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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[.]

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 4 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 2 o’clock and 3 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

inter-rogation 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

feasi-ble and associated with good outcome, complementing availafeasi-ble

adult data describing the utility of intensivist-driven cardiac

ultra-sound 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

identifica-tion 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 re-sponsiveness Dynamic measures are separated in time and rela-tive 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 out-flow 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 2 to 3 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 physi-ologic conditions: (1) the patient who is pharmacphysi-ologically para-lyzed 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 neuromuscu-larly 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 demon-strated that increasing airway pressure decreases respiratory varia-tion 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 com-pared 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 evalu-ation 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 ven-tricular 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 be-tween 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 re-sponsiveness in intubated pediatric ICU patients.90 , 91 In assessing

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

respira-tory cycle Variability of 14% is thought to indicate volume

respon-siveness Pitfalls of this technique include limited apical views

ow-ing 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

ap-propriate 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

ef-fusion 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,

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 func-tion 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 in-clude collapse of the low-pressure right heart chambers during filling periods in the cardiac cycle, namely, late diastole/early sys-tole in the right atrium and early diassys-tole in the right ventricle.92

Respiratory variation in the left ventricular inflow is also associ-ated with tamponade physiology, as demonstrassoci-ated 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 pericardiocen-tesis 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 applicabil-ity in the pediatric setting remains undetermined

Left Ventricular Function

Although the best metric for characterizing cardiac contractility remains elusive, acute care practitioners can estimate cardiac func-tion 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

•  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.

*

•  Fig. 15.21  ​Pericardial​effusion.​Asterisk​indicates​effusion.

V

24cm

*

(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

para-sternal short-axis view at the midchamber (papillary muscle) level

Area-based measurements of LV systolic function are also

use-ful 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

inaccu-rate 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 to-ward 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 excur-sion 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 6 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 ap-paratus 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 biven-tricular function Normal values have been published for pediat-ric populations.100

•  Fig. 15.22  ​E-point​ septal​ separation​ (EPSS).​ (A)​ Cursor​ alignment​ in​ two-dimensional​ imaging​ for​ (B)​

M-mode​measurement​of​EPSS.

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.

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

How-ever, 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

expla-nation 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)

Vol-ume overload is typically characterized by septal deviation

occur-ring 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

imag-ing 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

ma-chines are easily deployed to the patient’s bedside to augment

ongo-ing resuscitation efforts Ultrasound use in cardiac arrest requires

particular attention to patient and provider Ultrasound gel con-ducts electricity and must be wiped from the skin before defibrilla-tion The gel makes contact surfaces slippery for clinicians provid-ing chest compressions and may dislodge pads and monitorprovid-ing Echocardiography should be performed briefly during pulse checks

to minimize interruptions in chest compressions Primary use of subcostal views minimizes interference with compressors and defi-brillator pads on the chest Apical views may also be possible but are often more difficult to acquire, which may delay identifying revers-ible 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 exer-cised 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 poten-tial 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 transesoph-ageal 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 car-diac 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 rea-sons, 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

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 oph-thalmic 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 3 mm behind the vitreous humor and retina Careful measurement of the maxi-mum sheath diameter is important to decrease the risk of underes-timating 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

•  Fig. 15.24  ​Chronic​severe​right​ventricular​failure.​Note​thickened​right​

ventricular​ wall​ (1)​ and​ interventricular​ septum​ that​ bows​ into​ the​ left​

ventricle​(2).

2 1

child abuse cases and other traumatic and infectious pathologies

involving the eye.114 The diagnostic utility of this application

re-mains 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

assess-ment 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 1 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

in-terest From the peak systolic (SBF) and diastolic flow (DBF), the

resistive index (RI) can be calculated as RI 5 (SBF – DBF)/SBF

or the pulsatility index as PI 5 (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

So-ciety of Critical Care Medicine (SCCM) Ultrasound Certification

Task Force subsequently published suggestions116 for curricular

development and programmatic infrastructure based on

previ-ously 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

infre-quently discuss pediatric considerations This has allowed

pediat-ric critical care providers the opportunity to design local curpediat-ricula

based on applications to guide treatment in commonly

encoun-tered 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

man-agement.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

infra-structure—institutional credentialing processes, consistent

docu-mentation practices, secured image storage, and quality assurance

systems—which are frequently underdeveloped across PICUs

These are essential infrastructure elements for successful

ultra-sound 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 Implemen-tation of critical care ultrasound will have a powerful impact in clinical care: better procedural safety, timely and accurate under-standing of patient physiology, and, ultimately, better patient outcomes More research is needed to determine the role of ultra-sound in the ICU, but emerging indications for procedure and diagnosis and more incorporation into daily clinical care are likely

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