128 SECTION II Pediatric Critical Care Tools and Procedures level is useful for the pediatric intensivist to assess qualitative left ventricular function, as this location is where radial contractilit[.]
128 S E C T I O N I I Pediatric Critical Care: Tools and Procedures level is useful for the pediatric intensivist to assess qualitative left ventricular function, as this location is where radial contractility is most pronounced In this view, the beam transects the ventricle such that the papillary muscles are the primary intraventricular structures visible, with the inferoseptal papillary muscle appearing in the lower left and the anterolateral papillary muscle appearing in the lower right of the image A short-axis view of the heart can also be performed at the aortic valve level In this view the aortic leaflets are visible and roughly form a lambda sign in the middle of the appropriately centered image (Fig 15.19E) With careful targeting, the left or right coronary root is sometimes visible in this view On the screen, the left atrium is directly posterior to the aortic valve (6 o’clock) and moving clockwise is the right atrium (8 o’clock), tricuspid valve (9 o’clock), right ventricle (12 o’clock), and pulmonary valve (2 o’clock) with the pulmonary artery and its bifurcation sometimes visualized at the o’clock position Apical views (Fig 15.19F) are highly dependent on thoracic and abdominal structures Increased intrathoracic pressure, as in asthma or high airway pressure ventilation, can cause the apex of the heart to move caudad and medially Hyperinflated lung may obscure the apex Conversely, high intraabdominal pressure can push the diaphragm cephalad and displace the heart cephalad and laterally Apical views are insonated near the point of maximal impulse at the caudad aspect of the left pectoralis major muscle The axis of the probe should be aligned with the major axis of the heart and point toward the center of the mediastinum For the apical four-chamber view, the indicator is oriented toward the left flank, usually between the o’clock and o’clock positions From this view the four chambers and both atrioventricular valves of the heart should be visible In addition to the four chambers, this view allows visualization of both atrioventricular valves and their movement The left heart appears on the right of the screen and atria appear at the bottom when the probe face appears at the top of the screen From the four-chamber view the transducer can be fanned anteriorly so that the beam intercepts the LVOT for interrogation of outflow velocity This is called the apical five-chamber view From the four-chamber view, the probe can be rotated 60 degrees counterclockwise to obtain a two-chamber view of the left heart (left ventricle and atrium) for assessment of function Cardiac ultrasound can provide assistance in titrating fluid, inotropes, and vasopressors for persistent shock in children.79,80 Initial pediatric experience has demonstrated that its use is feasible and associated with good outcome, complementing available adult data describing the utility of intensivist-driven cardiac ultrasound for hemodynamic assessment It is reasonable to surmise that the value of focused cardiac ultrasound noted in adults81 might be similar or better in children, as views of the heart may be better given pediatric body habitus Most literature related to cardiac ultrasound for hemodynamic assessment in children has focused on volume status assessment Adult echocardiographers and pediatric nephrologists have used the collapsibility and distensibility of the compliant IVC as a noninvasive surrogate for volume status and estimation of dry weight for dialysis patients, respectively.82,83 Critical care physicians are most often interested in identification of patients who are volume responsive Volume responsiveness means that a fluid challenge will result in a subsequent increase in stroke volume Within literature, volume responsiveness is typically described as an increase in stroke volume by greater than 10% to 15% in response to a 10 to 20 mL/kg fluid bolus Surrogate markers for fluid responsiveness can be categorized as static or dynamic parameters Static parameters are point-in-time measurements—such as heart rate, central venous pressure (CVP), and blood pressure—and poorly correlate with fluid responsiveness Dynamic measures are separated in time and relative to a physiologic perturbation Most commonly, they are measures that are taken in response to changes in intrathoracic pressure during respiration Respiratory variation of aortic outflow velocity appears to be a promising measure indicating fluid responsiveness in critically ill pediatric populations.84 The diameter of the IVC is usually assessed in the sagittal view about to cm distal to the inferior cavoatrial junction