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    1. Background: what is a diving response?
    The diving response

    The diving response is well known from diving mammals such as seals and is considered to prolong diving by restricting oxygen consumption in tissues resistant to asphyxia. Thereby, oxygen is reserved for the most sensitive organs, the heart and brain. The response has two major features: a restricted blood flow to certain organs and a lowered heart rate. Our studies have focused on the following questions. Do humans have a diving response? How is this response triggered? How does it work? What is the adaptive value of the response? How does it compare with the response of other mammals? Thus, the general field of interest within our group is cardiovascular and respiratory regulation and function.

    Our research group
    Our research started in 1988 with the development of a laboratory and a model for studying the control and function of the human diving response. Methods were set up for the simultaneous and continuous recording of cardiovascular and respiratory parameters, including fast transient changes in heart rate, blood pressure, arterial and capillary blood flow, temperatures, and thoracic movements. This field of research was new at our department, and the development of the methods was based on our international contacts. The methods have been improved throughout the years. Our aim was to elucidate the control of the human diving response, especially the effects of temperature and different types of training on the response.

    2. Temperature and initiation
    Where are the receptors located?
    In previous studies, it had been established that apnea in combination with face immersion in cold water results in a diving response with approximately twice the magnitude of that obtained from apnea alone. The neural input for this augmented response had been discussed, some investigators assigning a role to water receptors in the area around the nostrils. No such receptors have been found. However, in our studies it was found that skin cold receptors are responsible for the augmentation of the response during immersion. By studying the diving response during apnea when individual parts of the face were stimulated by plastic bags containing cold water , the location of the cold receptors involved in the triggering of the response in humans could be revealed.
    It was found that stimulation of the forehead or eye region during apnea induced a diving response significantly greater than the response resulting from apnea alone. Taken together, the effects from chilling these two areas was equivalent to the effect from chilling the entire face. Chilling of other facial areas did not increase the response from that obtained during apnea alone. This indicates that the area innervated by the ophthalmic branch of the trigeminus nerve, e.g. the forehead and eye region, provides the most important sensory input for the augmented diving response elicited by face immersion. This location of the cold receptors involved implies that a mask covering the forehead and eyes, as is often used in modern breath-hold diving, will reduce the diving response towards that elicited by apnea alone. A suitable breath-hold diving mask should thus cover as little as possible of the forehead, just as the goggles used by traditional divers.

    Do tropical divers have a diving response?
    In several studies, it has been found that the temperature of the water is an important determinant of the magnitude of the diving response triggered. Water holding a temperature of about 10 degrees Centigrade has often been found most efficient. This has lead to the conclusion that the diving response will not be efficiently triggered in a tropical diver. Yet human diving is most likely to occur in warm water!
    In experiments using subjects acclimated to different ambient temperatures, we have found that both water temperature and ambient air temperature have significant, but opposite, effects on the magnitude of bradycardia developed during apneic face immersion. These results indicate that the diving response is negatively correlated to water temperature within a range that is determined by the ambient air temperature. This means that, in the range of temperatures most likely to be encountered by breath hold divers, the warmer the ambient air and the colder the water, the stronger the diving response will be.
    Thus, within a range of temperatures, the difference in temperature between air and water seems to be more important than the temperatures per se. This information should encourage the diver to keep his body warm and, thereby, keep the peripheral blood flow high between dives permitting a fast recovery of the face skin temperature. The mechanisms most likely to be responsible for this temperature effect were identified as 1) the relative vasoconstriction in cold ambient air and 2) the involvement of the dynamic cold receptor response in initiating the diving response.

    3. Does the human diving response conserve oxygen?

    The oxygen-conserving effect of the human diving response has been a matter of debate. Some studies do assign such an effect to the response, while others do not. In our studies we have found several results that indicate that an oxygen-conserving effect is present. The human diving response is therefore suggested to have an oxygen-conserving effect through reductions in cardiac output and peripheral blood flow during apnea. The apneic alveolar-to-capillary oxygen uptake is reduced compared to during normal breathing, due to the circulatory adjustments. Thereby, the lung oxygen store is preserved for vital organs during apnea, making longer apneic durations possible with an augmented diving response. In addition, especially during apneic exercise, the anaerobic metabolic rate may increase as a consequence of reduced peripheral oxygen delivery due to the diving response.

    We investigated the diving response and apneic time in nine groups of divers and non-divers. A positive correlation was found between the diving response and apneic time, with the trained divers showing the most pronounced diving response and the longest apneic time. Apneas were done both in air and with face immersion. Trained divers with a powerful diving response were found to prolong their maximal apneic time in water, when their diving response was most efficiently triggered.

