Controlled Emergency Swimming Ascents (CESA) are a fundamental component of diver training, simulating ascent without a gas supply. Now, a new study led by Olivier Castagna and colleagues at the French Military Diving School has raised questions about whether this vital drill might be more taxing on the lungs than previously assumed.

Subclinical stress

In this study, trained military divers performed standard CESA drills from depths of 5 and 10 metres. Though none reported symptoms, post-dive ultrasound scans revealed a significant accumulation of extravascular lung water in those who performed CESAs—evidence of subclinical pulmonary stress. The condition was absent following normal ascents.

Uneven expiration

Real-time spirometry showed that even experienced divers varied widely in how much they exhaled before and during the ascent. Some retained too much gas, risking overexpansion injuries, while others exhaled too much, reaching near-empty lungs mid-ascent. These mismatched strategies affected ascent speed and potentially increased the mechanical stress on alveolar structures.

Caution, not alarm

The study's authors stress that these changes are reversible and not necessarily dangerous on their own. Still, they call for better awareness and possibly objective tools to monitor how divers exhale during training.

CESA remains a cornerstone of diver safety, but this research underscores that even well-taught procedures may carry hidden risks—and that a one-size-fits-all approach may not serve all divers equally