We summarized the experience of ultrasound for the detection of COVID-19 pneumonia in Wuhan. The characteristic LUS findings of COVID-19 pneumonia included B lines, consolidations, and thickening or irregularity of the pleural line; pleural effusions were uncommon. LUS had good diagnostic performance for diagnosing COVID-19 pneumonia and showed a relatively low diagnostic accuracy for interstitial syndrome.
Traditionally, because of the air barrier, ultrasound did not apply to the diagnosis and management of most lung diseases and was limited to the study of superficial pleural conditions, such as tumors and effusions, and to guide invasive procedures[12, 13]. However, this situation has changed with literature supporting the use of LUS for multiple conditions[5–7]. The use of LUS was recently demonstrated for the detection of a broad range of pathologies characterized by a decrease in lung air content and an alteration of the tissue, air, and fluid ratio[14].
In the first part of our study, LUS signs of the 330 diagnosed patients included B lines; thickening or irregularity of the pleural line; consolidation, most of which appeared in severe and critical patients; and, rarely, pleural effusion. The percentage of these four signs increased with disease severity. The severe and critical groups had more areas with B lines and consolidation than the mild and common groups. Therefore, LUS may be useful for the preliminary evaluation of disease severity and follow-up.
The pathophysiological mechanisms of these signs can be explained using autopsy results[15]. At the start of the infection, leakage from pulmonary capillaries into the interstitium and alveoli leads to interstitial pulmonary oedema and pulmonary alveolar oedema. LUS reveals a gradual increase in the B lines. As leakage fluid continues to increase, resulting in a decrease in the air content of lung tissues, it causes pulmonary consolidation characterized by a heterogeneous subpleural hypoechoic region. When the inflammatory exudate involves the pleura and the thoracic cavity, it manifests as pleural thickening or irregularity and small amounts of pleural effusion.
In our study, LUS had a 92.70% diagnostic accuracy in detecting consolidation, which was similar to that reported in the literature[16]. Of the 20 consolidations missed on LUS, nine were located in the upper lobes (anterior-superior region) where lesions were easily missed on ultrasound due to its distance from the pleura and the influence of the spine and scapula. In 11 missed cases, the consolidations occurred deeply and did not extend up to the pleural surface. Thus, LUS is capable of detecting superficial pneumonia; however, it remains doubtful whether it can detect deep alveolar lesions[12].
LUS showed a relatively low diagnostic accuracy (82.10%) and a high false-negative rate for interstitial syndrome. Fifty-three patients with interstitial syndrome, all in the common group, were missed on LUS. Six patients in the mild group with no lung lesions were misdiagnosed with interstitial syndrome on LUS. Thus, further analysis of predictors of the diagnostic accuracy for interstitial syndrome was conducted; this is discussed later. LUS had an 86.30% diagnostic accuracy in detecting pleural thickening or irregularity. Forty-five cases were diagnosed with pleural thickening or irregularity on LUS that was absent on CT. The CT images of these 45 patients were re-analysed focusing on the pleura; 32 patients were found to be consistent with the LUS findings. In our opinion, LUS with high-frequency superficial probes was more sensitive to superficial tissue such as the pleura than was CT; however, subjectivity of LUS maybe the reason for false positive cases. LUS had a 98.80% diagnostic accuracy for detecting pleural effusion; the four missed cases were all critical patients who were unable to turn over to allow the examination of each area.
The diagnostic coincidence rates in mild, common, severe, and critical types were 93.18%, 68.64%, 100.00%, and 100.00%, respectively. In general, the lungs of severe and critical patients had more lesions than those of patients with mild and common types, which might be the reason for the good diagnostic coincidence of LUS for severe and critical types. In the mild group, six patients were positive on LUS and negative on CT. The images were re-analysed, and four cases showed two to three B lines near the diaphragm in the lower lobe of the lung. As reported in the literature, B lines can be found in the intercostal space of the chest near the diaphragm in normal lungs. Though they do not usually exceed two to three lines, they are easily misdiagnosed as positive cases[17]. The remaining two patients had a history of interstitial pneumonia. The diagnostic accuracy was the lowest in the common group.
Since LUS showed a relatively low DA for interstitial syndrome, a regression analysis was conducted on the data to find out the possible influencing factors.
First, according to our results, female patients were more likely to have inconsistent findings between LUS and CT than male patients. The missed female ultrasound images were re-read, and it was found that the missed area in female patients was mainly located in the anterior chest area, which corresponded to the deep surface of the breast. Female breast tissue increases the scan depth of the lungs, which may lead to false-negative results. LUS can detect superficial pneumonia, but the increase of depth from skin to lungs may lessen the accuracy [12]. To tackle this phenomenon, when the scanning anterior chest area in female patients, especially with plump breasts, we suggest that the probe frequency be reduced appropriately to increase the penetrability; careful scanning is necessary to avoid false-negative findings.
Second, the results showed that junior ultrasound doctors were more likely to miss lesions than their senior counterparts. As reported in the literature[18], lung scanning should be performed by well-trained operators to achieve high diagnostic accuracy. There appears to be a gap in the operating skills and experience of junior ultrasound doctors, especially in the challenging environment of the epidemic isolation ward. Therefore, we recommend that ultrasound doctors undergo systematic training and assessment before performing LUS.
Third, as the duration of the disease increased, the diagnostic coincidence rate of ultrasound decreased. In the early stage of disease, the coincidence rate of ultrasound was higher than that in the recovery stage, probably because with the prolongation of the disease duration and effective treatment, pulmonary lesions became smaller, thus reducing the ultrasonic concordant rate.
Fourth, according to the regression analysis, lesions located only in the upper or lower lobe were more likely to be missed than diffuse lung lesions. This could be due to the overlapping of adjacent regions. For example, the lower-left lobe of the lung may be obscured by the heart or gas in the fundus of the stomach. Gastrointestinal decompression, if possible, could be helpful for patients with severe flatulence. Lesions in the upper lobes of the lungs were more likely to be missed, and the reasons are not well-defined. A previous study[19] suggested that false-negative results might be due to the distance from the pleura, and the coverage of the spine and scapula might make it difficult to detect lesions. More detailed scanning from multiple angles is recommended in areas covered by surrounding organs or bones to reduce false negatives.