118 SECTION II Pediatric Critical Care Tools and Procedures • Fig 15 7 Effectof sweepspeedon time basedmodalities In this exampleofM mode imaging, the imagein(A)isrunningataslow[.]
118 S E C T I O N I I Pediatric Critical Care: Tools and Procedures A B • Fig 15.7 Effect of sweep speed on time-based modalities In this example of M-mode imaging, the image in (A) is running at a slower sweep speed than the image in (B) A B • Fig 15.8 Examples of linear array transducers (A) Conventional linear array probes (B) Carotid vascular (“hockey stick”) probe • Fig 15.9 Examples of phased array transducers CHAPTER 15 Ultrasonography in the Pediatric Intensive Care Unit patients with pulmonary hypertension can demonstrate pulsatile venous flow that confounds differentiation between vein and artery Though this finding is most pronounced in patients with a primary diagnosis of severe idiopathic pulmonary hypertension, it can also be seen in patients with acquired pulmonary hypertension from increased intrathoracic pressure or cardiogenic shock In the event that compressibility, Doppler flow, or pulsatility cannot identify a venous structure, this may mean that a vessel may no longer be continuous due to sclerosis or thrombosis Venous thrombi often occupy space in the vessel lumen, obliterate venous flow, and resist external compression Doppler modalities for identifying vessel occlusion are well described but have limited utility when thrombi are partially occlusive or in shock states when flow is very low Various approaches to central vessel cannulation using ultrasound exist Dynamic (real-time ultrasound visualization) approaches to the central veins include the transverse (Fig 15.10A) or longitudinal (Fig 15.10B) planes relative to the vessel An advantage of the transverse approach, with the view plane perpendicular to the vessel axis, is that it improves lateral steering of the needle to avoid arterial puncture A disadvantage is that needle tip identification is harder, because only a cross-section of the needle shaft is visualized Identification of the needle tip is crucial for successful access and prevention of inadvertent injury; it is performed by sliding the probe and staying over the needle tip throughout the procedure This sliding technique will help avoid inadvertent puncture of deeper structures (i.e., the pleura or carotid artery in internal jugular venous cannulation) The longitudinal approach, with the view plane parallel to the vessel axis, has reciprocal advantages and disadvantages It facilitates observation of the advancing needle tip throughout the initial puncture, but it does not provide a view of lateral structures For these reasons, transverse visualization of the vessels is often more intuitive and most frequently taught to novice practitioners The static approach uses the ultrasound to identify the vessel, followed by marking of the site for needle puncture without direct ultrasound visualization of the needle during the attempted cannulation This permits use of both hands for the technical performance of the procedure; however, patient positioning or physiology may change prior to venipuncture, rendering the mark inaccurate The dynamic approach has been shown to be superior to the static approach.5 Though most existing literature focuses on internal jugular venous access, experience with other vessel access using ultrasound guidance has been published There is limited adult patient experience reported on the placement of CVCs in the subclavian vein.8,9 Ultrasound guidance for subclavian venous access may be stymied by relative probe to patient size issues in pediatric patients In the longitudinal orientation, the transducer occupies the majority of the skin in the infraclavicular space and limits the area for target insertion The arc of the subclavian vein through the viewing plane also hampers this method A number of authors have described a supraclavicular approach to the subclavian or brachiocephalic vein.10–13 This approach uses a longitudinal transducer orientation to the vessel and can be limited by space above the clavicle in patients Ultrasound guidance for femoral vein CVC placement remains contentious Existing literature is methodologically limited and includes only small series or series that include femoral cannulation mixed with other catheterization sites.14–16 Though data on the efficacy of ultrasound in femoral cannulation is lacking, 119 A B • Fig 15.10 Sonographic approaches to vascular access (A) Transverse plane (B) Longitudinal plane operator comfort with ultrasound, similar technical performance, and similar complication concerns with the practice of internal jugular cannulation suggest meaningful benefits for ultrasound guidance in the femoral area.17–19 The literature includes few descriptions of central axillary vein CVCs,20 but this site can be useful for patients with limited options for central access When inserted in the axillary vein at a position in or distal to the axilla, the catheter takes a similar course to subclavian CVCs A longer catheter length is necessary so that it reaches from the axilla to the turn of the brachiocephalic vein into the superior vena cava (SVC) In contrast with other CVCs, the