in adults At this point the vessel diameter can be measured using M-mode or two-dimensional (2D) methods Available evidence regarding IVC assessment for volume status is largely related to two physiologic conditions: (1) the patient who is pharmacologically paralyzed and intubated receiving a set tidal volume at near normal airway pressure and (2) the spontaneously breathing patient IVC respiratory variation exceeding 12% to 18% in the neuromuscularly blocked adult patient85,86 has been described as a threshold for volume responsiveness Current convention in the adult is the use of maximum diameter observing for changes greater than 12% Similar criteria have not yet been established in children, and evaluating changes in caliber of smaller pediatric vessels may be prone to more measurement error It has also been demonstrated that increasing airway pressure decreases respiratory variation and can mask hypovolemia In the pediatric operating room, titration of positive-pressure ventilation as well as initiation of inhalational anesthesia changes the collapsibility of the IVC.87 Evidence indicates that IVC behavior is not equivalent between patients receiving sedation and positive-pressure ventilation compared with those who are spontaneously breathing Using a ratio of the size of the IVC to aorta (IVC:Ao) diameter in a transverse view may provide a useful assessment of volume status in children and avoids the challenge that results from the range of sizes in children An IVC:Ao ratio of 0.8 or less appears to suggest clinically significant dehydration, whereas a ratio greater than 1.2 suggests hypervolemia.82 Fluid responsiveness was not described; this study targeted diagnosis rather than evaluation of physiologic response to therapy However, in critically ill children, limited data suggest that IVC:Ao and IVC collapsibility are poor surrogates for CVP.88 In addition, despite its use in septic shock algorithms as a surrogate marker for intravascular volume depletion, CVP measurements have come under considerable scrutiny given their poor accuracy.89 Assessment of left heart performance as a surrogate for volume status has also been an area of interest in adults and children Velocity of blood flow across LVOT has been used as a surrogate of stroke volume and indicator of intravascular volume status Assessment of the LVOT using Doppler ultrasound is performed from the apical five-chamber view observing velocity of left ventricular ejection (Fig 15.20) Stroke volume can be approximated by obtaining the Doppler velocity tracing of flow across the LVOT over time, calculating its integral, velocity time integral, and multiplying this by the measured cross-sectional area of the LVOT The cross-section of the LVOT is usually measured from the parasternal long-axis view using the diameter measured between the aortic valve leaflets in midsystole Using the estimated stroke volume and multiplying by the heart rate allows approximation of cardiac output The LVOT Doppler tracing can also provide a beat-to-beat assessment of changes in stroke volume through the respiratory cycle Respiratory changes in peak LVOT velocity exceeding 14% appear to identify volume responsiveness in intubated pediatric ICU patients.90,91 In assessing CHAPTER 15 Ultrasonography in the Pediatric Intensive Care Unit 129 * A B • Fig 15.20 Doppler interrogation of the left ventricular outflow tract (LVOT) (A) Apical five-chamber view with LVOT identified (asterisk) (B) Pulsed-wave Doppler of the LVOT variability in the Doppler flow tracing, it is advisable to slow the sweep speed of the machine so that multiple cardiac cycles—in particular, aortic flow in systole—are observed through the respiratory cycle Variability of 14% is thought to indicate volume responsiveness Pitfalls of this technique include limited apical views owing to lung inflation and patient habitus Further concerns particular to pediatric patients include the need for a smaller phased array transducer to maintain skin contact and small LVOT for appropriate placement of the Doppler cursor, made more challenging in the dehydrated child with a hyperkinetic heart Effusion in the pericardial space appears similar to pleural effusion as a largely dark and anechoic space separating the heart from the reflective pericardium (Fig 15.21) Effusion may not be concentric and instead collect in dependent areas; therefore, V subcostal windows are often ideal for visualizing them Effusions can also be complex as a result of accumulated protein in an empyema, solidification of hemorrhage, or presence of tumor Tamponade is a clinical diagnosis; however, the presence of an effusion and subsequent changes to cardiac morphology and function suggestive of tamponade physiology are detectable using ultrasound The most specific indicator of tamponade is IVC