Ultrasound in the neonatal and pediatric intensive care unit

Overview

Children and neonates are ideal candidates for the implementation of the holistic approach (HOLA) critical care ultrasound concept (see Chapter 1 ). This is mainly due to their small body size, which permits the fast application of all possible ultrasound techniques from head to toe. However, in the individual pediatric patient, only a part of the available ultrasound techniques will be clinically indicated, and many sites of generic scanning will need to be omitted. Ultrasound is safe in imaging pediatric patients if used when clinically indicated and with the minimally necessary energy exposures, based on the ALARA ( a s l ow a s r easonably a chievable) principle. In the pediatric intensive care unit (PICU), HOLA ultrasound is easily scaled down to specific application profiles; some of those require expert input to interpret findings that should be processed under the light of clinical judgment or require special expertise. Elective and repetitive ultrasound scanning is often necessary for diagnostic and monitoring purposes, whereas goal-directed therapies can be guided by means of ultrasonography. ^, This chapter briefly outlines the role of ultrasound in the neonatal intensive care unit (NICU) and PICU.

Hemodynamic monitoring

Ultrasound is an essential tool in the evaluation of nontraumatic, symptomatic, undifferentiated hypotension in NICU and PICU patients. It is a useful adjunct to the physical examination, which may be inconclusive, and aids in bedside hemodynamic monitoring and management of pediatric patients. Cardiovascular morphology and function as well as volume status are basic parameters in the hemodynamic monitoring of both adults and children (see Chapter 36 ). ^, Bedside ultrasound can facilitate the assessment of the above-mentioned parameters and guide therapeutic interventions (e.g., administration of fluids or vasoactive agents).

Echocardiography is an essential tool of pediatric hemodynamic monitoring. The assessment of left ventricular (LV) global systolic function is mainly performed by means of LV fraction shortening (FS) and ejection fraction (EF), either by global visual assessment or objective numerical measurement. In pediatric patients, global eye assessment of LVEF is usually an easily applied and reproducible method. When the LV empties more than half of its volume, the EF is considered to be normal (EF > 55%). Mild (EF = 45%-55%), moderate (EF = 30%-44%), and severe (EF < 30%) impairment can be estimated accordingly. Color Doppler imaging and tissue Doppler indexes of transmitral flow, reflecting LV end-diastolic pressure (and thus left atrial pressure), can be also applied in the evaluation of cardiac function as analyzed in the echocardiographic section of this book. Echocardiography can be used in almost all emergency situations, including cardiac arrests.

Vena cava and global ventricular volume analysis, along with lung ultrasound, can be used in the evaluation of volume status in pediatric patients. By implementing the HOLA ultrasound concept in hemodynamic monitoring (see Chapter 36 ) and using echocardiography as a core tool, a simple four-step algorithm can be applied in the evaluation of pediatric patients in shock states.

• 1.

  Gross ruling out of preexisting cardiac disease (dilatation or hypertrophy of the left and right ventricle or valvular abnormalities) or genetic cardiovascular abnormalities (e.g., ventricular septal defect, aortic coarctation, right-sided aneurysmal aorta).

• 2.

  Vena cava analysis. A small inferior vena cava (IVC) with spontaneous collapse suggests hypovolemia (e.g., inspiratory collapse > 50% in spontaneous ventilation). Hypovolemia can be considered as “absolute” (hemorrhage) or “relative” (e.g., sepsis, anaphylaxis, third spacing). A distended and fixed IVC may suggest pulmonary hypertension or tamponade when associated with pertinent right ventricular (RV) echocardiographic signs (see step 3). The presence of an A-line profile in lung ultrasound usually rules out pulmonary edema at this stage. Cautious volume loading therapy can therefore be attempted.

• 3.

  Evaluation of RV function. In the presence of a fixed and distended IVC, a small and hyperkinetic right ventricle is suggestive of tamponade (especially if combined with the rapid accumulation of pericardial fluid), whereas a dilatated and hypokinetic right ventricle may signify pulmonary hypertension. The assessment of RV volume is rather difficult by usual two-dimensional echocardiography. However, RV end-diastolic area (EDA) can be easily assessed. Thus we should underline the effect of positive end-expiratory pressure (PEEP) in the hemodynamic equation. With increasing PEEP, RV EDA progressively increases, and thus this phenomenon should be co-evaluated when assessing RV function in a shocked mechanically ventilated pediatric patient ( Figure 47-1 ). Additional echocardiographic signs suggestive of tamponade are right atrial inversion/collapse in late systole, RV inversion/collapse in early diastole, swinging heart (clockwise rotation), and presence of fluids or clots around the heart. Additional Doppler echocardiographic signs suggestive of pulmonary hypertension are analyzed in detail in Chapter 33 . At this stage, other causes of obstructive shock, such as a pneumothorax, can be ruled out by lung ultrasound (see Chapter 19 , Chapter 20 , Chapter 21 ).

  

• 4.

  Assessment of LV function. A hypokinetic LV and a B-line profile depicted by lung ultrasound in a hypotensive pediatric patient with low blood oxygen saturation are suggestive of cardiogenic pulmonary edema, indicating thus the administration of diuretics and inotropes. Of note, the typical appearance of adult lung ultrasound signs, such as A-lines and B-lines may differ in pediatric patients, especially in neonates. In general, fluid administration in such cases should be done extremely cautiously.