redundancy of axillary tissues complicates venipuncture, in which case ultrasound guidance seems especially beneficial Arterial and Peripheral Intravenous Access Outside of central veins, ultrasound has also been useful for peripheral intravenous (IV) access and arterial access For patients 120 S E C T I O N I I Pediatric Critical Care: Tools and Procedures with difficult peripheral access, ultrasound facilitates targeting and site selection, leading to successful IV placement.21–24 Ultrasound can identify sites difficult to detect from the surface, particularly collateral vessels that may become engorged owing to obstruction of previously cannulated veins An important consideration is vessel depth, because deep infiltrates are not as readily recognized as shallow ones, resulting in soft-tissue injury and potential complications.25 IV catheters exceeding an inch in length are useful to access deeper vessels and to maximize length of catheter within the vascular lumen Their insertion benefits from placement in veins with sufficiently long straight courses Deeper vessels require a steeper angle of insertion, which increases the risk of penetrating the back wall of the vessel Increasing interest in ultrasound guidance has seen a concomitant increase in the production of longer-length catheters by manufacturers, some of which exceed 2.25 inches at the time of this writing Following the threading of the IV catheter into the vein under ultrasound visualization may help ameliorate this effect, as the operator can keep the device off the back wall of the vessel For these reasons, IVs deeper than cm from the skin surface are still often riskier candidate vessels A number of series have been published on ultrasound guidance for peripheral arterial access Though historically the arterial pulse has been relied on for arterial catheterization, both adult26 and pediatric studies27,28 demonstrate that ultrasound guidance using methods similar to venous procedures facilitates arterial access Recent literature also suggests that ultrasound use mitigates both patient and provider characteristics associated with difficult arterial line placement, demonstrating the technology’s ability to bridge the “experience divide” between those experienced and not with landmark-based techniques.27 Ultrasound can also help identify catheter position after CVC placement Direct visualization of other CVCs in the IVC in addition to umbilical catheters is possible in patients.29–31 However, identifying upper extremity CVCs is difficult because views of the superior cavoatrial junction are obstructed by lung artifact in older children and adults.32–34 Another technique that has been demonstrated to verify CVC position is use of agitated saline.35,36 Agitated saline is produced by aspirating a small blush of blood (,0.1 mL) into 10 mL of saline and rapidly flushing it between two 10-mL syringes attached to a stopcock connected to the CVC until the saline appears homogenous At this time, the saline is steadily and promptly flushed into the patient so that the contrast is delivered while an ultrasound probe images the superior or inferior vena cava or the right heart Agitated saline opacifies vascular structures and indicates continuity of the catheter within the proximal vessel or heart Failure of the bubbles to appear in the venous space suggests that the catheter is not in the correct vessel Pitfalls in this technique include slow injection, in which contrast may not reach the heart, and the presence of shunting, which could also diminish return of bubbles to the area being examined with ultrasound Agitated saline techniques should be used with acceptance that small bubbles might travel into the systemic circulation, where there is a small risk that they could obstruct distal organ perfusion Umbilical Access Umbilical access can also be facilitated by ultrasound in neonatal patients who may be in the pediatric ICU for specialized therapies, such as advanced cardiac care or extracorporeal membrane oxygenation.29,30 Umbilical catheters can be placed using sagittal positioning of a small curvilinear or phased array transducer over the IVC as it enters the right atrium Confirmation of tip position at the IVC–right ventricular (RV) junction is possible using this method because catheters are echogenic This technique can be employed in umbilical arterial catheter placement by identifying the catheter in the descending aorta at the level of the diaphragm Drainage Procedures Ultrasound is a useful adjunct to thoracentesis and thoracostomy Pleural fluid collection is highly amenable to ultrasound interrogation Simple effusions appear as an enclosed dark anechoic space sharply differentiated from surrounding tissues and often with underlying consolidated lung appearing to float within the fluid Ultrasound is useful for characterizing effusion complexity: proteinaceous septations and debris appear echogenic within the effusion Effusions can be visualized from several locations During a pleural examination, using a linear array, effusions are recognizable as a dark anechoic space below the parietal pleura Effusion can also be seen on echocardiographic views, particularly those capturing the dependent