engorgement secondary to impeded venous return to the heart Other ultrasound findings consistent with tamponade may include collapse of the low-pressure right heart chambers during filling periods in the cardiac cycle, namely, late diastole/early systole in the right atrium and early diastole in the right ventricle.92 Respiratory variation in the left ventricular inflow is also associated with tamponade physiology, as demonstrated by noticeably reduced mitral inflow during inspiration Conversely, inflow across the tricuspid valve may increase during inspiration Pericardiocentesis Imaging at the time of pericardiocentesis often facilitates effusion drainage Use of a phased array probe in the apical position may not allow visualization of the needle but permits monitoring effective drainage of the effusion and wire placement for drain placement Injecting a small quantity of agitated saline into the pericardiocentesis needle for sonographic contrast can help confirm that a needle is in the pericardial space rather than a vascular space, particularly if an effusion is bloody.93 Dynamic visualization techniques have been described for pericardiocentesis using a linear array transducer placed in the subxiphoid area in children94 or the left parasternal area in adults,95 with the indicator pointed cephalad These have been limited series; whether these techniques have broad applicability in the pediatric setting remains undetermined * 24cm • Fig 15.21 Pericardial effusion Asterisk indicates effusion Left Ventricular Function Although the best metric for characterizing cardiac contractility remains elusive, acute care practitioners can estimate cardiac function using qualitative and quantitative markers with reasonable accuracy when compared with echocardiography specialists.96,97 The motion of the left ventricle (LV) assessed through the cardiac cycle is useful for this assessment Visualizing the LV across the center of the chamber in multiple views, a sonographer can visually approximate or directly measure the excursion of the ventricular walls These values are then compared with known standards 130 S E C T I O N I I Pediatric Critical Care: Tools and Procedures B A • Fig 15.22 E-point septal separation (EPSS) (A) Cursor alignment in two-dimensional imaging for (B) M-mode measurement of EPSS (e.g., a normal left ventricular fractional shortening [FS] measures between 25% to 45% of the LV end-diastolic diameter) This measurement is optimally performed quantitatively in the parasternal short-axis view at the midchamber (papillary muscle) level Area-based measurements of LV systolic function are also useful These include changes in the cross-section of the LV seen in the short-axis or apical views (ejection fraction [EF]) by Simpson’s method of discs Area-based calculations are advantageous to single-dimension assessments such as FS because they reduce the effects of regional wall motion abnormalities on accuracy of EF measurement These calculations are prone to error from inaccurate inclusion of intracavitary structures such as the papillary muscles and trabeculae as well as various artifacts Apical views are also challenging in a child without appropriately sized transducers or if the lungs are hyperinflated, which can compromise accurate assessment of the LVEF Substantial mentored practice is strongly recommended for skill development and accurate results M-mode modalities, such as E-point septal separation (EPSS) and MV annulus plane of systolic excursion (MAPSE) can also approximate systolic function EPSS assesses the relative motion of the MV anterior leaflet during diastole (Fig 15.22) In the parasternal long-axis view, at early diastole the leaflet is readily visualized as being more proximal to the probe and moving toward the septum both in the early passive phase of ventricular filling and late phase with atrial contraction In the failing heart, end-systolic volume increases and diminishes the gradient across the MV for flow into the LV during diastole Therefore the excursion of the anterior MV leaflet is less pronounced and does not come as close to the septum The distance between the leaflet and septum is measured in the parasternal long-axis view by placing the M-mode cursor across the MV tips of the leaflets In the adult a normal EPSS is less than mm and an abnormal one exceeds 10 mm Normal values have been published in pediatric populations.98,99 MAPSE is assessed from the apical view of the MV by placing the M-mode cursor across the lateral end of the MV annulus (Fig 15.23) In M-mode the vertical excursion of the MV apparatus is quantified as an approximation of the longitudinal contraction of the heart The septal end of the MV can also be measured, though this tends to be a better indicator of biventricular function Normal values have been published for