Detection of pneumothorax
The sonographic detection of pneumothorax is one of the core applications of critical care ultrasound. In trained professionals, the literature unanimously supports the assertion that lung ultrasound outperforms chest x-ray for detection of pneumothoraces.1–4 International guidelines on lung ultrasound support the accurate and reliable use of lung US in detection of pneumothorax with level A evidence.5 This technology is being rapidly adopted worldwide as it fast, inexpensive, non-invasive, reliable and accurate.
The main finding in pneumothorax is lack of lung sliding,5 which indicates a lack of apposition of the parietal and visceral pleura in the presence of a pneumothorax. The sensitivity of lung ultrasound for pneumothoraces is ~88-100% depending on the cited study.1–4 In a supine patient, scanning non-gravitational dependent area (anterior chest wall), the finding of lung sliding, lung pulse or B-lines at multiple points has a very high negative predictive value of pneumothorax approaching 95-100% (for absence of pneumothorax).6 In comparison, CXR has a sensitivity of 25-75%, with limited rates of detection in the supine patient.4,7 The risk of false positive with lung US is exceedingly low, and can be further mitigated by switching to a high frequency linear probe to enhance visualization of the pleura. Ultimately, chest CT is the gold standard and should be sought if the clinical situation is uncertain.
The following image demonstrates the pleural line (white arrow) and reverberation artifacts (green arrows) called, "A-lines", indicating grossly normal pleura and parenchyma. Because this is an image and not a clip, we cannot describe lung "sliding", a dynamic feature. This clip is taken with a phased array probe in the para-sagittal plane, as we can see a narrow footprint with a wedge-shaped image. The linear probe offers more pleural visualization with a higher frequency. The curvilinear probe can also be used and will also cover multiple rib interspaces.
The following clip demonstrates, "lung sliding", a scintillating artifact in the near field from movement of the closely opposed visceral and parietal pleura. This rules out pneumothorax (~90-100% sens) at the site of image acquisition.
The absence of lung sliding (shown in this clip) is not specific to pneumothorax and can be seen in a variety of conditions. Conditions with absence of lung sliding including apnea, main-stem intubation (most often no slide on left), pneumonia or atelectasis (often with other findings), bullae and pleural scarring
Lung findings with high "rule-out" potential include lung sliding, B-lines and lung pulse. Lung pulse, is movement of the pleura in response to cardiac oscillation. Often this is more notable on the left lung and can be demonstrated in healthy patients with breath holding. This clip demonstrates lung pulse in a patient with bradycardia and two B-lines (vertical hyperechoic lines that travel from pleural line down to bottom of screen).
The specificity of lung US for pneumothorax approaches 98-100% if a "lung point" is found alongside the absence of lung sliding. This is a transition point where the pleura becomes non-opposed. This can be difficult to find depending on the patient position. This is not to be confused with the lung-cardiac point (loss of apposition from cardiac structures) or lung-diaphragm point (loss of apposition due to abrupt caudal transition to diaphragm).
1. Ianniello S, Di Giacomo V, Sessa B, Miele V. First-line sonographic diagnosis of pneumothorax in major trauma: accuracy of e-FAST and comparison with multidetector computed tomography. Radiol Med. 2014;119(9):674-680.
2. Nagarsheth K, Kurek S. Ultrasound detection of pneumothorax compared with chest X-ray and computed tomography scan. Am Surg. 2011;77(4):480-484.
3. Husain LF, Hagopian L, Wayman D, Baker WE, Carmody KA. Sonographic diagnosis of pneumothorax. J Emerg Trauma Shock. 2012;5(1):76-81. doi:10.4103/0974-2700.93116.
4. Ebrahimi A, Yousefifard M, Kazemi HM, et al. Diagnostic accuracy of chest ultrasonography versus chest radiography for identification of pneumothorax: A systematic review and meta-analysis. Tanaffos. 2014;13(4):29-40.
5. Volpicelli G, Elbarbary M, Blaivas M, et al. International evidence-based recommendations for point-of-care lung ultrasound. In: Intensive Care Medicine.Vol 38.; 2012:577-591. doi:10.1007/s00134-012-2513-4.
6. Blaivas M, Lyon M, Duggal S. A prospective comparison of supine chest radiography and bedside ultrasound for the diagnosis of traumatic pneumothorax. Acad Emerg Med. 2005;12(9):844-849. doi:10.1197/j.aem.2005.05.005.
7. Xirouchaki N, Magkanas E, Vaporidi K, et al. Lung ultrasound in critically ill patients: Comparison with bedside chest radiography. Intensive Care Med. 2011;37:1488-1493. doi:10.1007/s00134-011-2317-y.