Abstract

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 (SpO2) 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 SpO2 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 “shallow” and one “deep” constant weight dive with fins. Plethysmograms were recorded at 30 Hz, and SpO2 and HR were extracted. Mean ± SD depth of shallow dives was 19 ± 3 m, and 73 ± 12 m for deep dives. Duration was 82 ± 36 s in shallow and 150 ± 27 s in deep dives. All divers desaturated more during deeper dives (nadir 55 ± 10%) compared to shallow dives (nadir 80 ± 22%) with a lowest SpO2 of 44% in one deep dive. HR showed a “diving response,” 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 “free fall” phase. The underwater pulse oximeter allowed successful SpO2 and HR monitoring in freedives to 82 m depth – deeper than ever recorded before. Divers’ 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.

Highlights

  • Freediving is increasing in popularity both as a recreational and competitive sport (Divers Alert Network, 2017), where all activities are performed on one breath of air

  • Based on our observation of a slight decrease in heart rate (HR) during the last phase, we find it more likely that the resulting diving HR is a net-effect of both stimuli where the opposite influences of the diving response and exercise on HR balance oxygen conservation with local energetic demands

  • The average of 46% HR reduction observed in our study shows that the diving response in elite divers may be of similar magnitude as the 40% bradycardia observed in Steller sea lions (Hindle et al, 2010), while in gray seals average heart rate was shown to be reduced by nearly 90% (Thompson and Fedak, 1993)

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Summary

Introduction

Freediving is increasing in popularity both as a recreational and competitive sport (Divers Alert Network, 2017), where all activities are performed on one breath of air. While most freediving is done in the depth range down to 20 m, deep freediving is an increasingly popular extreme sport where elite divers reach far beyond that depth, exposing themselves to progressive hypoxia and to dramatic changes in pressure. These changes have several physiological effects, both by direct compression of air filled cavities (Boyles law) and on gas exchange, as partial pressure of oxygen changes proportionally with ambient pressure (Dalton’s law). The effect of pressure on gas exchange may in addition expose deep divers to increased risk of blackout resulting from the air re-expansion during ascent, which can dramatically reduce arterial oxygen saturation (SpO2), resulting in a specific case of blackout often called “shallow water blackout” (Lanphier and Rahn, 1963; reviewed in Schagatay, 2011)

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