pediatric populations.100 * * A B • Fig 15.23 Mitral annulus plane of systolic excursion (MAPSE) (A) Cursor alignment in two-dimensional imaging for (B) M-mode measurement of MAPSE Vertical excursion of the lateral side of the mitral valve annulus (asterisk) is measured to determine MAPSE CHAPTER 15 Ultrasonography in the Pediatric Intensive Care Unit Right Ventricular Function In the pediatric ICU, assessment of the RV can provide valuable information about the effects of pulmonary vascular resistance changes and mechanical ventilation on cardiac performance However, assessment of the RV can be difficult because of its position closer to the sternum and its triangular shape straddling the LV and making characterization of its movement through the cardiac cycle difficult Subtle signs of RV dysfunction can be seen with dilation and pulsatility in the IVC, although some pulsatility can be normal in both the IVC and subclavian veins From the apical position or parasternal right ventricular inflow or parasternal short-axis view at the aortic valve level, identification of a tricuspid valve regurgitant jet can potentially be used to evaluate RV systolic pressures using continuous-wave Doppler A more complete explanation of methods for measuring tricuspid regurgitation can be found in dedicated comprehensive echocardiography texts.101 Other clues to RV dysfunction include leftward interventricular septal deviation Septal deviation can be visualized in multiple views; however, it is most prominent and appropriately evaluated in the parasternal short-axis view at the mid-chamber (papillary muscle) level Septal position can reveal RV volume overload from pressure overload as a cause of RV dysfunction (Fig 15.24) Volume overload is typically characterized by septal deviation occurring in diastole but not systole, resulting in the LV assuming a predominantly circular conformation during systole It is easy to make the septum look falsely flat by not having the plane of imaging perpendicular to the axis of the ventricle Therefore, if a flat septum is seen, an effort should be made to sweep through the heart, sliding from base to apex, to ensure that the potential for a false-positive finding is limited In the progression of RV failure to pressure overload of the ventricle, septal deviation is seen through the entire cardiac cycle Cardiac Arrest During cardiac arrest, ultrasound may help to identify reversible causes, including critical hypovolemia and pericardial effusion with tamponade physiology.102 In the modern PICU, ultrasound machines are easily deployed to the patient’s bedside to augment ongoing resuscitation efforts Ultrasound use in cardiac arrest requires 131 particular attention to patient and provider Ultrasound gel conducts electricity and must be wiped from the skin before defibrillation The gel makes contact surfaces slippery for clinicians providing chest compressions and may dislodge pads and monitoring Echocardiography should be performed briefly during pulse checks to minimize interruptions in chest compressions Primary use of subcostal views minimizes interference with compressors and defibrillator pads on the chest Apical views may also be possible but are often more difficult to acquire, which may delay identifying reversible causes of arrest The sonographer should not be the code team leader If the code leader is the only provider with ultrasound skills, temporarily transfer the code leader role to focus on the ultrasound study While there are studies that demonstrate feasibility and value of ultrasound during resuscitative efforts,103 several studies have shown that the use of ultrasound may be associated with longer interruptions in chest compressions.104,105 Caution should be exercised so that ultrasound does not negatively impact CPR performance during cardiac arrest Best practice is to obtain the cardiac views during planned pulse checks or chest compression provider switches.106 Adult emergency medicine providers are exploring the use of transesophageal ultrasound during resuscitation The potential advantages of this approach are no need for chest compression interruptions, capability to visualize LV outflow obstruction to optimize the hand position of chest compression providers, and ability to monitor quality of compressions performed.107 These benefits are balanced with known risks associated with transesophageal ultrasound and material costs for the technology Absent cardiac contractility, termed cardiac standstill, has been described as highly indicative of unlikely return of spontaneous circulation103,108,109 in adults In contrast, in the pediatric setting, recovery of cardiac function after standstill has been described.110 In a study of providers polled on interpretation of potential cardiac standstill videos, the providers demonstrated