    We also compared apneas with a given duration, with and without face immersion, with respect to arterial hemoglobin oxygen saturation, oxygen uptake from the lung, and lactic acid concentration in the blood. Experiments were performed on both resting and exercising subjects. We found that the arterial hemoglobin was more saturated after apneas with face immersion, when the diving response was more pronounced. Also, during apneas with face immersion, the depletion of the lung oxygen store was smaller than during apneas without face immersion. During apneas without face immersion, when the diving response was not fully developed, more oxygen had apparently been used. In exercising subjects, higher levels of lactic acid in the blood accompanied the more pronounced diving response during apneas with face immersion. This indicates that the anaerobic metabolic rate was higher with a more pronounced diving response. Thus, our data favor the view that an efficient, oxygen conserving diving response is present in man, as in diving mammals.

    4. Training studies

    Short-term training by repeated apneas
    In both diving mammals and naturally diving humans, dives are performed in series with the duration of apneas and surface intervals adapted to the intended working depth. In humans, repeated apneas with short (less than 10 min) intervals have been shown to prolong apneic time. The mechanisms causing this "short-term training effect" are poorly understood. Previously, it has been suggested that the increased apneic time with repeated apneas is caused by an increased diving response, a progressive hyperventilation or an increased inspired lung volume throughout the series.

    In our studies, we have measured the duration of the period before the physiological breaking point. At the physiological breaking point, involuntary breathing movements are triggered by a high arterial carbon dioxide tension. We have found that both physiological and psychological factors contribute to the prolongation of apneas during short term training. Physiological factors contribute during the first three apneas, whereas psychological tolerance contributes until apneas 5 to 7. In our studies, neither an increased diving response, nor an increased inspired lung volume, could explain the physiological contribution to the increased apneic time during repeated apneas. Moreover, hyperventilation alone was not found to be responsible. We have shown that splenic contraction is one physiological factor involved in the short-term training effect in humans. During apnea, splenic contraction thus injects large quantities of the previously sequestered erythrocytes into the circulation. Increased blood gas storage capacity following splenic emptying and a facilitated recovery from apneas may be the involved mechanisms (see below).

    Long term training
    Why do trained divers have such a powerful diving response? Is this trait genetically determined or an effect of their daily training? If the trait is caused by training, what part of the training is responsible? To answer these questions, several studies concerning the effects of general and specific training regimes on the diving response were made in our laboratory. Longitudinal effects of general physical training and apnea training were studied in different groups of subjects. We found that physical training, leading to an increased maximal oxygen uptake and decreased resting heart rate, does not increase the diving bradycardia during simulated diving. The time period before the physiological breaking point, e.g. before involuntary breathing movements are triggered, was also unchanged after physical training. However, the apneic duration was prolonged by an increased duration of the phase after the physiological breaking point. This indicates that the psychological tolerance to apnea appears to be enhanced after physical training.

    Apnea training, on the other hand, was found to increase the diving response and prolong apneic time by postponing the physiological breaking point. This indicates that factors associated with the production or accumulation of carbon dioxide are affected by apnea training or that the response to a given stimulus by the chemoreceptors is altered. The results suggest that apnea training may be an essential factor in breath-hold diving training. After apneic training, the dive becomes not only longer, but also easier!

    5. Splenic contraction

    In the Weddell seal and other seal species, contraction of the spleen during diving releases stored red blood cells into the circulation. This will, in addition to immediately increasing the available oxygen stores, facilitate the uptake of oxygen and release of carbon dioxide during brief surface intervals between dives. Although the function of the human spleen as a red blood cell reservoir has been doubted, the spleen serves as a dynamic red blood cell reservoir in several other terrestrial mammals, e.g., dogs, cats, pigs, sheep, goats, and horses.

    The specific objectives of one of our investigations were to elucidate whether splenic contraction is a part of the human diving response and if it is involved in the short-term training effect of repeated apneas. Twenty healthy volunteers participated in the study. Ten subjects had previously been splenectomized due to various medical reasons. All subjects performed five maximal-duration apneas with face immersion in 10 degree Centigrade water, with 2-min intervals. In subjects with spleens, hematocrit and hemoglobin concentration increased over the serial apneas and returned to baseline 10 min after the series. A delay of the physiological breaking point of apnea was also seen in this group. Neither increases of hematocrit and hemoglobin concentration nor a delay of the physiological breaking point of apnea was observed the splenectomized group. Serial apneas thus triggered the hematological changes that have been previously observed in diving seals, the changes were rapidly reversed and did not occur in splenectomized subjects. This suggests that splenic contraction occurs in humans as a part of the diving response and splenic emptying may prolong repeated apneas.

    6. The "diving response" in pigs
    Knowledge of the diving response in mammals has, to a large extent, been obtained from studies of diving mammals, mostly seals. It is interesting to evaluate the human diving response in a mammalian perspective, especially compared with that of terrestrial mammals. The response has been shown to be present also in dogs and rats. Studies in our laboratory have shown that it is possible to train pigs to voluntarily perform apneas and snout immersion, and that they respond with a diving response involving a reduction of their heart rate and skin blood flow. In humans, the magnitude of the diving bradycardia varies between 15-30 % for untrained persons, which corresponds to the response in these trained pigs, and between 30-50 % for trained divers, which is comparable to the response found in many semi-aquatic mammals. Thus, among humans, non-divers react like pigs, while apneic divers respond like beavers!