CO2 Toxicity

[Arnaud]

I am trying to learn a little more about CO2 tox but haven't found much information so far. My basic understanding is that hypercapnia results from the difficulty for the body to release the CO2 it produces as a result of a slower metabolism caused by pressure.

Please feel free to correct the foregoing statement...as long as it's in lay terms

My questions are:

1. Is there an accepted threshold before CO2 tox kicks in (depth or depth & time, etc.)?

2. Do you know the shallowest depth of reported cases?

3. Does the gas (or gas mix) used by a diver impact his or her metabolism? In other words, are divers more or less subject to hypercapnia depending on what they breathe?

 

[AzAtty]

I'm not a doctor, but here's what I've read in anatomy texts and the like:

1. The "normal" partial pressure of CO2 in oxygenated blood is 40mmHg. The partial pressure of CO2 in deoygenated blood is 45mmHg. 1 ATM = 760mmHg. The partial pressure of CO2 in the atmosphere is 0.04mmHg. The acceptable level of CO2 in the breathing gas for diving operations is roughly 10.5mmHg.

As far as I understand, the level of CO2 in the blood is unaffected by ambient pressure. It is only affected by metabolism and the rate and depth of breathing. Increased activity increases the body's metabolism and generates more CO2. CO2 is eliminated from the blood by respiration. If a diver is not respirating efficiently, CO2 is not eliminated efficiently. Thus, CO2 levels in the blood increase which in turn lead to hypercapnia.

As for CO2 elmination becoming more difficult because of a decrease in metabolic rate, it would seem that if depth causes a reduction in one's metabolic rate, CO2 production would actually decrease.

2. I suppose that hypercapnia can occur at any depth--even at the surface--because CO2 levels in the blood are affected only by metbolic activity and respiration. If you engage in heavy exercise, skip breathing, breath holding or shallow breathing, CO2 levels will increase regardless of your depth. Additionally, a person who is in poor physical condition will generate higher levels of CO2 than a physically fit person. Hypercapnia probably isn't that uncommon. Symptoms include: feeling of constriction around the chest, heavy perspiration, lightheadedness, or headache. Sounds kinda like you feel after the first jog of the season after you've taken off the winter, eh?

3. If breathing difficulty increases because of increased pressure, then CO2 levels may increase because of a reduction in respiration efficiency and because of the increased metabolic activity the extra work requires. Tech divers will tell you that at greater depths, gas density becomes a concern, as moving gasses of increasing density in and out of your lungs becomes more of a task and respiratory efficiency is again reduced. So I suppose the breathing gas would affect the buildup of CO2 when the increase in gas density reaches the point that it becomes harder to breathe and respiratory efficiency drops.

Hope this helps, and I hope I'm not too far off.

 

[Arnaud]

Azatty, your answers are helping. I started to wonder about this subject after reading the "deep air" thread. PADI is pretty quiet on the subject.

If I understand your post correctly (I'm by no means a doc either) and based on what I've read, it seems that hypercapnia is an indirect consequence of going deep. The immediate consequence is denser gas which generates a more difficult breathing. This can be exacerbated by a rigid regulator, too. This situation will slow the metabolism and in turn, the release of CO2.

Correct?

 

[AzAtty]

I knew that I should have paid more attention in biochemistry. I should probably clarify terms that I've been using far too loosely. "Respiration" as I've been using it is really gas exchange in the lungs. "Respiration" to a biochemist is a process of deriving energy from the biological degradation of food molecules. "Metabolism" is the process by which living organisms acquire energy from external external sources and utilize it internally in order to carry out necessary cellular activities.

The metabolic process results in the production of carbon dioxide, which the blood carries to the lungs to be eliminated from the body. Increases in metabolic activity, such as that caused by heavy exercise, increases the amount of carbon dioxide produced by the body. Decreases in metabolic activity result in a decreased amount of carbon dioxide being produced.

Hypercapnia *can* be a result of going deep if (1) one's gas exchange is inefficient (shallow breathing, "dead air spaces" such as a large regulator); and/or (2) one's metabolic rate increases as a result of the extra lung work required to breath a gas whose density is increasing because of pressure. However, I do not believe hypercapnia is an inevitable consequence of deep diving. Instead, I think that the individual diver's physical condition determines how much carbon dioxide levels will increase as a diver's body responds to the addition work required to breathe a denser gas, and how efficiently the lungs will eliminate carbon dioxide.

I don't know whether one's metabolic rate increases, decreases or remains the same when diving to a greater depth using SCUBA. I know that marine mammals exhibit a decrease in metabolic rate in response to increasing depth (the so-called "diving response"). I also know of research concerning the human diving response, but the studies I'm aware of examined the effect of temperature and training on the diving response in breathhold divers, not scuba divers. I believe the key factor in triggering the diving response is apnea, or the cessation of breathing.

