The human diving response in a functional and comparative perspective
Erika Schagatay, Associate Professor
Department of Natural and Environmental Sciences
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.
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.
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.
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.
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.
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