Question Decreasing the Breathing Reflex....Possible?

[John C. Ratliff]

I'm going to disagree somewhat. No question that I can train myself (and did at one point) to ignore higher levels of CO2. If O2 is not the driver of breathing, and I agree it is not, then training oneself to ignore the need to breathe is in fact CO2 training.

I’m going to step in here, as I think we are getting very confused about the difference between apnea in breath-hold diving and scuba diving, where skip-breathing is sometimes practiced. These are physiologically very different scenarios. For breath-hold diving, training oneself to ignore the need to breath is quite a dangerous practice. Believe me, there is no warning about oxygen-deficient blackout in SWB. One moment everything is fine, the next, nothing. Also, if you have pre-programmed your brain to swim a distance in a pool, for instance, your body will take that command and continue swimming after blackout. Therefore, when someone sees a body at the bottom, it may already be very late to resuscitate that individual.

Now, skip breathing in scuba diving is an entirely different practice, and the CO2 levels really don’t play much role in this practice. Why? Well, because respiration continues before oxygen levels are fully depleted and CO2 levels have failed to build up to the “must breathe” signal. I sometimes skip-breath when photographing small fish. I also sometimes simply hold my breath while trying to get that close-up of small critters with my camera or GoPro. This is followed by a series of fairly rapid breaths to ventilate my lungs when I’ve gotten the photo (or missed it—that also happens).

But this thread has gotten off the original topic, which was talking about breath-hold diving. It’s sorta been hijacked to a completely different topic, of how nitrox might affect scuba diver’s breathing cycles, which are not based upon the “urge to breath” in the breath-hold diving context. The problem with doing this is that assumptions made in breath-hold diving may be dangerous for nitrox diving at depth, and (especially) vice-versa. I see a lot of wrong-headed thoughts above about training for altitude (which in fact requires a long acclimatization period for the body to adjust to diminished oxygen at altitude, by increasing red blood cells (RBCs), for instance, which takes weeks to accomplish). Also, hyperventilation does not and cannot increase oxygen levels, as the RBCs will simply be saturated with oxygen at 21% in air (the percentage doesn’t change with hyperventilation). (see below PS). So think about how different the two scenarios (breath-hold diving and use of nitrox with skip-breathing) are in a physiological way of thinking.

SeaRat

PS, I have just read Dr. Simon Mitchell’s paper referenced in my post below, and lung packing can increase the blood oxygen levels.

 

[John C. Ratliff]

Hello,
In fact, it does. Several other commentators on this thread have correctly pointed out that the most important influence on the drive to breathe is generated by carbon dioxide levels in the arterial blood - higher CO2 = greater drive to breathe. However, if you have a higher arterial oxygen level, then the drive to breathe generated by increasing CO2 levels is reduced. You can easily try this yourself if you have a cylinder of oxygen connected to a scuba regulator (oxygen clean of course!!). Time yourself holding your breath on air, and then try the same after washing oxygen into your lungs (eg 10 breaths on the scuba regulator). You will notice a significant difference in the time and the subjective perception of breath hold discomfort. To summarize this, although increasing CO2 level is the primary driver of breathing, this is definitely modified by oxygen levels (less drive to breathe is oxygen is high, more drive to breathe if oxygen is low).



Yes, this is essentially true although 'cold-pressed' is not really the right term. The lungs get compressed rapidly so even though the diver is apnoeic and consuming oxygen, the pressure of oxygen in the lungs increases during the rapid descent. We demonstrated the latter phenomenon in a 2021 study where we measured the arterial oxygen levels in an elite freediver at ~200' / 60m. The related paper has been published and can be read on PMC for free here.

Simon M

I would highly recommend that all divers interested in this topic read Dr. Simon Mitchell’s paper referenced above. It contained updated information that educated me to understand that “lung packing” does increase the pO2 in the diver’s bloodstream, and it also reinforces the problems of ascent and the hazard of shallow water blackout, with actual data on actual blood CO2 and O2 levels during the dive. Here is the conclusion to his paper:

Conclusions​

Predive ventilation routines used by deep free divers are effective in lowering PaCO2 and increasing PaO2 before diving. PaO2 increases markedly during a rapid free diving descent to extreme depth. It subsequently decreases precipitously during ascent, creating a significant hazard of hypoxic loss of consciousness in the latter part of ascent. This finding is confluent with those of other studies and long-standing beliefs around the pathophysiology of hypoxic loss of consciousness in free diving. PaCO2 increases during a rapid free diving descent to a degree unexplained by metabolic production of CO2. This probably arises as a result of compression of pulmonary gas with an increase in alveolar partial pressure of carbon dioxide (PACO2).

