Aside from a few `deep blue' champion free divers, no one would claim that man distinguishes himself in the category of `diving mammal', yet the classic dive response outlined by Irving and Scholander in the 1940s has been described in both human newborns and adults. The dive response is a suite of reflexes which conserves oxygen for the heart and brain during submergence;these responses include apnea (breath-holding), decreased blood supply to non-essential organs such as the peripheral muscles and gut, decreases in heart rate and cardiac pumping, and an increased dependence on anaerobic metabolism. These reflexes appear to some extent in all diving mammals and birds, with extreme reactions in forced-dived animals. But while the oxygen-conserving benefits of these adaptations are clear for natural divers,and for human newborns (who commonly endure low-oxygen conditions in utero and during the birthing process), the function of the dive response in human adults is less clear. Early studies in the late 1960s and early '70s were contradictory; some demonstrated an oxygen-conserving effect during the apnea and heart rate reduction of breath-hold diving, while other studies did not. The relatively poor diving abilities of humans compared to true diving mammals makes it difficult to determine if changes in heart rate and blood flow in man provide any real oxygen savings.
Andersson and colleagues answer the question of potential oxygen conservation in diving man by augmenting the effects of apnea alone with face immersion in exercising subjects. Earlier work in their lab had already demonstrated that breath-holding with a reduced heart rate led to slightly smaller decreases in arterial oxygen saturation than when apnea was not associated with a change in heart rate. The subjects were not exercising,however, so the changes in oxygen levels were too small to determine any true oxygen savings. In this study, the subjects exercised on stationery cycles to increase oxygen use overall, while breath-holding was augmented with face-immersion. The researchers predicted that the stronger dive response associated with cold-water immersion would more clearly delineate any oxygen conserving benefits of the diving reflex. (As in animal subjects, immersion of the face in cold water enhances the dive response beyond the responses to breath-holding alone, for example, a more exaggerated decrease in heart rate).
While the `dive response' occurred during both apnea and apnea with face immersion, the reflex was clearly greater during immersion, with a significantly larger reduction in heart rate, a greater increase in arterial blood pressure, and most importantly, a smaller reduction in arterial oxygen saturation. Taken together, the authors conclude that the dive response in man does indeed conserve oxygen. The reduced cardiac output and the redistribution of blood flow should preserve lung oxygen at the expense of tissue oxygen stores (though lactate was not measured), and thus delay the development of hypoxia in the heart and brain. It could be speculated, then, that the `diving reflex' is the common vertebrate response to apnea and immersion, which has been greatly extended and enhanced in true diving marine mammals and birds.
