The human diving response - effects of temperature and training

Abstract: The aim was to elucidate the cardiovascular response associated with breath-hold diving, especially the effects of temperature in its elicitation and the effects of different types of training on the human diving response (DR) and breath-holding time (BHT), and to evaluate the human DR in a mammalian perspective. A model for simulated diving by apnea and facial immersion in cold water was developed. Cardiovascular parameters were registred using non-invasive techniques. The bradycardia response to breath-holding and chilling of the forehead was found to be more pronounced than that obtained when other facial areas of the same size were chilled, indicating that the ophthalmic branch of the trigeminus nerve is the main sensory area for initiating bradycardia in humans. It was found that both water temperature and ambient air temperature had significant, but opposite, effects on the magnitude of bradycardia developed at apneic face immersion, and the results indicate that diving bradycardia is negatively correlated to wate temperature within a range that is determined by the ambient air temperature. Furthermore, it was found that there is an increase in BHT over five dives when diving is repeated with short intervals, due to a delay of the the physiologi- cal breaking point (PBP) up to dive three, while to an increased tolerance by the subject up to dive five. It was conclude that an increased diving response is not the cause of the prolonged BHT seen in repeated diving, as has been suggested. A positive correlation between individual BHT and maximum bradycardia at repeated diving was found. There was also a positive correlation between mean bradycardia, skin capillary blood flow reduction and BHT on the group level, with the trained divers showing the most pronounced DR and the longest BHT. In trained divers the increase in diving bradycardia seen during apnea with face immersion compared to during apnea in air was accompanied by an increase in diving time, which was not seen in untrained subjects. Physical training leading to increase in maximal oxygen uptake does not increase the time before the PBP or the diving bradycardia. However, the tolerance to IBM appears to be en- hanced after physical training, which prolongs the BHT. Apnea training leads to a delay of the PBP and an increased diving bradycardia and BHT, and may be an essential factor in dive training. It was concluded that a pronounced DR is not only genetically determined but appears as a result of training and disappears as a result of aging. Taken together, data suggest that the human DR may have an 02 conserving effect in trained divers. The mean DR of trained human divers is in the range of that of the beaver, a semiaquatic mammal, while that of untrained humans is in the range of the pig, a terrestrial species. It is hypothesized, based on the results concerning temperature dependence of the trigemi- nus cold-response in adults, that this response may be involved in the events leading to sudden infant death syndrome.