Interpleural location of chest drain on ultrasound excludes pneumothorax and associates with a low degree of chest drain foreshortening on the antero-posterior chest X-ray

Background The role of chest drain (CD) location by bedside imaging methods in the diagnosis of pneumothorax has not been explored in a prospective study yet. Methods Covid-19 ARDS patients with pneumothorax were prospectively monitored with chest ultrasound (CUS) and antero-posterior X-ray (CR) performed after drainage in the safe triangle. CD foreshortening was estimated as a decrease of chest drain index (CDI = length of CD in chest taken from CR/depth of insertion on CD scale + 5 cm). The angle of inclination of the CD was measured between the horizontal line and the CD at the point where it enters pleural space on CR. Results Of the total 106 pneumothorax cases 80 patients had full lung expansion on CUS, the CD was located by CUS in 69 (86%), the CDI was 0.99 (0.88–1.06). 26 cases had a residual pneumothorax after drainage (24.5%), the CD was located by CUS in 31%, the CDI was 0.76 (0.6–0.93), p < 0.01. The risk ratio for a pneumothorax in a patient with not visible CD between the pleural layers on CUS and an associated low CDI on CR was 5.97, p ˂0.0001. For the patients with a steep angle of inclination (> 50°) of the CD, the risk ratio for pneumothorax was not significant ( p < 0.17). A continued air leak from the CD after drainage is related to the risk for a residual pneumothorax (RR 2.27, p = 0.003). Conclusion Absence of a CD on CUS post drainage, low CDI on CR and continuous air leak significantly associate with residual occult pneumothorax which may evade diagnosis on an antero-posterior CR.