1、 INTRODUCTION
Lung ultrasound originated in adult critical care medicine. In 1990,
Avni EF et al.1 first reported that lung ultrasound
might be an alternative to chest radiography for the diagnosis of
neonatal respiratory distress syndrome. Since then, neonatal and
paediatric lung ultrasound has also developed rapidly. The
ultrasound appearances of neonatal
respiratory distress syndrome
(NRDS)2, transient tachypnoea of newborns
(TTN)3, neonatal
pneumonia4, pneumothorax5 and
meconium aspiration syndrome (MAS) 6have been clearly
described. It was found that lung ultrasound had high specificity,
sensitivity, positive predictive value and negative predictive value and
showed high consistency with chest X-ray in detecting the above
disorders7. International evidence-based guidelines on
point-of-care ultrasound (POCUS) for critically ill neonates and
children were issued by the POCUS Working Group of the European Society
of Paediatric and Neonatal Intensive Care in 20208.
Given that bedside lung ultrasound is more sensitive to tiny foci close
to the pleura than chest radiography is and that lung ultrasound is a
low-cost, radiation-free, and easy-to-operate tool that can be
repeatedly performed at the bedside, lung ultrasound has been widely
used in neonatal intensive care units (NICUs). In recent years, lung
ultrasound has become the preferred examination for NRDS, TTN, neonatal
pneumonia, pneumothorax and MAS instead of chest X-ray in the neonatal
department9. Every neonatologist has grasped the
essential operational skill sets, and interpretation of ultrasound
images by training and bedside lung ultrasound has been widely used in
our NICU in the past two years, gradually replacing bedside chest X-ray.
However, according to the application of lung ultrasound, reading of the
relevant literature, and problems encountered in actual clinical
practice, bedside lung ultrasound also has limitations in some other
lung diseases and cannot fully substitute for chest X-ray or CT.
LIMITATIONS OF LUNG ULTRASOUND
2.1 Pulmonary disease
distant to the pleura
Lesions distant to the pleura, such as congenital pulmonary airway
malformation (CPAM) and interlobar pulmonary sequestration, are missed
by lung ultrasound. CPAMs are polycystic immature alveolar tissues
formed by abnormal branching development of the lung in utero,
disruption of the lung architecture and excessive growth of terminal
bronchioles. Because the foetal lung is filled with fluid, lesions can
be detected by perinatal ultrasound examination. The number of CPAMs
detected in utero is gradually increasing with the promotion of
perinatal ultrasound. Neonatal lung ultrasound is limited in that it
misses lesions because of air-filled lungs. Hence, lung CT examination
is required for postnatal diagnosis. The ultrasound appearances of CPAM
after birth have also been gradually studied with the development of
lung ultrasound. Yousef et al.10 described the
ultrasonic manifestations of CPAM, such as a single large cystic lesion,
multiple tiny cysts, and/or irregular consolidation. However, a single
large cyst can also be found in certain congenital pulmonary cyst cases,
which cannot be differentiated by lung ultrasound. L. Merli et
al.11 reported a case of prenatal suspected CPAM. It
presented multiple cystic lesions, of which the largest lesion was found
to be distant from the pleura by CT scan, while postnatal lung
ultrasound examination only revealed subpleural consolidation. The
reason was that the alveoli around the lesions were normally inflated,
and total reflection was formed between the normal alveoli and the
probe. Thus, lesions away from the pleura could not be clearly observed,
and normal or slightly abnormal sonograms were mainly yielded by lung
ultrasound.
Pulmonary bullae
Pulmonary bullae cannot be detected by lung ultrasound. In our NICU, a
case of a preterm infant with a gestational age of 25 weeks who was
diagnosed with severe bronchopulmonary dysplasia and had persistent
dyspnoea with non-invasive respiratory support was suspicious for
pulmonary bullae. Serial bedside lung ultrasound
showed subpleural focal
consolidation and alveolar–interstitial syndrome of the bilateral
posterior lungs (shown in Fig. 1a, b). The artefacts of the lung
ultrasound were not consistent with the patient’s respiratory status. So
chest radiograph was performed which showed a roundish air-containing
space in the upper right lobe (shown in Fig. 1c). Lung CT scans showed a
round air cavity (3.5 cm×5.5 cm) in the upper and middle lobes of the
right lung (shown in Fig. 1d). The cavity was confirmed by surgery to be
full of gas. The failure of bedside lung ultrasound to diagnose this
case is related to the imaging principle of ultrasound. When the probe
emits an acoustic beam to perforate through the tissues and organs,
physical phenomena such as reflection, refraction, and scattering occur.
The penetration of acoustic beams is related to the components of
tissues and organs. When the acoustic beam encounters gas, it will be
totally reflected. Certain lesions cannot be found by lung ultrasound
due to the influence of gas in front of the lesion. When pulmonary
bullae form, lung ultrasound cannot show the presence of pulmonary
bullae due to the total reflection of the acoustic beam caused by a
large amount of gas in the bubble. Similarly, subcutaneous emphysema and
pneumomediastinum may also affect the results of lung ultrasound due to
the same mechanism described earlier where there is a large amount of
gas interfering with the acoustic beam.