CO 2 and Nitrox

[Lawman]

would Nitrox have any effect on CO 2 and the
breathless feeling you get from CO 2 buildup?

 

[The Iceni]

Hi Lawman,

Very basically there is only one source of CO2 and that is from metabolism where the body's fuels are "burned" to produce CO2 and H2O.

In health respiration is controlled by the levels of CO2 in the arterial blood. As CO2 production increases with effort, when more fuel is burned, there is a very powerful reflex increase in the depth and rate of breathing, which increases the excretion of CO2 thus returning it to "normal" levels in the body.

The metabolic rate is not passive and is very accurately controlled. Metabolism is not like a fire, where a higher oxygen delivery passively results in greater consumption and CO2 production. Thus, even when Nitrox is breathed the body only uses the oxygen it needs and consequently produces an equivalent amount of CO2. It follows that when breathing Nitrox the expired air has more unused oxygen in it than when breathing air.

The fitter the individual the more efficient they are and less energy is neededed to produce an equivalent amount of work. Accordingly, and quite perversely, cardiovascular fitness results in an apparently lower metabolic rate and marginally lower production of CO2 for the same work done.

On the other hand, there is some anecdotal evidence that surface air consumption is marginally reduced when Nitrox is used. If so this could result in CO2 retention but I am afraid I know of no evidence, one way or the other.

I hope this answers you question. :doctor:

 

[Lawman]

That's what I wanted to know. It sounds like
big, fat, old, desk bound lawyers should look
into those 120cf tanks!

Thanks again.

 

[MikeFerrara]

There are some studies that I refered to (I posted some stuff) in another thread. If I remember right the studies indicated an increase in CO2 when breathing high PPO2 gas.

see the last post on the page I posted a good article there

Here

 

[The Iceni]

Yes Mike,

I had come across a lot of this work but there are one or two factors that lead one to believe much of the findings quoted, based on "hard hat divers" will not necessarily apply to modern recreational scuba. For example one of the recommendations (from 1957!) is that Nitrox should be avoided as a means of reducing deco obligations.

CO2 retention does occur (especially when skip breathing) and some individuals do become acclimatised to high PPCO2. I wondered if I am one since I did lose consciousness for no good reason.

I have taken the liberty of plagiarising the link on your post!;

ACCLIMATIZATION YOU DON'T WANT...

Effects of carbon dioxide retention in diving.
Could you be a retainer?

by Jolie Bookspan, Ph.D. and Rev. Edward H. Lanphier, M.D. 1995


WHAT'S GOING ON WITH MIXED GAS?

The year was 1952 at the U.S. Navy Experimental Diving Unit. The EDU, then located in Washington, D.C., was
ordered to work out a system using "mixed gas" to reduce decompression requirements . . .

Dives with nitrox mixtures appeared to produce an unusual number of problems compared with those using
previously worked-out oxygen limits. Furthermore, these problems did not occur when using helium-oxygen
mixtures with the same oxygen pressure. The only plausible explanation involved carbon dioxide. There was no
CO2 in the mixes, and dead space in the breathing apparatus was minimal; but data from an earlier study
indicated that, at depth, some divers breathed less than others during similar exertion. Divers who breathed much less probably did not eliminate CO2 adequately. This was of particular concern from the standpoint of susceptibility to oxygen convulsions. CO2 excess increases brain blood flow, and that increases the "dose" of oxygen to the brain.

Lanphier and Dwyer . . provided an estimate of levels of CO2 in arterial blood. At depth, end-tidal CO2 was definitely
high in certain individuals, particularly when N2-O2 mixtures were used. An independent study in 1995 repeated the EDU conditions and confirmed the results. Investigators looked at CO2 retention during hyperbaric exercise while breathing 40/60 nitrox. They determined that CO2 retention "is not expected to be globally aggravated by breathing nitrox down to 30 meters, but that some individuals could be
so affected."


STARTLING RESULTS WITH HELIUM-OXYGEN MIXTURES

Continued work it made it clear that while breathing nitrogen-oxygen mixtures at depth, carbon dioxide retention
occurred, whereas with helium-oxygen, ventilation was essentially unimpaired and CO2 levels stayed close to
normal. Conclusions reached following the 1956 and 1957 studies included the following:

(1) Retention of carbon dioxide during working dives at moderate depth is a definite reality.

(2) Only when the breathing medium is a helium-oxygen mixture is an increase in body carbon dioxide tension
absent or small.