areas of the lower thorax, such as subcostal or apical views (Fig 15.11A) Pleural effusion can also be seen in abdominal examinations of the right and left upper quadrant if the view captures the posterior aspects of the diaphragm In these cases, the effusion appears as an anechoic space cephalad to the diaphragm An advantage of imaging an effusion using a phased array or curvilinear probe is that a large portion of the effusion tends to be visible in the far field of the sonographic sector, allowing identification of a deep effusion pocket, its extent, and its complexity (Fig 15.11B) Color Doppler ultrasound may assist with effusion identification because its motion within the potential space will be detectable Pleural fluid collections can be easily targeted for drainage using ultrasound to ensure safe needle insertion above the diaphragm Though dynamic (i.e., real-time) ultrasound guidance can be used during the drainage of an effusion, the static technique (i.e., marking the location for needle insertion) simplifies the procedure without exposing the patient to increased procedural risk There is presently no pediatric evidence to suggest clear superiority of one technique over another, though an adult study demonstrated a reduction in complications (e.g., pneumothoraces) when using the dynamic technique compared with the static technique.37 Thoracostomy tubes can also be identified in the effusion space, but they are less visible once fluid is drained Prior to attachment of a thoracostomy tube to suction, visualization of the effusion space and position of the chest tube within can sometimes help in determining whether the tube travels anterior or posterior to the lung Abdominal fluid collections are also characterized as simple or complex, and drainage of the peritoneal space is possible using ultrasound guidance An important consideration in performing paracentesis is locating and avoiding the inferior epigastric arteries traveling from the external iliac arteries cephalad along the inside of the anterior abdominal wall at the lateral edge of the rectus abdominis muscle These vessels are identifiable on ultrasound and the machine can assist in planning a safe needle insertion With ultrasound, paracentesis is most easily performed from the lateral approach After appropriately preparing the patient and identifying fluid for drainage, the arteries are identified Rather than performing a traditional z-track entry, the needle can be introduced obliquely into the peritoneal cavity with the probe oriented CHAPTER 15 Ultrasonography in the Pediatric Intensive Care Unit 121 * * B A • Fig 15.11 Examples of pleural effusions seen with a phased array probe (A) Simple effusion (asterisk) (B) complex effusion (asterisk) * the literature,38,39 though some hypothesize that it is beneficial in patients who have difficult landmarks, such as older and obese patients40 and children with spine abnormalities (e.g., scoliosis) Such patients can benefit from ultrasound-guided lumbar puncture due to difficulty in determining surface landmarks and insertion depth secondary to lumbar muscularity or adipose tissue The technique can be performed under ultrasound visualization, but owing to the near-perpendicular angle of needle insertion into the skin as well as reduced ability to directly visualize the dura, subarachnoid space, and spinal cord as children age, the procedure is often best performed under static guidance using insertion markings (Fig 15.13) This process involves visualizing the spine in two planes, both sagittally along the spine to identify the appropriate entry level and axially (transversely) across it to center insertion in the midline Views of the spine can also be used for measuring angle and depth of needle insertion to reach the dural space so that an appropriate needle can be selected Diagnostic Modalities Pulmonary Ultrasound • Fig 15.12 Ultrasound-guided paracentesis Asterisk indicates the needle longitudinally over the needle for dynamic guidance (Fig 15.12) The oblique track helps reduce the chance of leakage, is easier under ultrasound than the z-track, and allows direct visualization of needle entry into the peritoneum to avoid underlying solid and hollow organ puncture A temporary catheter can then be placed using the Seldinger technique for either one-time or ongoing drainage of ascites fluid Lumbar Puncture Ultrasound-guided lumbar puncture provides a limited advantage over landmark-based techniques in children as demonstrated in Historically, pulmonary ultrasound has been considered impractical owing to air-filled alveoli impeding ultrasound transmission.41 Yet ultrasound images are produced within the thoracic cavity by the complex interaction of the ultrasound beam with interfaces between air- and water-filled structures Seminal work done by intensivists examining the lungs of critically ill adults and children led to descriptions of pulmonary artifact patterns that reflect lung pathology.42 Age can affect visualization of the thoracic cavity in the pediatric patient Infants have excellent windows for thoracic visualization because of shallow imaging depths to reach the pleural space, higher body water content, and limited thoracic cage