The
human diving response in a functional and comparative perspective
Erika Schagatay, Associate
Professor
Department of Natural and
Environmental Sciences
Mid
e-mail: Erika.Schagatay@mh.se
Introduction
The diving response is well
known from diving mammal (Elsner and Gooden 1983; Kooyman 1989). It leads to a
redistribution of blood flow assuring the brain and heart a constant supply of
oxygen, leading to a longer apneic duration without the risk of asphyxia. This
is acheived by constriction of blood vessels and reduced oxygen consumption in
more tolerant tissues and a drop in heart rate. The diving response is also
present in humans, where apnea and facial chilling e.g. by immersion are the
essential stimuli for eliciting the response (Gooden 1994).
The diving response occurs
to some extent in all mammals tested, showing that it is an old general defence
mechanism against hypoxia (Elsner and Gooden 1983), however, the number of
terrestrial species tested in voluntary dives is small. The magnitude of the
response is greater in diving than in terrestrial species. In seals, the
response may reduce the heart rate by up to 80% (Kooyman 1989).
It has been demonstrated
that human subjects untrained in apneic diving react with a heart rate
reduction of between 20 and 30% from the resting level, at diving or apnea with
face immersion in cold water (Schagatay and Andersson 1998). The levels of
heart rate reduction and vasoconstriction of these untrained subjects have
sometimes been considered to be to weak to prolong
apneic duration (Lin and Hong 1996). Findings of a strong temperature
dependence of the human diving response have lead to the assumption that the
response may not be triggered in a tropical diver (Mukhtar and Patrick 1986).
We therefore aimed to evaluate the human diving response with respect to its
possible oxygen conserving function in untrained and trained divers, and to
study whether the response is triggered in the natural, tropical diver.
Methods
In a series of reports from
our laboratory we studied the diving response in trained apneic divers, as well
as in non-divers, using simulated dives by apnea and facial immersion during
prone rest (Fig 1). This model allows a high level of control over the imposed
stimuli, permitting comparisons between studied conditions, individuals and
groups. A similar method was developed to evaluate the diving response in pigs
(Fig 2). The studies reviewed here contain data from 254 human subjects and
four pigs.
Figure
1. Method used for simulated dives by
apnea and facial immersion during prone rest.
Figure
2. Pig
trained to perform apnea with snout immersion.
Results
Our results have shown
that, while the diving response magnitude is 20-30% in untrained subjects, it
is 40-50% in trained divers (Schagatay 1991, Schagatay and Andersson 1998).
We have further found that,
while apneic duration is largely determined by psychological factors in
untrained humans, the diving response prolongs maximal apneic duration in
trained apneic divers (Schagatay and Andersson 1998). In accordance with this
finding was the observation that the arterial oxygen saturation declined less
after an apnea with a powerful diving response (during apnea with face
immersion), than after an apnea with a weak diving response (during apnea;
Andersson and Schagatay 1998). Concerning whether or not the response may be
efficiently triggered in the diver, we found that the neural imput is derived
from chilling of the upper part of the face, innervated by the ophthalmic
region of the trigeminal nerve (Schagatay and Holm 1988). This shows that the
response will be triggered during diving but not during swimming. We also found
that its magnitude will depend on the difference between ambient and water
(i.e. skin) temperature (Schagatay and Holm 1996), thus the response will be
elicited also by warm water if the ambient air is warmer. Further, the
individual diving response can be increased in untrained individuals by apnea
training (Schagatay el al 2000). We also found that when simultaneous, opposing
thermal stimuli occur, the diving response overrules other thermal response
during apnea (Andersson et al 2000). Finally, when comparing the human diving
response in untrained and apnea trained subjects to the response in pigs
trained to perform apnea and snout immersion (Fig 1), the trained pigs
responded with bradycardia of 31%, i.e. in the range of untrained humans
(Schagatay and van Kampen 1995), while the response in the trained humans was
in the range of that of semiaquatic mammals like beavers and otters.
Conclusions
It was originally stated by
Hardy (1977) that the human diving response may be a remnant of an aquatic
past. The response had then been described for aquatic mammals, and also
observed in man. Many studies have since then increased our understanding of
the function of this mechanism in humans. The studies reported here have shown
that the diving response facilitates apneic diving, is triggered during natural
apneic diving also in a tropical environment, and that it can be trained to
allow for prolonged diving. Whatever its origin, it makes modern humans
physiologically well suited for apneic diving. From a biological point of view,
it is a plausible conclusion that evolutionary pressure during a semiaquatic
phase of early evolution has favoured these adaptations.
Acknowledgements
I wish to thank all the
human volunteers of these studies and also the pigs, which did not volunteer
but clearly enjoyed the diving practice. I also thank Dr Boris Holm, Dr Johan
Andersson and the other co-workers of the studies of our laboratory cited here.
References
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