Babies

In humans, the intensity of the response to the diving reflex decreases dramatically with age. It appears early in development as a safeguard against hypoxia during birth, and then is thought to almost disappear after 6 months of age (Panneton et al., 2010). However a study conducted on infants found that although there is a slight decline in the bradycardia reflex with increasing age, infants over 6 months of age still retained the diving reflex(Rosengren et al., 2002).

This heightened stage of the diving reflex in babies is potentially linked to sudden infant death syndrome. Though most often attributed to a lack of musculature, it may be the override of homeostatic reflexes that contributes to death because the babies do not attempt to breath despite changes in their blood chemistry (Dahms et al., 2010,).

Fig.2 from (Rosengren et al., 2002) with permission from the author. Mean heart response while diving in infants of up to 12 months of ages. Greatest response seen in youngest age group.

Drowning

Most terrestrial mammals, including humans, are very sensitive to changes in oxygen. After 3-4 minutes of not breathing, suffocation leads to severe hypoxia that results in a loss of consciousness within seconds (Gooden, 1992). However, the diving reflex provides some protection to the central nervous system against hypoxia. Bradycardia prevents an excessive rise in blood pressure, thus decreasing the oxygen necessity. Vasoconstriction is also actively allocating blood flow to tissues most sensitive to oxygen deprivation such as the brain (Sarnaik and Vohra, 1986).

The diving reflex also interacts with hyperthermia in many instances of near drowning. Case studies highlight young children who fall in into cold water and emerge without substantial harm after being submerged for as long as 2 hours. It is debatable how much of this success is due to the diving reflex or to hypothermia, but there is a definite overlap in the responses. During the first 10 minutes, the diving reflex is thought to protect the brain from hypoxia by redirecting oxygen stores and reducing cardiac consumption. After this point, the effects of hypothermia have a greater impact. Survival depends upon on a specific temperature that balances these two responses (Gooden, 1992).

Fig. 4 from (Gooden, 1992) with permission from the author. Hypothetical interplay between the diving reflex and passive hypothermia over time, and the effect upon metabolic rate.

An understanding of the effects of the diving reflex is also essential to the rescue of drown victims. An imperceptible pulse from a combination of bradycardia and peripheral vasoconstriction could make the victim appear dead even when resuscitation could ensure survival (Sarnaik and Vohra, 1986).