Arterial Blood Gas Analysis In Breath Hold Divers At Depth
Authors: Gerardo Bosco, Alex Rizzato, Luca Martani, Simone Schiavo, Ennio Talamonti, Giacomo Garetto, Matteo Paganini, Enrico M. Camporesi and Richard E. Moon
DOI / Source: https://doi.org/10.3389/fphys.2018.01558
Date: 05 November 2018
Reading level: Intermediate
Why This Matters for Freedivers
This paper shows that real blood gases don’t always match the “textbook” prediction: some divers can be relatively hypoxic even at depth, likely due to lung compression and gas-exchange mismatch, and CO₂ changes may be smaller than expected. It’s a strong reminder that safety isn’t just about how you feel—technique, conservative depth progression, and avoiding heavy pre-dive hyperventilation all matter, especially because the highest blackout risk is typically near the end of ascent and right after surfacing.
Synopsis
A lot of freediving “truths” are based on theory: pressure goes up at depth, so oxygen pressure should go up too; carbon dioxide should rise during a long breath-hold; and the dangerous part is usually the end of the dive. This study is exciting because it does something almost nobody manages to do safely: it measures arterial blood gases during an actual dive at 40 meters, not just before and after. The researchers placed a small arterial line in the diver’s wrist (radial artery) and took blood samples at three points: 10 minutes before the dive, right at 40 m, and within 2 minutes after surfacing. Six trained divers completed the protocol in the Y-40 deep pool. 
What did they find? First, the “depth = guaranteed hyperoxia” idea mostly held… but not for everyone. In 4 out of 6 divers, oxygen pressure in arterial blood (paO₂) increased at depth as expected. But 2 divers had the opposite: their paO₂ at 40 m was lower than their pre-dive value, meaning they were relatively hypoxic even at the bottom, when theory predicts plenty of oxygen pressure. The authors suggest a plausible reason: lung compression at depth can create uneven gas exchange (parts of the lung may partially close or perfuse poorly), so oxygen doesn’t transfer into blood as well even though the pressure is high. In other words, depth can “hide” a gas-exchange problem. 
Second, carbon dioxide didn’t behave as dramatically as many people would expect. At depth, paCO₂ rose only slightly on average, and in 2 divers it actually fell, likely because they hyperventilated more than the protocol intended after the first sample was taken. The paper explains why CO₂ can be “buffered” during dives: CO₂ dissolves and gets stored in blood and tissues much more easily than oxygen changes, so arterial CO₂ doesn’t always spike the way simple gas-law math suggests. Finally, the post-dive oxygen values were generally lower than pre-dive (and one diver had low oxygen saturation), reinforcing the practical point that the ascent/surface phase is where hypoxia becomes most dangerous—even if a diver looks fine during the dive itself.