Diving bradycardia: a mechanism of defence against hypoxic damage

Abstract

A feature of all air-breathing vertebrates, diving bradycardia is triggered by apnoea and accentuated by immersion of the face or whole body in cold water. Very little is known about the afferents of diving bradycardia, whereas the efferent part of the reflex circuit is constituted by the cardiac vagal fibres. Diving bradycardia is associated with vasoconstriction of selected vascular beds and a reduction in cardiac output. The diving response appears to be more pronounced in mammals than in birds. In humans, the bradycardic response to diving varies greatly from person to person; the reduction in heart rate generally ranges from 15 to 40%, but a small proportion of healthy individuals can develop bradycardia below 20 beats/min. During prolonged dives, bradycardia becomes more pronounced because of activation of the peripheral chemoreceptors by a reduction in the arterial partial pressure of oxygen (O2), responsible for slowing of heart rate. The vasoconstriction is associated with a redistribution of the blood flow, which saves O2 for the O2-sensitive organs, such as the heart and brain. The results of several investigations carried out both in animals and in humans show that the diving response has an O2-conserving effect, both during exercise and at rest, thus lengthening the time to the onset of serious hypoxic damage. The diving response can therefore be regarded as an important defence mechanism for the organism.

Apnea Training Specificity and its Implications for Performance in Aquatic Sports: Case Study Reports

“…These findings contradict what the diving response would have predicted i.e. brady cardia [43], and an elevated blood pressure.…”

The dive response redefined: underwater behavior influences cardiac variability in freely diving dolphins

“…These original landmark studies remain the cornerstone in our understanding of the cardiovascular adjustments required for conserving oxygen and prolonging the duration of submergence by air-breathing vertebrates. In general, the physiological response to submersion is qualitatively similar for aquatically adapted and terrestrial mammals, with diving-induced bradycardia considered to be the major mechanism to regulate blood oxygen depletion rate, thereby conserving oxygen for the brain and heart (Harrison and Tomlinson, 1960;Scholander, 1963;Van Citters et al, 1965;Ridgway et al, 1975;Alboni et al, 2011).…”

Pathophysiology of Drowning

“…In these animals, it has been demonstrated that both apnea alone and facial immersion alone can induce vagal activity, but the combined situation enhances the effects [ 12 ]. Within 10-40 s there is selective vasoconstriction with less perfusion in the peripheral vascular beds of skin, muscles, and viscera.…”