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BREATH-HOLDING ON PURE O2 – HOW THE CO2/O2 BALANCE AND THE ONCOMING OF CONTRACTIONS IS AFFECTED

by Sina Schieweck

Table of Content
1. Introduction
2. Breathing physiology – short repetition
3. Self test on pure O2
4. Additional influences on breath holds on pure O2
5. Questionable theories in the freediving world
6. Resumee

1.Introduction

Freedivers normally perform breath holds after breathing the surrounding atmospheric air, which consists of 21% Oxygen and 79% Nitrogen.
The Oxygen diffuses through the Alveoli membranes into our blood stream, where most of it bonds to haemoglobin and the smaller amount gets in suspension with the blood plasma. The Nitrogen in the inhaled air has no physiologic use for our body.
When the O2 gets used by our Metabolism for example in the muscles, the waste product is CO2. The level of CO2 is monitored by the medulla oblongata (the respiratory center of our brain, that’s analyzing the concentration of different substances in the cerebral fluid that are connected to the concentrations in the blood stream) which gives us the urge to breathe, as soon as the CO2 concentration exceeds a certain level. During Freediving training we try to build up the tolerance of our body to high levels of CO2 to increase this level.< br/>> The world record of static apnea is 10 minutes 12 seconds, held by Tom Sietas. He also is the world record holder of holding the breath after breathing pure oxygen (100% O2), which is 16minutes and 14 seconds.
Since the common opinion in the freediving world is, that the only known trigger of the urge to breathe is the CO2 concentration in the blood, we have to think about the fact, that nothing changed in the production of CO2 because of breathing pure Oxygen.
As a result of this the breath hold shouldn’t be longer than the breath hold on normal air. To be more precise: the urge to breath should occur at the same time on a breath hold on pure oxygen as is does on a atmospheric air breath hold.
Therefore the physiological differences in breath holds on normal air and pure oxygen will be compared.

2.Breathing physiology – short repetition

Diffusion is the main principle of the gas exchange in our lungs.
It is defined as “The natural tendency of a gas to move from an area of high concentration to an area of low concentration”.
This means that the oxygen in the inhaled air will diffuse through the alveoli membrane, because the concentration in the blood is lower than in the air in the lungs at that moment. On the other hand the carbon dioxide will diffuse from the blood , the place of higher concentration into the air in the lungs, the place of lower concentration.
If we had a perfect system and could wait for a long enough period of time, the process of diffusion would go on until the concentrations on both sides of the membrane are the same. If we now have a look at the normal composition of inspired and expired air compared to pure oxygen, we see that only 4,5% oxygen is actually used during a normal breath.
Additionally 4,0% carbon dioxide is expired together with 0,5% water which is another waste product of the breathing process.

	Inspired	Expired	Pure oxygen
Nitrogen	79%	79.0%	0%
Oxygen	21%	16.5%	100%
Carbon dioxide	0%	4.0%	0%
Water vapor	0%	0.5%	0%

Since the atmospheric air contains only 21% oxygen the amount of oxygen that theoretically can be used while breathing pure O2 is nearly 5 times more. This would lead to the assumption that the breath-hold-time should be 5 times longer on pure O2 than on atmospheric air (if we set the same diffusion rate ), because of the higher amount of oxygen that is available for diffusion if oxygen was the limiting factor.
The limiting factor for a breath hold in athletes (and humans in general) is the urge to breathe, which at some point can’t be consciously resisted any more (the person starts to breathe) or will lead to a black out (loss of consciousness) due to lack of oxygen in the brain, if the concentration of CO2 exceeds a certain level.
This level is measured by the medulla oblongata – the respiratory center which is located in our brain stem. Special nerve cells, so called chemoreceptors continuously measure the ppCO2 in the arterial blood and the cerebral fluid.
The normal breathing-stimulus is reached at ppCO2=0.04bar which can build up to an invincible-strong breathing-stimulus at ppCO2=0.08bar.
This ppCO2s are the inducing pressures in untrained humans.

The chemoreceptors are “sensitive to acid (hydrogen ions). When blood laden with carbon dioxide reaches the brain, the medulla sends nerve impulses to the diaphragm and other respiratory muscles, making them contract and causing inspiration and the subsequent removal of carbon dioxide with expiration.”
The sensibility to acid additional to the ppCO2 is an interesting point that will be considered in the possible explanations of the differences in breath holds with and without pure oxygen.
Due to Henrys Law “The concentration of a gas that is dissolved in a liquid is proportional to the partial pressure of that gas in contact with the liquid”.
This means that a higher partial pressure of oxygen in the lungs will result in a greater concentration of oxygen in the blood plasma. At normal pressure and temperature only very little oxygen is dissolved.
The percentage of bonding to haemoglobin is 97,5% at the normal PP of 0,21, which is assumed to be fully saturated.
Even if a higher saturation could be achieved, it can therefore only be increased by 2,5% to 100% saturation at a PP of 1.

3.Self-Test on pure O2

In theory – considering the facts we reviewed in the short repetition – the urge to breathe should occur at the same time during a breath hold on pure O2 as it does on atmospheric air.
In a self test with three freedivers doing several statics on pure O2 over two days we found out, that initially the contractions start at the same time they would start on a normal static. The breath hold time slightly increased anyways on pure O2.
The reasons for that could be psychological – the person knows, that he has stored unusual high amounts of oxygen and therefore is able to cope more easily with the contractions. On the other hand were the contractions described as “not as bad” as on a normal static..
After a very short period of time the breath hold times progressively increased significantly and the oncoming of contractions delayed to a point where the first contraction was felt at 5:30 min compared to 2:30min on a atmospheric air breath hold.
The body seems to learn or adapt to the new breath hold scenario. Apparently the top athletes who are training breath holds on pure oxygen delay the oncoming of contraction on pure O2 for several minutes compared to the breath holds on atmospheric air.
What could be the reason for this adaption and which other physiological mechanisms have to be considered for this scenario?

