Researchers have taken arterial blood samples from freedivers at depths down to 80m, revealing how oxygen and carbon dioxide levels change during deep breath-hold dives—and producing some unexpected results.
Professor Simon Mitchell takes blood gas at a depth of 80 metres.
A study involving elite freedivers has provided the deepest direct measurements yet of arterial blood gases during breath-hold diving, challenging assumptions about what happens as divers descend beyond about 40m.
Researchers took arterial blood samples from three highly experienced freedivers during dives to 20, 40, 60 and 80m off Dominica. The divers included multiple world-record holder William Trubridge.
The unusual experiment required the freedivers to descend with arterial catheters already inserted. Support divers using closed-circuit rebreathers then drew blood samples underwater at the target depths.
Oxygen peaks earlier than expected
Increasing water pressure compresses the lungs during descent, which might be expected to progressively increase arterial oxygen and carbon dioxide levels as the diver goes deeper.
Instead, arterial oxygen pressure peaked at around 40m and subsequently declined despite increasing depth. The researchers suggest this corresponds approximately with the point at which lung compression approaches residual volume, limiting further pressure-related changes in gas exchange.
Carbon dioxide also did not rise steadily with depth. After initially plateauing, it increased at 80m, apparently reflecting accumulating metabolic carbon dioxide rather than further lung compression.
The findings help provide a clearer picture of the extreme physiological changes occurring during deep freediving. They also reinforce the significance of ascent, when falling ambient pressure can cause arterial oxygen levels to decline rapidly—a mechanism associated with hypoxic or so-called shallow-water blackout.
An unusual experiment
The research was led by Thomas Scott and Professor Simon Mitchell of the University of Auckland, working with an international team that included researchers from Divers Alert Network.
Mitchell, a diving physician and hyperbaric medicine specialist who has previously contributed to X-Ray Magazine, was also among the support divers collecting blood samples at depth.
The study, “Arterial blood gas changes in progressively deeper breath-hold dives,” was published in the Journal of Applied Physiology in July.