ossification As children get older, their imaging windows may become more difficult as a result of development, including increasing ossification and decreasing body water content, though imaging windows remain largely accessible If areas of the pleura are difficult to visualize owing to ribs, rotating the probe parallel to the ribs within the intercostal space can improve the view Lung motion is still appreciated as artifacts across the pleural plane 122 S E C T I O N I I Pediatric Critical Care: Tools and Procedures * * A * B • Fig 15.13 Views of the lumbar spine (A) Longitudinal view (B) Transverse view Asterisks indicate spinous processes Alternatively, a microconvex or phased array transducer can be used to see the pleural line, though these lower-frequency probes often perform better in evaluating deep structures Thoracic ultrasound of the pleural space identifies overlying skin and soft tissue nearest to the probe, followed by deeper rib and intercostal structures, and then the pleural space The visceral pleura slides against the parietal pleura through a thin and often invisible layer of normal pleural fluid Though the exact physiologic mechanisms for ultrasound lung artifacts is often unclear, their similarity to the artifacts associated with pathology in other organ systems suggest their origins Reverberation artifacts that replicate the pleural line at regular intervals from proximal to distal field are called A-lines These can appear regardless of pathology or whether tissue or air is underlying the parietal pleura; therefore, they need only be recognized as artifacts in the image B-lines and Z-lines are generated radially from interfaces near the pleural line and appear to pierce through the lung parenchyma B-lines reach the end of the viewable sector and obliterate A-lines, whereas Z-lines are limited reverberatory extensions from the pleural line (Fig 15.14) • Fig 15.14 Ultrasound of the thorax using a linear probe 1, A-line 2, B-line A pattern of increased B-lines suggests increased interstitial lung water In the adult population, this is consistent with pulmonary edema.43 It is reasonable to infer that it would bear similar implication in children However, studies also suggest that B-lines indicate the presence of neonatal respiratory distress syndrome or transient tachypnea of the newborn in the neonatal intensive care setting as well as bronchiolitis in children in the emergency department (ED) setting.44–46 Further elucidation of the significance of B-line patterns in critically ill children is lacking B-lines in children may also represent lung consolidation Consolidated lung appears increasingly granular with the appearance of liver-like parenchyma, commonly referred to as hepatization owing to the augmented visibility of consolidated lung parenchyma and vascular architecture appearing similar to hepatic structures In the case of infectious pneumonia, diffuse geographic areas of consolidation, air, and necrosis may develop, resulting in what some authors term the shred sign The diagnostic value of these lung consolidation patterns in critically ill children has not been well quantified to date Identification of pneumothorax with ultrasound has been described as a dissociation of the visceral from parietal pleura by ultrasound-obstructing air, such that the parietal pleura is no longer visible (Fig 15.15).43 Loss of pleural sliding is indicative of pneumothorax; identification of the limit of a pneumothorax as the point where pleural sliding abuts an area where it is absent is described as the lung point Identification of the lung point is highly specific for pneumothorax.47 M-mode ultrasound is also useful for identification of pneumothorax Placement of the Mmode cursor through the pleural surface characterizes movement of the lung parenchyma as a granular-textured echogenic area distal to the pleural line In contrast with the more horizontal linear pattern of the proximal chest wall, this assumes a seashorelike appearance (Fig 15.15B) When the M-mode cursor overlays pneumothorax, no lung parenchyma movement is seen, and the entire M-mode tracing over time appears to be a pattern of horizontal lines, otherwise described as a barcode or stratosphere sign (Fig 15.15D) Bedside ultrasound used to detect pneumothorax among high-risk neonates has been shown to be highly accurate and timely.48 Ultrasound views of diaphragm movement can confirm tracheal placement of an endotracheal tube (ETT) during intubation This is potentially useful in the management of a ventilated patient requiring rapid confirmation of tracheal placement, such ... confounds differentiation between vein and artery Though this finding is most pronounced in patients with a primary diagnosis of severe idiopathic pulmonary hypertension, it can also be seen in patients... through the viewing plane also hampers this method A number of authors have described a supraclavicular approach to the subclavian or brachiocephalic vein.10–13 This approach uses a longitudinal transducer... which exceed 2.25 inches at the time of this writing Following the threading of the IV catheter into the vein under ultrasound visualization may help ameliorate this effect, as the operator can keep