4.Additional influences on breath holds on pure O2

The chemoreceptors in the medulla are sensitive to the blood pH. The blood pH is resembled by all blood components which influence each other.
A higher CO2 concentration lowers the blood pH and makes it more acidic while a higher concentration of dissolved O2 makes the blood more alkaline. Therefore the overall balance between CO2 and O2 influences the blood pH.
This could lead to a longer tolerance time of the body against up building of CO2, because the high O2 concentration in the blood initially increases the blood pH.
Additionally high ppO2 lead to more dissolved oxygen in the plasma. “In the extreme case the total oxygen requirement may be carried in solution and haemoglobin may still be saturated with oxygen in the venous blood. This stops the transport of carbon dioxide in the form of carbaminohaemoglobin.”
Therefore less CO2 is able to enter the blood stream from the tissues which means the produced CO2 that would normally bind to haemoglobin doesn’t affect the actual blood pH or ppCO2 as long as the metabolism of the body can be run by the O2 that is dissolved in the plasma.
As soon as the O2 which is bond to haemoglobin is used, the haemoglobin in the venous blood will bond to CO2. CO2 is transported in the blood in three different ways: most of it is carried as bicarbonate, a smaller part is dissolved and the third small part bonds to haemoglobin and plasma proteins.
There are two different kinds of chemoreceptors that affect our breathing. The first ones are the central chemoreceptors in the medulla oblongata, which are the most important ones for the breathing impulse and which can desensitize though training.
They are measuring the pH and don’t respond to a drop in oxygen. The second group is made of the peripheral chemoreceptors, which are divided in the Aortic body (located in the Aorta) and the Carotid body (located in the Carotid). The Carotid body detects changes in blood oxygen, carbon dioxide and pH.
The Aortic body only registers changes in blood oxygen and carbon dioxide, not in the blood pH.

5.Questionable theories in the freediving world

Since the average freediving medical knowledge teaches that the urge to breathe is triggered by a high level of CO2 in the blood – and this statement contradicts the experiences of divers with pure oxygen breath holds – there are different theories discussed that are not yet verified by medical research.
One possible theory is that it’s the actual balance between oxygen and carbon dioxide which triggers the urge to breathe not the level in total. This would be an explanation for the longer breath hold times on pure oxygen.
If we for example assume that the Aortic body detects the balance between oxygen and carbon dioxide, it would take much longer to get the signal from the Aortic body to the diaphragm than on a breath hold on atmospheric air.
The amount of usable oxygen is much higher, so that the body would have to produce far more carbon dioxide to get to similar levels it gets on a normal breath hold.
The medical literature doesn’t support this opinion and always states that fixed numbers of ppCO2 or ppO2 or pH induce the signals from the chemoreceptors instead of quantities in comparison of oxygen to carbon dioxide.
Another discussed opinion is that the contractions on a breath hold on atmospheric air are a mixture of signals induced firstly by a high CO2 concentration but secondly also induced by a low O2 concentration.
The contractions following the low O2 concentration are the ones that are described as bad or unbearable in the end of a long breath hold.
Since the oxygen level on a pure O2 breath hold will take a long time to fall to a concentration where actually the ppO2 is inducing contractions, only the contractions induced by high ppCO2 will occur.
This is seen as explanation why the contractions during a pure O2 breath hold are easier to stand.
This line of thoughts can’t be supported by any medical research that was accessible for this presentation.
Eric Fattah states that the amount of oxygen in the body on a pure O2 breath hold is enough to still have a oxygen saturation of 98% at the end of the breath hold. So the only limiting factor is the carbon dioxide.
“High O2 pressures in the body blunt the CO2 breathing reflex. A CO2 level of 10-11% on a regular air apnea would result in an inhuman urge to breathe- the same CO2 level in a pure O2 static is much more controllable.”
This theory would be supported by a statement from a rebreather diving website, where they say that high ppO2 leads to O2 bindings on CO2 receptors which normally would indicate the urge to breath.
Fattah sees CO2 narcosis as the limiting factor for a breath hold on pure oxygen and refers so again to the carbon dioxide tolerance of the trained freediver.
This possibility seems to be the most likely one out of the non-medical explanations and might be supported after some medical research.

6.Resume

There is no explicit explanation for the prolonging of the oncoming of contractions on a pure oxygen breath hold.
That on the first tries contractions almost occur at the same time as on atmospheric air could be explained by the bodies memory and the diver who is used to get contractions at a certain point.
More scientific research would be useful to answer the outstanding questions.
The self test and the discussions with divers who are actually training breath holds on pure oxygen show, that even though the buildup of CO2 must be the same contractions occur later.
Therefore the ppCO2 can’t be the main or only trigger for the urge to breathe.

References

•AIDA 2-Star Freediver Presentation 2009.
•AIDA 3-Star Freediver Presentation 2009.
•AIDA 4-Star Freediver Presentation 2009.
•Diving and subaquatic Medicine, Edmonds, Lowry & Pennefather, 1983.
•Oxygen First Aid, Lippmann, John, 2006

Internet References

•http://forums.deeperblue.com/general-freediving/64739-new-wr-pure-o2-static.html
•http://Rebreather.de/rebreather/O2_apneu.htm © Karl Kramer
•http://www.cvphysiology.com/Blood%20Pressure/BP014.htm
•http://www.wikipedia.org
•http://www.aida-international.org
•http://www.tomsietas.de