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 (as low as reasonably achievable) 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.1,2 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).3,4 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.5
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.6 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 Chapters 19–21).
Figure 47-1 Effects of positive end-expiratory pressure (PEEP) of 5, 10, and 15 cm H2O, respectively, on the right ventricular end-diastolic area (EDA) in a mechanically ventilated pediatric patient (bottom, measurements performed at end inspiration).
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.
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.7 Such ultrasonography also may be used in periendotracheal intubation procedures.8
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.9 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
