Written by Chris Thom
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Emphysema can result in significant air trapping and reduced pleural movement, which can lead to POCUS findings that mimic pneumothorax. In this retrospective study of lung POCUS in emphysema patients, there was a high false-positive rate for pneumothorax.
COPD makes barcode and seashore signs a bit more complicated
Lung ultrasound has terrific sensitivity and specificity for pneumothorax, far surpassing supine chest x-ray in the setting of trauma (1). Pneumothorax can be readily identified via the absence of sliding lung on B-mode (normal 2D ultrasound imaging) and the presence of the barcode sign on M-mode. Emphysema is one of a few pathologies where the specificity of these findings for pneumothorax can be lower. In this study, the authors evaluated a cohort of 48 patients with known severe emphysema who were about to undergo a scheduled outpatient procedure. They obtained 159 sets of images from various lung zones and had all imaging data reviewed by two expert pulmonologists and one chest radiologist. The B-mode and M-mode lung ultrasound findings were compared to high resolution chest CT (HRCT) and pulmonary function tests (PFTs). 14% of images had barcode appearance on M-mode and 20.3% had indeterminate or absent lung sliding on B-mode. Correlation of false positive lung ultrasound findings with PFTs and HRCT was poor, with only a nonsignificant association with increased forced vital capacity identified.
How will this change my practice?
Severe emphysema is associated with increased air trapping and reduced lung excursion, which may affect the test characteristics of lung ultrasound for pneumothorax. The current study suggests that we should be cautious when interpreting pleural slide and M-mode images on patients with severe emphysema, as the specificity appears to be lower than in other patients. However, the lung pulse sign was present in all of the study patients with barcode appearance on M-mode, which suggests that careful evaluation of the lung windows for this finding can still lead to the correct diagnosis.
Ultrasound acquisition pro-tips
The lung ultrasound exam for pneumothorax is generally performed via anterior lung windows around the mid-clavicular line. Often one or two windows per side is adequate for obtaining high accuracy in pneumothorax detection, though sensitivity may increase with additional intercostal spaces. The use of M-mode is considered an adjunct to live B-mode imaging and is only necessary when there is diagnostic uncertainty from the B-mode imaging. Indeed, one large study demonstrated that M-mode increases diagnostic accuracy for novice users that have performed < 250 lung ultrasounds but provides no utility for those with more experience (2). The lung pulse is often an unsung hero with lung ultrasound, as it can differentiate the poorly aerated lung from a true pneumothorax (3). The lung pulse occurs due to the cardiac pulsation physically moving the lung in the thoracic cavity, which transmits to the pleura except in cases of true pneumothorax.
Source
Advanced emphysema leads to high false positivity rate for pneumothorax in point of care ultrasound. Respir Med. 2024 Dec;235:107860. doi: 10.1016/j.rmed.2024.107860. Epub 2024 Nov 16. PMID: 39557207
Additional References
- Chan KK, Joo DA, McRae AD, et al. Chest ultrasonography versus supine chest radiography for diagnosis of pneumothorax in trauma patients in the emergency department. Cochrane Database Syst Rev. 2020;7(7):CD013031. Published 2020 Jul 23.
- Avila J, Smith B, Mead T, et al. Does the Addition of M-Mode to B-Mode Ultrasound Increase the Accuracy of Identification of Lung Sliding in Traumatic Pneumothoraces?. J Ultrasound Med. 2018;37(11):2681-2687.
- Alerhand S, Tsung JW. Unmasking the Lung Pulse for Detection of Endobronchial Intubation. J Ultrasound Med. 2020;39(11):2105-2109.
