2.3 The volume of the pneumothorax
Although lung ultrasound is sensitive for the diagnosis of pneumothorax, it is unable to measure the volume of the pneumothorax. Jing Liu et al.12 conducted a study to explore the accuracy and reliability of ultrasonography for the diagnosis of neonatal pneumothorax and found that the main ultrasonographic signs of pneumothorax included disappearance of lung sliding, presence of a pleural line and A-line, presence of lung points in children with mild to moderate pneumothorax, absence of lung points in children with severe pneumothorax, absence of a B-line and lung consolidation in the area of pneumothorax. The sensitivity, specificity, positive predictive value and negative predictive value of lung sliding disappearance and the presence of pleural lines and A-lines in diagnosing pneumothorax were 100%. Raimondi et al.5reported that lung ultrasound had high accuracy in detecting pneumothorax, was superior to clinical evaluation and shortened the time of diagnosis and treatment. International evidence-based guidelines also indicated that lung ultrasound was helpful to accurately detect pneumothorax in neonates and children (Grade B)8. It is worth noting that the absence of lung sliding was also observed in other disorders, such as apnoea, acute respiratory distress syndrome, pulmonary atelectasis, deep pneumonia, decreased lung compliance, and foreign body aspiration. As a result, the diagnosis of pneumothorax cannot rely on the disappearance of lung sliding alone, but the presence of lung sliding can exclude pneumothorax. In addition, the usefulness of lung ultrasound in the determination of pneumothorax size was not certain. Jing Liu et al.9found that lung points existed in children with mild to moderate pneumothorax and disappeared in children with severe pneumothorax. Volpicelli et al.13 proposed that lung ultrasound could quantify the pneumothorax volume by locating lung points on the chest wall, and the lateral progression of lung points on the chest wall corresponded to an increase in the extent of pneumothorax. However, the thickness of pneumothorax cannot be measured due to the total reflection caused by the gas, and the usefulness of lung ultrasound for quantifying the pneumothorax size needs further validation in practice.
Pulmonary interstitial emphysema
Pulmonary interstitial emphysema cannot be diagnosed by lung ultrasound. One case study reported14 the role of lung ultrasound in the follow-up of localized interstitial emphysema. The infant again presented tachypnoea after continuous positive airway pressure for 72 hours. Lung ultrasound revealed a normal pattern of the right lung and non-coalescent B-lines on the right side, while thoracic computed tomography showed localized interstitial emphysema of the left upper lobe. This ultrasonic finding (non-coalescent B-line) was observed in other lung diseases and was not specific for emphysema. In patients with emphysema, lung ultrasound often showed normal sonographic manifestations with the presence of lung sliding and an A-line due to decreased elastance of airway distal to terminal bronchioles, overinflation of the lung, increases in lung volume, and a widened intercostal space. Therefore, lung ultrasound cannot diagnose emphysema.