The implementation of the above-mentioned four-step approach could guide therapy (vasoactive agents vs. volume loading) and monitor its results (e.g., improvement of LV function) in pediatric shock states.

Lung-pleural ultrasound and mechanical ventilation

Lung ultrasound pediatric applications are similar when compared with the applications of the method in adult patients. Hence diagnosis and management of lung disorders, especially lung interstitial syndrome, pneumothorax, lung consolidation, and pleural effusions, by means of lung ultrasound are routinely performed in the NICU and PICU. Regular follow-up lung ultrasound examinations facilitate the early detection of lung and pleural disorders and enhance their therapeutic management. When using lung ultrasound as a routine diagnostic and monitoring tool, children and neonates are largely spared the detrimental ionizing effect of repetitive chest radiography or computed tomography scans. Such ultrasonography also may be used in periendotracheal intubation procedures.

Sometimes, lung ultrasonographic findings may be difficult to interpret in the NICU because their appearance is slightly different compared with the usual one known in adults. For example, the hyperechoic B-lines in neonates and infants may be confluent (not comet tail–like), which makes counting them impossible. A global assessment of the percentage of white (hyperechoic)/black (hypoechoic) areas in the ultrasound screen divided by 10 usually gives a correspondent number of B-lines. This may aid in guiding diuretic or positive-pressure ventilation (PPV) therapy in cardiogenic pulmonary edema.

In general, PPV supports the function of an impaired LV by reducing the transmural pressure across the LV free wall (LV afterload is reduced). In contrast, PPV is usually a functional burden on an already impaired RV function because of the reduction of preload and increase of afterload, respectively (see Figure 47-1 ). Ultrasonography can help in optimizing PPV to achieve the maximal benefit in oxygenation, while avoiding its side effects on cardiac function. An advanced HOLA protocol would be combined lung and cardiac ultrasound to estimate the optimum level of PEEP . Optimization of PEEP could facilitate the recruitment of collapsed alveoli, and the improvement in pulmonary functional residual capacity thus reduces the pulmonary vascular resistance. In turn, this could minimize the burden on RV function. Thus general chest ultrasound (lung and cardiac ultrasound) evaluation could guide both ventilatory and circulatory support. The optimization of heart-lung interaction could enhance the therapeutic effect of mechanical ventilation and also facilitate the weaning process (see Chapter 34 ).

In pediatric patients, diaphragmatic and vocal cord paralysis can lead to extubation failure. Ultrasound can easily detect the above disorders in the PICU. The ultrasound examination of the vocal cords is performed while the child is breathing spontaneously, in a supine position with minimal neck extension, to allow a space for the transducer to be placed and gently manipulated on the cricoid cartilage and trachea. A high-frequency linear or curvilinear probe is used. The diaphragm normally moves downward on inspiration and upward on expiration. When paralyzed, the diaphragm either remains immobile in a fixed position (upward) or may exhibit paradoxic movement (moves upward during inspiration). When paretic, the diaphragm is moving in the right direction; however, the excursion is limited. Fluoroscopic examination is the gold standard in assessing diaphragmatic motion. Recently, ultrasound examination has been successfully integrated in the study of diaphragmatic motion. Both hemidiaphragms of the child should be ultrasonographically assessed (on both longitudinal and transverse planes) during quiet respiration by pleural ultrasound. High transverse scans must be undertaken, with both diaphragms in view, for movement comparison. Important technical points follow.

• 1.

  In children older than 2 years, a 3- to 5-MHz frequency transducer is preferred, whereas a 5- to 8-MHz frequency curvilinear transducer is used for children younger than 2 years.

• 2.

  Subcostal transverse views with the transducer pointer oriented at 9:00 o’clock are not ideal for the visualization of the ipsilateral hemidiaphragm but do aid in a general preview (the pointer should be oriented in the right side of the screen, the posterior aspect of the diaphragm is usually visualized, stomach air prevents the clear visualization of the hemidiaphragm).

• 3.

  Longitudinal views with the transducer pointer oriented at 12:00 o’clock (cephalic) are usually applied for visualizing the ipsilateral hemidiaphragm (coronal plane at the midaxillary line (diaphragmatic level) and axial plane at the subcostal position).

• 4.

  Next, M-mode recordings ( Figure 47-2 ) are performed when the cursor is almost vertical on the hemidiaphragm (while moving with a maximum deviation of 20 degrees from the vertical position). M-mode tracings are obtained, in the axial subcostal or in the coronal midaxillary views on either side with the transducer pointer oriented at 12:00 o’clock (cephalic). The amplitude of the movement of the hemidiaphragmatic copula should be greater than 4 mm and oriented toward the transducer (during inspiration).

  

Procedural ultrasound

Common procedures (e.g., ultrasound-guided vascular access) are accomplished in the emergency room/ICU with ultrasound guidance. Additional ultrasound-guided procedures can be performed in the NICU and PICU, such as lumbar puncture ( Figure 47-3 ), paracentesis of fluid collections, pericardiocentesis ( Figure 47-4 ), and placement of peripherally inserted central venous catheters. Finally, abscess, gall bladder and suprapubic drainage, or the placement of transhepatic lines can also be performed with ultrasound guidance in pediatric patients.