only moderate agreement in correctly identifying cardiac standstill.111 This can be practically even more difficult when only a few or even just one limited 2D view is obtained during resuscitation For these reasons, currently there is a lack of supportive evidence for using ultrasound to primarily decide the termination of resuscitation efforts in pediatric practice While bedside ultrasound is a robust implement for identifying reversible causes, caution should be exercised regarding how ultrasound result should be used to guide resuscitative efforts Neurosonology • Fig 15.24 Chronic severe right ventricular failure Note thickened right ventricular wall (1) and interventricular septum that bows into the left ventricle (2) Neurosonology encompasses sonography of the central nervous system, peripheral nervous systems, and cerebral circulation.112 In the PICU, neurosonology use has most commonly been described in evaluation of cerebral blood flow More recently, there has been interest in ultrasound of the ocular orbit and optic nerve sheath Distention of the optic nerve sheath demonstrated via direct ophthalmic ultrasound can potentially provide evidence of increased intracranial pressure.113 The eye is imaged from the front over the closed eyelid to visualize the sheath approximately mm behind the vitreous humor and retina Careful measurement of the maximum sheath diameter is important to decrease the risk of underestimating pressure Copious gel, preferably made for ophthalmic use, is advised to reduce eye irritation There are inconsistent data about normal or abnormal values for children of different ages or different intracranial disorders A similar view for the posterior retinal space has also been described for retinal hemorrhages in 132 S E C T I O N I I Pediatric Critical Care: Tools and Procedures child abuse cases and other traumatic and infectious pathologies involving the eye.114 The diagnostic utility of this application remains under investigation Use of transcranial Doppler for assessing cerebral blood flow has become common for monitoring vasospasm in patients with subarachnoid hemorrhage and other disorders encountered in the adult neurocritical care unit There is considerable interest in its application in pediatric neurocritical care as a noninvasive assessment tool for cerebral perfusion Insonating the middle cerebral artery from the temporal window lateral to either eye requires a low-frequency transducer that can operate near MHz It is also possible to insonate the anterior cerebral artery in patients with an open anterior fontanelle, and the vertebrobasilar system can be insonated from the foramen magnum with some occipital support and care not to disrupt a critical airway or cervical spine Color Doppler is used to image the cranial vessels; pulsed-wave Doppler can subsequently be used to identify velocities in the vessel of interest From the peak systolic (SBF) and diastolic flow (DBF), the resistive index (RI) can be calculated as RI (SBF – DBF)/SBF or the pulsatility index as PI (SBF – DBF)/(time-averaged mean velocity) An increasing RI or PI is suggestive of vascular constriction and a greater difference between systolic and diastolic flow Translation to Practice In 2011 an international consensus document on training in critical care ultrasound was endorsed and published by 13 critical care societies.115 These societies agreed that general critical care ultrasound, including basic-level echocardiography, should be a required component of training for critical care trainees The Society of Critical Care Medicine (SCCM) Ultrasound Certification Task Force subsequently published suggestions116 for curricular development and programmatic infrastructure based on previously published and successfully implemented guidelines from the American College of Emergency Physicians.117 Guidelines for ultrasound applications in critical care practice have also been published by the SCCM.118,119 Societal guidelines regarding critical care applications infrequently discuss pediatric considerations This has allowed pediatric critical care providers the opportunity to design local curricula based on applications to guide treatment in commonly encountered clinical scenarios amenable to ultrasound interrogation Within pediatric critical care, translating ultrasound education to the care of critically ill children has proven beneficial in the clinical setting, bringing meaningful findings that change management.120 Recent studies based on surveys suggest that most academic pediatric critical care departments in the United States have an ultrasound machine, and many providers have access to ultrasound education and are performing both diagnostic and procedural studies.121,122 These studies suggest standardized infrastructure—institutional credentialing