(3) Although increased breathing resistance and dead space both favor carbon dioxide retention, keeping these
factors to a practical minimum does not eliminate the problem.

(4) Some individuals are much more likely to develop high carbon dioxide tensions than others, but all individuals
show a tendency in this direction especially when breathing a nitrogen-oxygen mixture. There is no sharp dividing
line between "retainers" and "normals."

(5) The most effective method of minimizing the complications caused by carbon dioxide retention is to use
helium-oxygen mixtures for "mixed gas" dives.

The recommendations of Research Report 7-58 [ June, 1958] can be reproduced verbatim:

"It is recommended that:

(1) Attempts to use high-oxygen nitrogen-oxygen mixtures be abandoned as a means of reducing the
requirements of decompression.

(2) Studies leading to the use of helium-oxygen mixtures for "mixed gas" diving be carried forward as
rapidly as possible."

NORMAL CO2 PRODUCTION AND REMOVAL

Normally, arterial CO2 is held, almost without exception, within 3 mmHg during both rest and exercise, a very
tight range. How does your body do this? How much, and how deeply you breathe, is regulated by your arterial
oxygen pressure, carbon dioxide tension, pH, by reflexes in your lung and chest wall, and through control by your
brain.

Having not enough oxygen in your breathing mixture enhances the ventilatory drive; there is a hypoxic drive to
breathe.

As I understood it oxygen has very little effect on respiratory drive??

CO2 is an even more profound respiratory stimulant. Of all the various inputs, your arterial CO2 is the
most influential. Rising production of CO2 with exercise increases how much and how fast you breathe,
regulating your CO2, so that CO2 does not normally rise at all, even during heavy exercise. . .

for varying periods, sometimes hundreds of times per night. Sleep apnea sufferers are often overweight, heavy
necked males. For long-term treatment, losing weight is often very effective.

MECHANISMS OF CO2 RETENTION

Normally, no great rise in your CO2 level occurs during rest or exercise. Sometimes it does rise, however. Why is
this? Several variables seem to impair the CO2 response during underwater work. Lanphier found . . that but high inspired oxygen levels knock out the
chemoreceptor response to lactic acid, which helps explain CO2 retention in working divers who at least are
verging on anaerobic threshold. Most of the elevation of partial pressures of carbon dioxide, or PC02, in the blood
is accounted for by the increased work of breathing at depth.

Work of breathing is made more difficult by the higher gas density at depth. Your body compensates by reducing
ventilation - easily demonstrated by trying to breathe through a narrow tube. . . Early work at EDU seemed to show that the critical factor in the CO2 problems was the
higher gas density of the nitrogen mixtures compared to helium mixtures.

WHY CO2 ACCLIMATIZATION?

. . .

There is evidence that the tendency to retain CO2 increases with chronic exposure to high CO2 environments,
such as those encountered during specific diving situations. The body gets used to higher levels, allowing them to
occur without the usual autoregulation that would correct the situation.

In the first EDU studies, almost all of the subjects had been experienced "hard hat" divers. The volumes of air
needed for adequate ventilation of a helmet are very great, particularly at significant

depth. Adequate ventilation of a helmet is unlikely, so acclimatization to CO2 may have been an occupational
necessity. . .

There may be some sort of selection, where those who tolerated high CO2 levels via a blunted chemoreceptor or
other adaptive response, self-select to continue with their diving career. That situation must be less prevalent
today, so the number of CO2-tolerant divers from that source may be considerably smaller. Perhaps, a number of
the CO2-retaining divers are sleep-apnoeics, who routinely experience high carbon dioxide levels during sleep. The
large, heavy, body types of many divers suggests this.

In some cases, CO2 retention occurs in subjects with no experience with high-CO2 environments, but who may
be exposed in other ways, most notably a learned adaptation from breathing patterns that regularly produce
elevated internal CO2 levels. When scuba diving first became prevalent, "skip breathing" was often taught or
popularized by word of mouth as a means of conserving the air supply in open-circuit scuba. . .


AVOIDING CO2 RETENTION

If the solution of the CO2 retention problem does not lie in personnel selection, what other avenues are open?
Avoiding "skip breathing" and any other attempt to conserve air seems obvious, but this may be easier to
recommend than to accomplish. Providing ventilatory assistance to divers may deserve investigation.

The material in this article was presented at the 13th Meeting of the United States - Japan Cooperative
Program in Natural Resources (UJNR) Panel on Diving Physiology in Yokosuka, Japan, 23-25 October
1995.