Mid Sweden University

Freediving, underwater diving while breath-holding, imposes unique physiological challenges to the human body. This includes immersion, changes in hydrostatic pressure, hypercapnia, and hypoxia. Severe hypoxia can cause loss of consciousness, known as hypoxic blackout, and without immediate assistance drowning may occur. 

The aim of this thesis was to identify factors increasing the risk of hypoxic blackout, to determine if physiological responses mitigated these risks and to explore potential interventions that could promote safe freediving. This was done by utilizing a prototype water- and pressure-proof pulse oximeter (SUB) to measure heart rate (HR) and arterial oxygen saturation (SpO2) under various real-world freediving conditions, and through laboratory investigation of the effects of apnea- recovery pacing on repeated apneas, focusing on arterial, cerebral, and muscle oxygenation. 

Study 1 demonstrated the SUB's ability to record HR and SpO2 during deep sea dives up to 82 meters, marking an important advancement in underwater monitoring technology. Study 2 showed that deeper dives resulted in greater oxygen desaturation, potentially increasing the risk of hypoxic blackout, not only due to pressure effects on gas exchange, but also from increased physical exertion. In addition, some individuals experienced hypoxia upon reaching maximum depth, when hyperoxia is expected, suggesting that gas exchange may be compromised, which could increase the risk of blackout during ascent. In Study 3, a persistent cardiac arrhythmia preceded a blackout, suggesting that arrhythmias may be a contributing factor to increased risk of blackout. Study 4 showed that brain oxygen homeostasis was maintained across a series of submaximal apneas with equal dive-to-rest ratio, without progressive oxygen desaturation occurring, suggesting that pacing strategies can be effectively used to maintain safety in repeated freediving and should be individually tailored. Collectively, these studies confirm that well- trained freedivers exhibit a remarkable tolerance to hypoxia. However, the risk of hypoxic blackout is highly individual, suggesting that establishing a definitive blackout threshold based on SpO2 may be challenging. It is therefore concluded that there is a need for enhanced safety protocols in freediving, including personalized physiological monitoring, which could be enabled by innovative wearable technologies like the SUB to mitigate the risk of blackout in freediving.