Written by Chris Thom
Spoon Feed
In this cross-sectional study, there was moderate interrater reliability for B-mode and M-mode images in the diagnosis of pneumothorax. B-mode had higher overall accuracy than M-mode.
Do you B or M mode your sliding lung?
This was a cross-sectional study of twenty EM, IM, anesthesia, and critical care resident and attending physicians. Each physician reviewed 20 pre-selected B-mode videos and M-mode lung ultrasound images from the patient imaging database. These images were taken with the low frequency phased array probe. Approximately half of these were pneumothorax-positive cases. Participants were asked to note whether lung sliding was absent or present on each clip and image. An intraclass correlation coefficient (ICC) was calculated to assess interrater reliability in imaging interpretation.
90% of respondents reported using POCUS at least once weekly in their practice. The overall ICC for B-mode clips was 0.44 and 0.43 for M-mode images. A subcohort analysis was done on the POCUS users with higher self-reported experience and usage frequency, which found no significant difference in ICC. The overall accuracy of B-mode for pneumothorax was 78%, while overall accuracy for M-mode was 68%.
How does this change my practice?
While we love teaching M-mode lung ultrasound for pneumothorax, simple B-mode is more accurate. A prior study quantified this well, finding that M-mode offers minimal benefit over B-mode, particularly in those individuals with a history of more than 250 lung POCUS exams (1). The second is that identifying sliding lung is not always as easy as we’d like it to be, especially with a diverse group of operators. Training is key here in order to achieve the >90% accuracy rates found in many studies. And I suspect AI for lung sliding diagnosis will be here shortly, as it is an attractive way to potentially minimize variability in interpretation amongst a diverse group of users. Lastly, I would point out that a pitfall of the current study was the use of the phased array probe on the abdominal setting. For sliding lung, the linear transducer is often preferred and can help distinguish the finding much more easily.
POCUS pro-tips and clips:
Use the linear transducer over the anterior chest wall for the pneumothorax evaluation. One to two intercostal spaces is adequate to catch the majority of pneumothoraces, though more subtle cases can be found with more extensive anterior chest wall scanning. Whether you call it “shimmering”, “ants on a log”, or other, you’re looking for the movement of the parietal and visceral pleura against each other. M-mode will reveal a “seashore” sign in normal cases and a “barcode” sign in pneumothorax. Another great use case is the “lung pulse” sign, which can help rapidly identify right main stem intubation with good accuracy (2).
Source
Interrater Agreement of Physicians Identifying Lung Sliding Artifact on B-Mode And M-Mode Point of Care Ultrasound (POCUS). POCUS J. 2025 Apr 15;10(1):92-98. doi: 10.24908/pocusj.v10i01.17807. PMID: 40342679
Works Cited
- 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.
