Nick Pelios
Freediver, Creator
Share this on
Nick Pelios
Freediver, Creator
As freedivers, we need to learn the best use of our body oxygen and the best way of doing it, so the mammalian dive response is one of the keys to be understood. Rafael Nolasco explains.
For those starting freediving, it is essential to understand that progression in this sport is not different from any other. This means you need to learn the principles and their foundation to progress. From the experiences of some freediving friends and myself, I understood that physiology is the most important fact to learn before moving to another level. You may ask why, the reason being is because you need to understand the changes your body would go through, like the transition it will do to achieve the mammalian dive response. Although I knew that freediving physiology was a critical tool for progressing, I should have spent more time to fully understand it. When I was coached by one of the top freedivers in the world, I was told to start from zero and learn more about the MDR. I did not think it was so important, but it did help me a lot with my freediving journey.
The mammalian dive response involves a considerable decrease in heart rate during submersion compared to breathing at the surface for marine animals. The same can be said for humans as we also have MDR. It is the body's physiological response to being submerged in cold water or breath-hold, which involves shutting down body sections to preserve oxygen consumption. The mammalian dive response is a critical physiological mechanism for maximizing underwater excursions. The oxygen-conserving reaction comprises three metabolic and cardiovascular changes: noticeable bradycardia, selective peripheral vasoconstriction, and enhanced blood flow to the viscera.
Freediving elicits a mammalian response in all air-breathing vertebrates, including peripheral vasoconstriction due to sympathetic activity, which is associated with initial hypertension, and a vagally caused bradycardia with a decrease in cardiac output. Cooling of the face and/or hypoxia amplify these circulatory alterations. In addition, apnea has been shown to increase peripheral vascular resistance in freediving with proportionally significant bradycardia and decreased cardiac output.
Bradycardia is a reflex response to freediving. However, there have been observations that the increase in blood pressure precedes the slowing of the heart rate, suggesting that baroreflex induction and chemoreceptor stimulation from hypoxia during the later part of the breath-hold may play a role in the development of bradycardia. The diving reaction is thought to induce blood and lung oxygen reserves to be transported preferentially to the heart and brain. The impact of breath-hold dives on increasing the hemoglobin content of circulating blood through the splenic contraction, which happens early in the diving-response cascade, precedes the bradycardia, has gotten much attention in recent years.
According to an article in the journal of physiology, it states that; one research reported higher hemoglobin levels in breath-hold divers than in non-divers, suggesting that the observed 24% rise in erythropoietin levels may cause long-term elevation of hemoglobin levels in breath-hold divers. In another investigation, top breath-hold divers were found to have average hemoglobin concentrations and total hemoglobin mass, according to the journal of physiology. To the extent that breath-hold divers have higher hemoglobin levels, it could be the same as in people with obstructive sleep apnea, where blood changes correlate with the severity of hypoxia during sleep, despite the noticeable quantitative differences in hypoxia exposure between the two groups.
When I teach freediving, I explain how to induce the mammalian dive response and how rapidly it may be activated when your brain detects that your body needs to be switched to safe mode. For example, to demonstrate how quickly my heart rate would be lowering, I would fully exhale and hold my breath while utilizing an oxygen saturation device. Another approach demonstrated was submerging my face in a bowl of cold water, which efficiently induced bradycardia in a few seconds.
The mammalian dive response is highly variable among humans during rest and exercise. Systematic differences also exist depending on age and presence or lack of diving experience. The diving bradycardia is quite pronounced in children 4–12 months and may have survival value during hypoxic episodes proximal to birth. The diving response weakens with advancing age and is more marked among habitual breath-hold divers than non-divers. The diving response is relatively more pronounced during exercise than during rest.
The combination of bradycardia and cardiac arrhythmias shown in the human diving response is owing to vagal inhibition of atrioventricular conduction mixed with sympathetically induced augmentation of automaticity in other latent pacemakers permissive to ectopic beats. If you want to hold your breath longer, you must master your mammalian dive response.