processes, consistent documentation practices, secured image storage, and quality assurance systems—which are frequently underdeveloped across PICUs These are essential infrastructure elements for successful ultrasound practice implementation in PICUs Conclusion Ultrasound use is increasingly common in pediatric critical care medicine for procedural guidance and real-time clinical assessment at the bedside Optimal ultrasound use in the PICU requires an infrastructure for provider education and credentialing process, quality assurance, documentation, and image storage Implementation of critical care ultrasound will have a powerful impact in clinical care: better procedural safety, timely and accurate understanding of patient physiology, and, ultimately, better patient outcomes More research is needed to determine the role of ultrasound in the ICU, but emerging indications for procedure and diagnosis and more incorporation into daily clinical care are likely Key References Aouad MT, Kanazi GE, Abdallah FW, et al Femoral vein cannulation performed by residents: a comparison between ultrasound-guided and landmark technique in infants and children undergoing cardiac surgery Anesth Analg 2010;111:724-728 Barbier C, Loubi res Y, Schmit C, et al Respiratory changes in inferior vena cava diameter are helpful in predicting fluid responsiveness in ventilated septic patients Intensive Care Med 2004;30 (9):1740-1746 Bouferrache K, Amiel J-B, Chimot L, et al Initial resuscitation guided by the Surviving Sepsis Campaign recommendations and early echocardiographic assessment of hemodynamics in intensive care unit septic patients Crit Care Med 2012;40:2821-2827 Brass P, Hellmich M, Kolodziej L, et al Ultrasound guidance versus anatomical landmarks for internal jugular vein catheterization Cochrane Database Syst Rev 2015;(1):CD006962 Brass P, Hellmich M, Kolodziej L, et al Ultrasound guidance versus anatomical landmarks for subclavian or femoral vein catheterization Cochrane Database Syst Rev 2015;(1):CD011447 Breitkreutz R, Price S, Steiger HV, et al Focused echocardiographic evaluation in life support and peri-resuscitation of emergency patients: a prospective trial Resuscitation 2010;81:1527-1533 Byon HJ, Lim CW, Lee JH, et al Prediction of fluid responsiveness in mechanically ventilated children undergoing neurosurgery Br J Anaesth 2013;110:586-591 Czarnik T, Gawda R, Perkowski T, et al Supraclavicular approach is an easy and safe method of subclavian vein catheterization even in mechanically ventilated patients: analysis of 370 attempts Anesthesiology 2009;111:334-339 Dennington D, Vali P, Finer NN, et al Ultrasound confirmation of endotracheal tube position in neonates Neonatology 2012;102:185-189 Durand P, Chevret L, Essouri S, et al Respiratory variations in aortic blood flow predict fluid responsiveness in ventilated children Intensive Care Med 2008;34:888-894 Epelman M, Navarro OM, Daneman A, et al M-mode sonography of diaphragmatic motion: description of technique and experience in 278 pediatric patients Pediatr Radiol 2005;35:661-667 Faingold R, Daneman A, Tomlinson G, et al Necrotizing enterocolitis: assessment of bowel viability with color Doppler US1 Radiology 2005;235:587-594 Feissel M, Michard FDR, Faller JP, et al The respiratory variation in inferior vena cava diameter as a guide to fluid therapy Intensive Care Med 2004;30(9):1834-1837 Fields JM, Dean AJ, Todman RW, et al The effect of vessel depth, diameter, and location on ultrasound-guided peripheral intravenous catheter longevity Am J Emerg Med 2012;30(7):1134-1140 Fox JC, Boysen M, Gharahbaghian L, et al Test characteristics of focused assessment of sonography for trauma for clinically significant abdominal free fluid in pediatric blunt abdominal trauma Acad Emerg Med 2011;18:477-482 Froehlich CD, Rigby MR, Rosenberg ES, et al Ultrasound-guided central venous catheter placement decreases complications and decreases placement attempts compared with the landmark technique in patients in a pediatric intensive care unit Crit Care Med 2009;37:1090-1096 Galicinao J, Bush AJ, Godambe SA Use of bedside ultrasonography for endotracheal tube placement in pediatric patients: a feasibility study Pediatrics 2007;120:1297-1303 ... the respiratory cycle Variability of 14% is thought to indicate volume responsiveness Pitfalls of this technique include limited apical views owing to lung inflation and patient habitus Further... specialists.96,97 The motion of the left ventricle (LV) assessed through the cardiac cycle is useful for this assessment Visualizing the LV across the center of the chamber in multiple views, a sonographer... ventricular fractional shortening [FS] measures between 25% to 45% of the LV end-diastolic diameter) This measurement is optimally performed quantitatively in the parasternal short-axis view at the