1. Safety of Breath holding
Patrick McKeown

2. During a maximum breath hold, the partial pressure of oxygen decreases in the blood, which causes the body to conserve any available oxygen for the heart and brain by constricting the blood vessels that supply non-essential organs.

3. For example, your arms and legs may feel cold as the blood vessels close and the body diverts blood away from them. Another effect is bradycardia, which is the slowing of the heart, causing peripheral blood vessel to constrict, blood pressure to increase, and the spleen to contract, resulting in the ‘diving response’.

4. The diving response is experienced by all air-breathing vertebrates, and is an automatic response to a drop in oxygen supply. It is what allows babies and young children to instinctively hold their breath when underwater, and is generally more pronounced in adults who regularly practise breath holding

5. During breath holding, the arterial partial pressure of oxygen decreases from its normal level (100 mmHg), while carbon dioxide increases to above its normal level (40 mmHg).

6. ‘Breaking point’, or the point at which an individual must release a breath hold, is when oxygen decreases to 62 mmHg and carbon dioxide is 54 mmHg.

7. While it is extremely difficult for adults to hold their breath to the point of fainting, it has been estimated that consciousness is lost when the oxygen drops to below 27 mmHg and carbon dioxide increases to between 90 and 120 mmHg.5

8. The body uses inbuilt safety mechanisms like the diving response and fainting to ensure that we do not deprive the brain of oxygen for too long, as doing so can result in brain injury.

9. Although breath hold training increases tolerance of carbon dioxide, it is interesting to note that it does not blunt the brain’s safety reaction to oxygen deprivation.

10. This is where deliberate breath hold exercises differ vastly from the physiological condition of sleep apnoea, where the breath is held unintentionally during sleep, sometimes leading to severe health problems and even death.

11. Research by Ivancev et al
Research  by Ivancev et al. looked at the breath holding ability and carbon dioxide sensitivity of breath hold divers, whose sport potentially puts them at risk of severe oxygen deprivation. With repeated practise, these divers are able to sustain very long breath holds, inducing a severe drop in oxygen without causing brain injury or blackouts.  
Ivancev V, Palada I, Valic Z, Obad A, Bakovic D, Dietz NM, Joyner MJ, Dujic Z Cerebrovascular reactivity to hypercapnia is unimpaired in breath-hold divers.. J Physiol Jul 15;582(Pt 2): Epub 2007 Apr 5.

12. Ivancev et al. tested the hypothesis that repeated breath holds, which are an integral part of breath hold diving, blunt cerebrovascular reactivity to hypercapnia. The study noted that breath hold divers had a greater tolerance to carbon dioxide, largely the result of lower breathing frequency.
Ivancev V, Palada I, Valic Z, Obad A, Bakovic D, Dietz NM, Joyner MJ, Dujic Z Cerebrovascular reactivity to hypercapnia is unimpaired in breath-hold divers.. J Physiol Jul 15;582(Pt 2): Epub 2007 Apr 5.