Gas exchange and pulmonary stress variations during SCUBA and breath-hold diving in open seawater

Understanding of pulmonary gas exchange measurements in divers at sea is incomplete. In this study, arterial blood gases (ABGs) were measured in SCUBA divers breathing compressed air and pedalling at depths of 15 or 40 m in seawater (msw). In breath-hold divers (BHDs), ABGs were obtained before, at 15, 25 or 40 msw, and at the surface before breathing. Lung ultrasound was also performed in both groups before, at 15 msw, and after all the dives. Blood syndecan-1 (SDC-1) and heparan sulfate (HS) were also measured. Among 10 SCUBA divers (one female; ages 32-57), PaO₂ increased at depth as predicted. Among 12 BHDs (three female, ages 33-62), PaO₂ rose at depth and decreased on surfacing; two participants at 15 msw and one at 25 msw did not develop bottom hyperoxaemia. Lung ultrasound was normal at 15 msw, while interstitial oedema or pleural irregularities were found after surfacing in most SCUBA divers and BHDs. In SCUBA divers, significant post-dive increases occurred in SDC-1 and HS; in BHDs, a significant increase was found in HS after the 15 and 25 msw dives, while SDC-1 increased after all depths. Compared with warm-freshwater experiments, ABG values in SCUBA divers were similar, while in BHDs relative hypoxaemia at depth was less common. Elevated levels of glycocalyx markers were consistent with endothelial stress, possibly providing a mechanism for fluid to accumulate in the pulmonary interstitium and explaining the ultrasound abnormalities. KEY POINTS: The understanding of lung-environment interactions during open-sea diving remains limited. We integrated underwater and surface arterial blood gases, lung ultrasound and endothelial glycocalyx markers (syndecan-1, heparan sulfate) to quantify gas-exchange perturbations and lung stress in SCUBA and breath-hold divers (BHDs). SCUBA: arterial oxygen (PaO₂) increased at depth and returned to baseline at the surface; BHDs: PaO₂ increased at depth (except in three participants), then values fell to hypoxaemia on surfacing. Post-dive lung ultrasound showed subclinical interstitial oedema - from focal B-lines to diffuse patterns - and pleural irregularities more marked after deeper dives and in BHDs than in SCUBA. Circulating glycocalyx markers increased post-dive, consistent with endothelial stress.