{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,9,30]],"date-time":"2026-09-30T16:11:42Z","timestamp":1790784702443,"version":"4.1.0"},"reference-count":33,"publisher":"Frontiers Media SA","license":[{"start":{"date-parts":[[2021,4,1]],"date-time":"2021-04-01T00:00:00Z","timestamp":1617235200000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/501100004359","name":"Swedish Research Council","doi-asserted-by":"publisher","id":[{"id":"10.13039\/501100004359","id-type":"DOI","asserted-by":"publisher"}]}],"content-domain":{"domain":["frontiersin.org"],"crossmark-restriction":true},"short-container-title":["Front. Physiol."],"abstract":"<jats:p>Deep freediving exposes humans to hypoxia and dramatic changes in pressure. The effect of depth on gas exchange may enhance risk of hypoxic blackout (BO) during the last part of the ascent. Our aim was to investigate arterial oxygen saturation (SpO<jats:sub>2<\/jats:sub>) and heart rate (HR) in shallow and deep freedives, central variables, which have rarely been studied underwater in deep freediving. Four male elite competitive freedivers volunteered to wear a newly developed underwater pulse oximeter for continuous monitoring of SpO<jats:sub>2<\/jats:sub> and HR during self-initiated training in the sea. Two probes were placed on the temples, connected to a recording unit on the back of the freediver. Divers performed one \u201cshallow\u201d and one \u201cdeep\u201d constant weight dive with fins. Plethysmograms were recorded at 30 Hz, and SpO<jats:sub>2<\/jats:sub> and HR were extracted. Mean \u00b1 SD depth of shallow dives was 19 \u00b1 3 m, and 73 \u00b1 12 m for deep dives. Duration was 82 \u00b1 36 s in shallow and 150 \u00b1 27 s in deep dives. All divers desaturated more during deeper dives (nadir 55 \u00b1 10%) compared to shallow dives (nadir 80 \u00b1 22%) with a lowest SpO<jats:sub>2<\/jats:sub> of 44% in one deep dive. HR showed a \u201cdiving response,\u201d with similar lowest HR of 42 bpm in shallow and deep dives; the lowest value (28 bpm) was observed in one shallow dive. HR increased before dives, followed by a decline, and upon resurfacing a peak after which HR normalized. During deep dives, HR was influenced by the level of exertion across different diving phases; after an initial drop, a second HR decline occurred during the passive \u201cfree fall\u201d phase. The underwater pulse oximeter allowed successful SpO<jats:sub>2<\/jats:sub> and HR monitoring in freedives to 82 m depth \u2013 deeper than ever recorded before. Divers\u2019 enhanced desaturation during deep dives was likely related to increased exertion and extended duration, but the rapid extreme desaturation to below 50% near surfacing could result from the diminishing pressure, in line with the hypothesis that risk of hypoxic BO may increase during ascent. Recordings also indicated that the diving response is not powerful enough to fully override the exercise-induced tachycardia during active swimming. Pulse oximetry monitoring of essential variables underwater may be an important step to increase freediving safety.<\/jats:p>","DOI":"10.3389\/fphys.2021.651128","type":"journal-article","created":{"date-parts":[[2021,4,1]],"date-time":"2021-04-01T05:10:06Z","timestamp":1617253806000},"update-policy":"https:\/\/doi.org\/10.3389\/crossmark-policy","source":"Crossref","is-referenced-by-count":17,"title":["Using Underwater Pulse Oximetry in Freediving to Extreme Depths to Study Risk of Hypoxic Blackout and Diving Response Phases"],"prefix":"10.3389","volume":"12","author":[{"given":"Eric","family":"Mulder","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Arne","family":"Sieber","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Erika","family":"Schagatay","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1965","published-online":{"date-parts":[[2021,4,1]]},"reference":[{"key":"ref1","doi-asserted-by":"publisher","first-page":"1005","DOI":"10.1152\/japplphysiol.01057.2002","article-title":"Cardiovascular and respiratory responses to apneas with and without face immersion in exercising humans","volume":"96","author":"Andersson","year":"2004","journal-title":"J. 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