Spirometric indices and the risk of pulmonary barotrauma in submarine escape training

Between 1975 and 1997, a total of 115,090 ascents, from depths of between 9 and 28 m, have been made by trainees at the submarine escape training tank HMS Dolphin. During this 22-yr period, 53 incidents have occurred in which, after an ascent, the trainee required hospital or recompression therapy or both. Scrutiny of the incident records revealed unequivocal evidence of pulmonary barotrauma in six incidents with an additional four in which, despite a negligible gas burden, a confident diagnosis of acute neurologic decompression illness with short latency could be made. No causative mechanism other than arterial gas embolism following pulmonary barotrauma can be implicated in these four cases despite the absence of clinical or radiographic evidence of lung injury. In all 10 cases the forced vital capacity (FVC) of the trainees was less than the predicted value for their age and height, revealing a statistically significant (P< 0.01) association between values of FVC below predicted and pulmonary barotrauma. The median FEV1 for the 10 cases was also significantly (P < 0.05) less than the predicted value after allowing for age and height. No such association was found for the FEV1:FVC ratio. FVC would thus seem to be the measure of lung function most closely associated with increased risk of pulmonary barotrauma. Possible reasons for this finding are discussed. It is concluded that although the association between low FVC and pulmonary barotrauma is statistically significant, it is insufficiently specific for low FVC to serve as an exclusion criterion for submarine escape training.