SeaRat

 

[Celt]

I never actually hold my breath underwater, I’m breathing all the time but it’s long relaxed breathing. A pause then inhalation, pause then long exhalation, very natural, not forced at any time, and never starving for air. I’m not trying to save air I always have plenty for the planned dive. I NEVER use the exercises that I use on land underwater.

 

[Dr Simon Mitchell]

There is no way I am aware for oxygen to become the dominant factor in the respiratory drive.

Hello,

There certainly is one way: if there is severe hypoxia oxygen certainly becomes the "dominant factor in the respiratory drive" - at least in some subjects. We are currently demonstrating this in a series of experiments in which we make subjects hypoxic, but we are not the first to have demonstrated it.

The point remains. CO2 is simply the driver of respiration. And affects from O2 are not a significant component.

From a practical perspective this is largely true, but not as completely so as I suspect you think it is. I have linked to a paper from Comprehensive Physiology in which we consider perturbation of respiratory control during diving in some detail. Much of page 172 is given to discussion of the hypothesis that increased arterial oxygen levels can promote a tendency to retain CO2. There is some evidence that elevated inspired PO2 may cause an increase in arterial CO2, but (as you suggest) probably not to levels that are practically significant.

You will find detailed discussion of the much greater effects of gas density elsewhere in the paper, and in a more readable chapter by Gavin Anthony and myself on "Respiratory physiology of rebreather diving" in the Rebreathers in scientific diving workshop proceedings you can download here.

Simon M

 

[Gelirfella]

Hello,

There certainly is one way: if there is severe hypoxia oxygen certainly becomes the "dominant factor in the respiratory drive" - at least in some subjects. We are currently demonstrating this in a series of experiments in which we make subjects hypoxic, but we are not the first to have demonstrated it.

Thanks so much for the references.

From a practical perspective this is largely true, but not as completely so as I suspect you think it is. I have linked to a paper from Comprehensive Physiology in which we consider perturbation of respiratory control during diving in some detail. Much of page 172 is given to discussion of the hypothesis that increased arterial oxygen levels can promote a tendency to retain CO2. There is some evidence that elevated inspired PO2 may cause an increase in arterial CO2, but (as you suggest) probably not to levels that are practically significant.

You will find detailed discussion of the much greater effects of gas density elsewhere in the paper, and in a more readable chapter by Gavin Anthony and myself on "Respiratory physiology of rebreather diving" in the Rebreathers in scientific diving workshop proceedings you can download here.

Simon M

Thanks so much. What is the proposed mechanism for hypoxic respiratory control? The central chemoreceptors are all HCO3 driven correct? It’s just the carotid sinus receptors with paO2 sensitivity I thought? Physiologists again breaking a long held assumption?

Will add, just a dumb surgeon here, we do a fair bit of critical care training in the US but far, far from a pulmonologist in my understanding of the principles and largely clinical knowledge. I’ll read your papers. Interested to see how hypoxic you have to get them to see the effects and also curious how you got IRB approval for it. Unless it’s military.

 

[Duke Dive Medicine]

I’m going to step in here, as I think we are getting very confused about the difference between apnea in breath-hold diving and scuba diving, where skip-breathing is sometimes practiced. These are physiologically very different scenarios. For breath-hold diving, training oneself to ignore the need to breath is quite a dangerous practice. Believe me, there is no warning about oxygen-deficient blackout in SWB. One moment everything is fine, the next, nothing. Also, if you have pre-programmed your brain to swim a distance in a pool, for instance, your body will take that command and continue swimming after blackout. Therefore, when someone sees a body at the bottom, it may already be very late to resuscitate that individual.

Now, skip breathing in scuba diving is an entirely different practice, and the CO2 levels really don’t play much role in this practice. Why? Well, because respiration continues before oxygen levels are fully depleted and CO2 levels have failed to build up to the “must breathe” signal. I sometimes skip-breath when photographing small fish. I also sometimes simply hold my breath while trying to get that close-up of small critters with my camera or GoPro. This is followed by a series of fairly rapid breaths to ventilate my lungs when I’ve gotten the photo (or missed it—that also happens).

But this thread has gotten off the original topic, which was talking about breath-hold diving. It’s sorta been hijacked to a completely different topic, of how nitrox might affect scuba diver’s breathing cycles, which are not based upon the “urge to breath” in the breath-hold diving context. The problem with doing this is that assumptions made in breath-hold diving may be dangerous for nitrox diving at depth, and (especially) vice-versa. I see a lot of wrong-headed thoughts above about training for altitude (which in fact requires a long acclimatization period for the body to adjust to diminished oxygen at altitude, by increasing red blood cells (RBCs), for instance, which takes weeks to accomplish). Also, hyperventilation does not and cannot increase oxygen levels, as the RBCs will simply be saturated with oxygen at 21% in air (the percentage doesn’t change with hyperventilation). (see below PS). So think about how different the two scenarios (breath-hold diving and use of nitrox with skip-breathing) are in a physiological way of thinking.

