Triggering of the diving response

THE TRIGGERING OF THE HUMAN DIVING RESPONSE

Erika Schagatay, Johan Andersson and Boris Holm
Department of Animal Physiology, University of Lund, Sweden

Originally published in: Gennser, M. (ed.) XXIV Annual Scientific Meeting of the European Underwater and Baromedical Society. National Defence Research Establishment, Stockholm, Sweden. FOA Report: FOA-B--98-00342--721--SE: 88-91.

INTRODUCTION

A series of cardiovascular and respiratory adaptations permit aquatic air breathers to leave the surface for extended periods. Among them is the ”diving response”, which is shared by man. This adaptation, consisting of selective vasoconstriction and bradycardia, serves to limit over all oxygen consumption and protect the heart and brain from asphyxia (2). In humans the response is induced by breath-holding while immersing the face in cold water. The magnitude of heart rate reduction is an often used measure of the magnitude of the diving response. Trained human divers exhibit a bradycardia of 40-50%, which is in the range of the responses found in some semiaquatic mammals. As the response in man has often been reported to reach its maximum in 10°C of water, it has been concluded that the diving response in humans is not effective in diving, as most apneic diving occurs in relatively warm waters (4). The face has been recognized as the area involved in the triggering of the response, but the conclusions concerning the location of the cold receptors involved have varied. While chilling of the face evokes a parasympathetic effect on the heart, chilling of other areas of the body are known to trigger tachycardia (3). The study of the interactions between the diving response and other cardiovascular reflexes is limited. The aims of the studies reviewed here were to further investigate the neural mechanisms responsible for triggering the human diving response, specifically I) the influence of air and water temperature (5), II) the location of the facial cold receptors involved (7) and III) the interaction between the diving response and the response to chilling of the arm (6). A general aim was to make an evaluation of whether the response is triggered in the human apneic diver.

METHODS

Seventy healthy volunteers participated in these ethically approved studies. Experi-ments were performed with the subjects in a prone position, and the orders of the various exposures were randomized in all tests. The diving response was elicited by apnea and facial chilling by cold water bags or by immersion, sometimes in combination with arm immersion. Cardiovascular and respiratory parameters were continuously recorded non-invasively. To evaluate the magnitude of the diving response the main parameter was the reduction of the heart rate during apnea from the heart rate obtained prior to each apnea.

Study I) Twenty three volunteers were exposed for 1 h to 10, 20 and 30°C ambient air, after which they performed apnea and immersed the face in 10, 20 and 30°C of water in the climatic chamber.

Study II) The faces of 22 subjects were divided into six areas of equal size (Fig 2a.), which were chilled with cold water bags while the subjects performed apnea. In addition, apnea without immersion and apnea with full immersion of the face in cold water were performed.

Study III) While snorkel breathing, nine subjects performed immersions in 10°C water of 1) the arm, 2) the face 3) the arm and the face. Sixteen other subjects performed the same proceedures during simultaneous apnea.

RESULTS

Study I) Both air and water temperatures influenced the magnitude of the diving response developed. In 10 and 20°C ambient air, there were significant cold responses at face immersion in 10°C of water, but not at immersion in 20 or 30°C of water. However, in 30°C ambient air both 10 and 20°C triggered significant cold responses (Fig 1., adapted from 5).

Fig 1. Relationship between ambient air and water temperature in the triggering of the diving response.

Study II) When the reduction of the heart rate for chilling of each area was com-pared to the heart rate reduction at apnea alone, the only significant increase in heart rate reduction was found when the forehead or eye regions were chilled. When the responses from the forehead and eyes were added together they were equivalent to the response obtained when the whole face was immersed (Fig 2a., adapted from 7).

Study III) During breathing, simultaneous immersion of the face and arm in cold water resulted in a small tachycardia. During apnea, however, chilling of the arm had no effect on the magnitude of the bradycardia obtained when the face was immersed into cold water (Fig 3., adapted from 6).

Fig 3. Heart rate responses to chilling of face or arm or both, at eupnea and apnea.

CONCLUSIONS

We concluded that: I) The human diving response will be triggered also in a warm environment, as long as the water is colder than the ambient air. Thus the tropical diver in warm water is likely to develop a diving response if the ambient air to which the head is exposed between dives is warmer than the water. II) The elicitation of the diving response by cold stimulation is derived from the upper part of the face innervated by the ophthalmic part of the trigeminal nerve (forehead and eye area). This will ensure that the response is triggered during full facial immersion (diving, Fig 2c.) but not during swimming (Fig 2b). III) During breathing, the influence from cold stimulation of the extremity will dominate over the influence from the stimulation from the upper face, resulting in tachycardia, while during apnea the influence from the upper face will abolish the influence from the arm and produce bradycardia.This suggests that the diving response is triggered at apneic diving even with simultaneous body chilling.

The three studies indicate that the sensory inputs serve well for triggering of the diving response during human apneic diving and that this possibly oxygen con-serving response (1) may dominate over other cardiovascular reflexes during apnea.

REFERENCES

1) Andersson, J. and E.Schagatay (1998). Effects of the human diving response on oxygen consumption. This volume.
2) Elsner, R. and B. Gooden (1983). Diving and asphyxia: a comparative study of animals and man. Physiol. Society monograph 40. Cambridge Univ. Press, Cambridge.
3) Frey, M.A.B., E.A. Selm and J.W Walther (1980). Reflex cardiovascular responses to cold exposure of the face or foot. Jpn. Heart J. 21: 665-679.
4) Mukhtar, M.R. and J.M. Patrick (1986). Ventilatory drive during face immersion in man. J. Physiol. 370: 13-24.
5) Schagatay, E. and B. Holm (1996). The effects of water and ambient air temperatures on human diving bradycardia. Eur. J. Appl. Occup. Physiol. 73: 1-6.
6) Schagatay, E., J. Andersson and B. Holm (in press). The conflicting stimuli of chilling the face and the forearm on cardiovascular regulation. Proceedings from the International Symposium on Problems with Cold Work, Nov. 16-20, 1997, Stockholm, Sweden.
7) Schuitema, K.E. and B. Holm (1988). The role of different facial areas in eliciting human diving bradycardia. Acta Physiol. Scand. 132: 119-120.

Published by courtesy of "European Underwater and Baromedical Society".

Return to "Publications"