procedures with skilled operators before attempting them independently Maneuvers for identification of vessel patency are helpful for targeting veins for cannulation The primary and most reliable method is observing for compressibility of the venous structure and absence of pulsatility It is important to recognize that ... monitoring and further guide management decisions emphasizes an additional clinical benefit of this unique technology Ultrasound Physics and Basics of Image Optimization The physical principles... (movement toward the transducer) or lower (movement away from the transducer) In the human body, this can correspond not only to blood but also to the motion of effusion fluid and other potential... dependent than traditional Doppler, thereby permitting improved visualization of vessel branching For this reason, power Doppler can be a useful modality for vessel identification in procedural guidance