If increasing depth does result in a decrease of metabolic activity, I would think that there would be a corresponding decrease in the amount of carbon dioxide created by metabolic processes.

I think that sort of answers the question.

 

[Arnaud]

AzAtty, if you didn't pay enough attention in biochemistry, I probably skip the whole course alltogether! You obviously know a great deal on the subject. I studied Law at the Sorbonne University and CO2 was not part of the curiculum...

Thanks for your post!

 

[AzAtty]

I eventually studied law, too, but I was in my last semester in college, I had already been admitted to my MBA program, and I needed three hours of science to finish up my undergraduate degree. So I enrolled in Chemistry 100, thinking that it would be "bonehead" chemistry. It seems my college career was marked by similarly bad assumptions, such as picking my major (economics) believing that it only required a few math classes, and ending up in Ph.D level econometrics.

Anyhow, it turned out that Chem 100 was biochemistry and was a "weed out" class for the pre-med program. Suffering from the severe trauma inflicted by the class, I vowed that I would exact my revenge upon all the evil doctors by becoming a lawyer and suing them if they so much as jaywalked. Well, that's not really how it happened, but that rendition is much more exciting than the real story.

Although I appreciate your compliments concerning my purported knowledge, I really don't know too much about the subject. I'm just waiting for one of the aforementioned evil doctors to stomp all over my posts and point out just how wrong I am. But those evil doctors are sneaky, wily creatures, and I suspect they're giving me just enough rope to hang myself. Doctors love a good ol' fashioned lawyer lynching.

 

[Arnaud]

Well, they didn't seem interested before, but if you're appealing to their gentle side...

I thought the lawyers were evil.

 

[John Reinertson]

In normal breathing in normal humans, the primary drive to breathe comes from carbon dioxide levels in the blood. The body tightly regulates the level of carbon dioxide in the blood, partially as a way to regulate pH (acid/base balance) in the blood and partly to maintain physiologic levels of CO2.

If you were to sit down and breathe a gas mix of 90 percent oxygen and 10 percent carbon dioxide, you would very shortly be breathing as fast and deep as possible, even though the oxygen level in your blood is higher than normal, as your body frantically tried to get rid of the excess CO2.

If you breathed a mix of 88 pct nitrogen and 12 pct oxygen, you would either breathe at your normal rate or very slightly deeper, even though this is only 55% of the oxygen your body is used to at sea level.

In some individuals, the CO2 reflex is less sensitive, and in these individuals, they can hypoventilate ("underbreathe") as long as O2 levels are ok. There is no free lunch for these people, though, since anytime CO2 levels go up, chemoreceptors in the blood vessels in the brain cause the vessels to dilate. If they go up enough, they can trigger a headache similar to a migraine. (There are other metabolic effects that create problems with high CO2 levels, but they are beyond my ability to address.)

This is not a rare occurrence at sport diving depths, and represents what is referred to as a CO2 headache. (see archives)

At deep air depths, there may be additional reflex impairment by the numbing or anesthetic effects of nitrogen, but that is getting beyond my area of expertise.

Keep blowing those bubbles

john

 

[Arnaud]

Thanks John! I guess this explains also why people who have a sleep apnea problem usually wake up with a headache, right?

 

[The Iceni]

AzAtty once bubbled...
. . . the level of CO2 in the blood is unaffected by ambient pressure. It is only affected by metabolism and the rate and depth of breathing. Increased activity increases the body's metabolism and generates more CO2. CO2 is eliminated from the blood by respiration. If a diver is not respirating efficiently, CO2 is not eliminated efficiently. Thus, CO2 levels in the blood increase. . .

Yes, Azatty effectively sums it up. This is most often seen in sufferers of emphysema who have respiratory failure due, most often to cigarettes!

As John R says the "archives" do carry quite a number of discussions about the toxic effects of CO2. (CO2 narcosis, for example?)

If the body's CO2 level rises there is a reflex urge to breath more deeply and rapidly. "Air hunger." I can confirm that this urge is overwhelming and very distressing. (For the medically qualified reader I would suggest that PEEP really is a four-lettered word - highish CO2 levels are used to wean ITU patients off the ventilator!). There is some evidence that this CO2 trigger level is higher in experienced divers.

John R failed to clarify that if you breath a mix giving a ppO2 of less than 0.1 bar (10% fO2 at surface pressure) there is indeed no urge to breath more, even though this level of oxygen is very dangerous and it cannot support life. Unconsciousness inevitably results with the diver totally unaware of the danger he is in.

As for sleep apnoea. I am no respiratory specialist but as this is a form of respitratory failure, CO2 retention must be a feature.

I do not think I am speaking out of turn in stating that significant CO2 retention is uncommon in scuba with efficient modern regulators, particularly when Trimix is used. However, elevated CO2 levels do significantly contribute to the onset and severity of nitrogen narcosis and CNS oxygen toxicity.

(I didn't study law so this advice/opinion is quite free!!)

HTH!!