Mid Sweden University

Breath-hold diving or freediving exposes the body to stressors such as low oxygen and high carbon dioxide levels in the blood, increased hydrostatic pressure, and cold. The reduction in blood oxygen levels is considered a primary factor for loss of consciousness while diving. This is known as hypoxic blackout. My thesis aimed to enhance the understanding of the factors that lead to hypoxic blackout in serial freediving. This includes studying the physiological mechanisms that protect the body against hypoxia and exploring how hyperventilation affects those mechanisms, with a specific focus on repetitive dives. 

The protective mechanisms, splenic contraction (Studies I, II) and diving bradycardia (Studies II and IV), were tested in experimental conditions. The effect of hyperventilation on simulated serial dives (Study II) was investigated, and a strategy to identify actual hyperventilation during diving was developed and explored (Study III). A static apnea test allowing identification of individuals at particular risk for blackout was developed (Study IV).

The results showed that hypoxia-induced splenic contraction occurs rapidly enough to be protective in long-duration breath-holds (Study I). Serial simulated dives, preceded by short-term hyperventilation, lead to longer apnea duration and progressive oxygen desaturation (Study II). Despite the more severe desaturation resulting from hyperventilation, these consecutive apneas do not show an augmented diving response or splenic contraction. Hyperventilation was also observed when divers intended to avoid it (Studies II, III, and IV). Therefore, the possibility of estimating hyperventilation from the signal of a force sensor attached to a chest belt was explored (Study III). A stronger diving bradycardia and a bigger spleen were found to be protective against hypoxia and are related to slower oxygen desaturation (Study IV). 

The main conclusion is that oxygenation is impaired by the accumulation of an oxygen debt when consecutive dives are preceded by hyperventilation. The protective mechanisms, such as splenic contraction and the diving response, are beneficial at the individual level but do not offset the increased risk across a series of dives. However, splenic contraction does offer protection even during the first dive of a series. A chest belt-mounted force sensor could be used to identify when freedivers are hyperventilating. Additionally, a novel static apnea ramp test is effective in identifying freedivers who are at a high risk of faster desaturation during freediving.