SeaRat

PS, I have just read Dr. Simon Mitchell’s paper referenced in my post below, and lung packing can increase the blood oxygen levels.

John, a small point here, more for posterity than for you. I would differentiate what you described as skip breathing, which is simply not breathing momentarily in order to obtain a better photo, from the practice of skip breathing over the length of a dive in order to conserve air. The former, while still not aligned with the basic tenet of not holding one's breath while diving, could be safely pulled off by an experienced individual with an open glottis. The latter is a dangerous practice that leads to increased CO2 levels which, when complicated by exertion or the gas density that @Gelirfella mentioned above, could result in incapacitation and death.

Re lung packing, while it can slightly increase the arterial pO2, those who practice it also risk pulmonary barotrauma.

Best regards,
DDM

 

[John C. Ratliff]

I thought you'd like to see what I was talking about by "skip breathing." As DDM says above, it is not done throughout the dive, but rather to photograph fish or, in one case, sneak up on ducks. Always, there is a period after the breath-holding that I ventilate my lungs. This short video of underwater life in the Clackamas River shows these breathing techniques off a bit. You will also notice that I breath deeply, and that there is usually a second or two space between my inhalations and exhalations. This, to me, is normal breathing underwater that I've developed after 50+ years of diving. Again, this is for scuba diving, not breath-hold diving.


SeaRat

 

[Angelo Farina]

John, a small point here, more for posterity than for you. I would differentiate what you described as skip breathing, which is simply not breathing momentarily in order to obtain a better photo, from the practice of skip breathing over the length of a dive in order to conserve air. The former, while still not aligned with the basic tenet of not holding one's breath while diving, could be safely pulled off by an experienced individual with an open glottis. The latter is a dangerous practice that leads to increased CO2 levels which, when complicated by exertion or the gas density that @Gelirfella mentioned above, could result in incapacitation and death.

Best regards,
DDM

Well, actually in the seventies I was trained with CC rebreathers, and the inspiratory pause (aka "skip breathing") was done for INCREASING the pulmonary ventilation efficiency, spending more time with maximum gas exchange surface.
Those single-hose ARO CC rebreathers were very poor in capturing CO2, and headache due to poor ventilation was very common.
For avoiding this, we were taught to use a very profound ventilation cycle, with an inspirator pause of 5-10s. This was proven to REMOVE CO2 better than continuous breathing.
Of course, a too-long inspiratory pause becomes counter-productive, and the duration of the inspiratory pause had to be adjusted on the muscular effort (increasing the effort causes more CO2 production, hence the need to shorten the inspiratory pause).
But a proper ventilation cycle, including a short inspiratory pause, is very effective in removing CO2 and avoiding hypercapnia... Particularly if compared with the PADI-like recommendation of "just breath normally"; which results is a short respiratory cycle with no pause.

 

[Duke Dive Medicine]

Well, actually in the seventies I was trained with CC rebreathers, and the inspiratory pause (aka "skip breathing") was done for INCREASING the pulmonary ventilation efficiency, spending more time with maximum gas exchange surface.
Those single-hose ARO CC rebreathers were very poor in capturing CO2, and headache due to poor ventilation was very common.
For avoiding this, we were taught to use a very profound ventilation cycle, with an inspirator pause of 5-10s. This was proven to REMOVE CO2 better than continuous breathing.
Of course, a too-long inspiratory pause becomes counter-productive, and the duration of the inspiratory pause had to be adjusted on the muscular effort (increasing the effort causes more CO2 production, hence the need to shorten the inspiratory pause).
But a proper ventilation cycle, including a short inspiratory pause, is very effective in removing CO2 and avoiding hypercapnia... Particularly if compared with the PADI-like recommendation of "just breath normally"; which results is a short respiratory cycle with no pause.

Angelo, kudos for surviving that. What you're describing might have been the lesser of two evils for that particular piece of diving equipment, but would not apply to open circuit diving or a properly functioning modern-day CCR. A 5-10 second inspiratory pause would not result in better CO2 elimination with modern diving equipment. Rather, it would likely result in CO2 retention.

Best regards,
DDM

 

[John C. Ratliff]

For those still skeptical about the mechanism of pulmonary barotrauma contributing air to the arterial system, I just found some of the PPT slides from my Tulane University course in Occupational Pulmonary Disease from Douglas A. Swift, M.D., M.S.P.H. This particular slide shows the "Ultrastructure of Pulmonary Alveoli and Capillaries," and shows graphically the wall thicknesses of the two. A tear that ruptured the alveolus membrane as well as the blood capillary membrane (one cell thick each) would easily provide a pathway for air into the arterial system. This was for my M.S.P.H. degree from Tulane University.

SeaRat