• Neil Mukherjee and Scott G Taylor.
• Emergency Department, Glasgow Royal Infirmary, Glasgow, UK.
• Eur J Emerg Med. 2008 Jun 1;15(3):166-8.

Background And ObjectivesWe encountered a case whereby an 18-year-old boy presented to the Emergency Department with a stab wound to the left posterior chest. Chest X-ray (CXR) showed what appeared to be a pneumothorax and chest drain insertion was considered. It was confirmed subsequently that this apparent pneumothorax was due to a linear artefactual projection from the edge of the oxygen mask reservoir bag. We set out to investigate whether our colleagues would have misdiagnosed this artefact and what their initial treatment plan would have been.MethodsFour clinical scenarios were presented with accompanying radiographs, one of which was the case described above. Doctors were asked to examine the CXRs and accompanying scenarios, describe the radiograph findings and describe initial treatments they would perform.ResultsTwenty-three doctors (two consultants, five middle grades, and 16 senior house officers) were recruited. Two (9%) doctors indicated the 'mask sign' as a possible artefact. Nine (39%) reported the CXR as demonstrating a pneumothorax and recommended large bore chest drain insertion.ConclusionOur results show that almost half of our colleagues would have carried out tube thoracostomy when no pneumothorax was actually present. In a situation where a pneumothorax is a clinical possibility we would recommend either temporarily removing the oxygen mask if clinically stable, or fixing the reservoir bag outwith the field of the CXR by means of adhesive tape to avoid any misinterpretation of this 'mask sign'.

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