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.