Using Underwater Pulse Oximetry in Freediving to Extreme Depths to Study Risk of Hypoxic Blackout and Diving Response Phases
Authors: Eric Mulder, Arne Sieber, Erika Schagatay
DOI / Source: https://doi.org/10.3389/fphys.2021.651128
Date: 01 April 2021
Reading level: Intermediate
Why This Matters for Freedivers
This paper puts real numbers on something divers talk about all the time: you can feel “fine” at depth and still crash near the surface. Their underwater SpO₂ tracking shows how fast oxygen saturation can drop during the last part of ascent, which supports why buddying, safety diver timing, smart pacing, and good recovery breathing matter—especially in deep CWT training.
Synopsis
Most freediving physiology data is collected at the surface or in a lab, because following someone down a line to extreme depth with instruments is normally impossible. This study is exciting because it does something that hasn’t really been done before: it measures oxygen saturation (SpO₂) and heart rate (HR) underwater during real deep sea dives—down to 82 meters—using a custom-built, pressure-proof pulse oximeter.
Four elite male competitive freedivers wore a new system with two sensors placed on the temples (under the hood) and a recording unit under the wetsuit. During normal training (not a strict lab protocol), each diver completed at least one “shallow” constant weight dive with fins (around 19 m) and one “deep” dive (around 73 m). The researchers then pulled out second-by-second trends for SpO₂ and HR and compared shallow vs deep.
What happened to oxygen saturation
The big headline is how low SpO₂ got during deep dives. On average, the lowest SpO₂ during deep dives was about 55%, compared with about 80% in shallow dives. The lowest single value recorded was 44%. For non-divers, numbers like that would usually mean severe impairment or loss of consciousness—but these athletes surfaced successfully, showing how extreme hypoxia tolerance can become in top-level divers.
Even more important than “how low” is when it got low: for many dives, SpO₂ stayed fairly stable early on and then dropped hard toward the end, often near surfacing (sometimes just after surfacing because of circulation delay). This supports the idea that the final part of ascent is a danger zone: as pressure decreases, the oxygen pressure in the lungs drops quickly, and oxygen can stop moving into the blood as effectively—so saturation can fall fast right when you need alertness and coordination the most.
What happened to heart rate (and why phases matter)
Heart rate showed a classic diving response pattern: an increase during pre-dive preparation, then a decline after the dive starts, and a big recovery peak after surfacing. Interestingly, the lowest HR was similar in shallow and deep dives (around 42 bpm on average), with one extreme low of 28 bpm.
But deep dives had much more “shape” to them, and the phase breakdown is super useful for coaching: - HR dropped during the descent, and dropped again during free-fall (when effort is low). - HR rose during the hard-working part of the ascent (when the diver is swimming strongly against negative buoyancy). - HR dipped again when buoyancy helped near the end.
The simple takeaway is: the diving response is real, but it doesn’t magically override exercise. Your heart rate is the result of a tug-of-war between oxygen-saving reflexes and the demands of movement. When you’re relaxed and gliding, the bradycardia can deepen. When you’re fighting the water, HR rises—and oxygen cost rises with it.
Why the paper is a big deal
Beyond the physiology, this is a technology milestone: it proves it’s possible to record meaningful SpO₂ and HR data at extreme depth. The authors point toward a future where live, real-time monitoring could help safety teams prepare for rescue earlier—because a diver’s numbers could warn you before their behavior does.
Abstract
This study used a newly developed underwater pulse oximeter to continuously record arterial oxygen saturation (SpO₂) and heart rate (HR) during real sea freedives, including extreme depths. Four elite male competitive freedivers performed one shallow and one deep constant weight dive with fins while wearing dual temple sensors connected to a data logger. Deep dives produced substantially greater desaturation than shallow dives, with very low SpO₂ values occurring near the end of dives, consistent with increased blackout risk during ascent as pressure decreases. HR showed a diving response in both shallow and deep dives, and phase analysis suggested HR reflects a balance between diving-related bradycardia and exercise-driven increases, with a second HR decline during passive free-fall. The results demonstrate feasibility of underwater pulse oximetry at extreme depth and highlight its potential value for understanding physiology and improving freediving safety.