Unraveling the mammalian diving reflex (Part I)

Useful words:

  • Bradycardia: reduced heart rate
  • Residual Volume: air volume remaining in the lungs after exhaling as deeply as possible.
  • Mammalian Diving Reflex: Physiologic responses to immersion that include (a) bradycardia, (b) decreased cardiac output, (c) lactate accumulation in underperfused muscle, (d) increased peripheral vasoconstriction.

When freedivers dive deep they activate a set of physiological reflexes that act as the first line of defense against hypoxia. Collectively termed either the diving response or mammalian diving reflex (MDR), the mechanism controls the shift of blood to the brain and heart, functioning in tandem as an important oxygen-conserving mechanism, not only in freedivers, but also in whales, penguins and seals (Figure 1). In fact, this amazing physiological phenomenon was first observed in deep diving mammals as early as the 1900’s, but until as recently as the 1950’s it was thought that humans did not possess the reflex.??

["Bradycardia Graph" center]

Fig. 1. Bradycardia observed in a diving seal. Data adapted from R.S. Elsner (1998)

Although the mechanisms that control this reflex are only partially understood, it is generally agreed that its success is dependent on certain key factors, all of which have been observed in freedivers. These mechanisms include vasoconstriction, preferential shunting/blood shifts/thoracic filling, and bradycardia, and it is the latter of these that is the focus of this first article.

Upon descent, one of the first observed physiological mechanisms that comprise the MDR is the bradycardial response, which is a scientific way of saying the heart begins to slow down. However, for the deep-diving freediver, it is important to be aware of the various factors that influence its onset, since some of these can be manipulated with training.

The temperature of the water, the physical condition of the athlete, lung volumes, the body position during immersion, the length of the breath-hold, the psychological state of the individual, and the depth of the dive are factors that influence this heart rate response. The age and gender of the athlete are also variables, but are slightly more contentious, and less studied so we’ll leave those alone!??

Water temperature If you happen to be a freediver living in the frigid conditions of either the UK or Canada, and are considering attempting a personal best dive, should you spend money and travel to the Caymans or will you achieve a similar result at home? Well, the studies that examined the degree of bradycardia in summer and winter agree that the bradycardiac response is inversely proportional to the water temperature. In other words, the magnitude of breath-hold bradycardia is significantly greater in the winter than in the summer.

Lung Volumes This is a gray area, but some studies have shown an inverse relationship between heart rate and lung volume. Perhaps of more importance is whether the freediver is exhaling or inhaling, since it is known that diving mammals dive in the expiratory position: seals, for example, exhale before diving. It seems that by doing this they accentuate the bradycardial response?? – as well as avoiding the bends: but that is another topic of discussion. In humans however, this has not been reliably demonstrated, although there is agreement that breath-holding at residual volume tends to increase rather than decrease heart rate.

Exercise The mechanism that controls bradycardia is a strong one and is a difficult one for the body to override, although an exact physiological explanation for this is still obscure. Even during the extreme physical exertion observed in elite freedivers, bradycardia has persisted, leading in some cases to a reduction in heart rate of more than 50%. There are changes to the response that are determined by the type of exercise being performed but due to problems associated with collecting data during a typical descent, these are poorly understood, although during static apnea the bradycardial response tends to be much lower than when performing a deep dive. The mechanisms that contribute to these responses, such as preferential shunting and vasoconstriction, are alluded to in the second part of this article