CO2 retention and air consumption

[DivingDoc]

Dr. Deco:

We know that at depth, divers may retain CO2 because of the greater density of the air they are breathing and hence, increased work of breathing, right?

I also understand that some divers retain more CO2 than others.

We also know that some people are better at air consumption than others (ie, they suck less air than others on the same dive).

Question: is there any relation between those who consume less air and those who retain CO2? Is air consumption more related to the person's efficiency of oxygen utilization (fitness level, pehaps) and metabolic rate or to their minute ventilation?

I guess my question mainly relates to whether those who have low air consuption are those who hypoventilate and therefore tend to retain more CO2.

 

[Dr Deco]

Dear Diving Doc:

Carbon dioxide retention is not something with which I have much acquaintance. Ventilation by divers is more a function of training and “ease in the water,” though I would suspect that fitness level plays a role.

Did you have something specific in mind regarding CO2 retention? It is not a governing factor in most scuba activities.

Dr Deco :doctor:

 

[The Iceni]

Hi Diving Doc,

I am sure, as a respiratory physician, you know that normally ventilation is driven by the partial pressure of CO2 in the blood (detected by the chemoreceptors in the respiratory centre of the brain).

Experienced and practiced divers learn to overide much of this respiratory drive, somehow. So, not only do more experienced divers become fitter, more comfortable and more relaxed, their drive to ventilate is reduced. However, I very much doubt they retain enough CO2 for it to become hazardous since as you know, CO2 retention produces numeorus unpleasant symtoms in divers. I suspect an hour's worth of hypoventilation at depth would produce significant hypercapnia but these "anaerobes" appear to be perfectly well and symptom-free at the end of their dives.

Apart for metabolic fitness I believe much of this considerable variation between individuals may be due to body size. Someone like me will always be a gas guzzler as I am over 6ft tall and weigh in at about 200 lbs. One of the girls in our club weighs 120 lbs and seems to use no gas at all (even when she was a novice). I presume this is because of a larger metabolising mass of tissue requires proportionately more actual oxygen and therefore produces proportionately more CO2. To remain within normal limits my respiratory minute volume must consequently be higher (even though my ventilation rate is actuallly less.)

Try as I might I cannot get my RMV to below 17 litres per minute and find it exceedingly difficult to slow down my breathing pattern underwater. I think it has a lot to do with the Hering Breuer reflex and the fact that divers are mouth breathing. Does nasal sensation have a role to play in respiratory control, I wonder?

Perhaps RMV is reduced by using a full face mask but I know I will never beat my female buddy?

 

[DivingDoc]

Dr. Thomas, as usual, has added some good insights to this discussion.

I have read in several places that it is common for divers to become hypercapneic or hypercarbic (your choice) at depth and that this phenomenon is somewhat variable from diver to diver. One such article is the following from the DAN website:

http://www.diversalertnetwork.org/medical/articles/article.asp?articleid=29

In particular, this section:

"Carbon dioxide acts as a respiratory stimulant and can cause depression of the central nervous system (CNS). The effect depends on the level of carbon dioxide in the blood. Deep diving produces elevated blood carbon dioxide levels for several reasons, which include:

the resistance to breathing caused by breathing denser gas through a regulator and against a higher ambient pressure;
reduced ventilation efficiency due to the denser breathing gas; and reduced transport, and, hence, elimination of carbon dioxide.
Hypercapnia increases narcosis and the likelihood of CNS oxygen toxicity. In addition, it may increase heat loss, alter heart rhythm and predispose to decompression illness. If the carbon dioxide level gets too high, and it can on deep scuba dives -- especially if a diver is very anxious and / or exerting him/herself -- the diver may go unconscious without warning. Certain divers are more susceptible to severe hypercapnia for a variety of reasons and are therefore more at risk."

Here is an abstract that suggests that divers may fall in the lower part of the normal range for CO2 sensitivity:

: Undersea Biomed Res 1980 Mar;7(1):61-74 Related Articles, Books, LinkOut


Ventilatory and occlusion-pressure responses to hypercapnia in divers and non-divers.

Sherman D, Eilender E, Shefer A, Kerem D.

Ventilatory (VE/PCO2) and occlusion-pressure (P0.1/PCO2) responses to progressive hypercapnia (rebreathing method of Read (1)) were estimated in 20 normal subjects and 22 scuba divers. Indexes of CO2 sensitivity (slopes of response curves) and absolute response values under strong CO2 drive (PCO2 = 60 mmHg) were significantly lower in the diver group. Individual CO2 sensitivity did not correlate with either diving experience or current diving activity. Positively skewed (log-normal) frequency distribution curves of individual CO2 sensitivities were drawn for the divers and for a larger sample of normal controls (using data from other studies). All divers' values fell in the lower range of non-diver control values and about one-third were below the normal range (mean +/- 2 SD) for CO2 sensitivity. We concluded that our divers did not represent a distinct population different from the normal one but rather a group of normal healthy subjects with either an inherent or acquired relatively low CO2 response. The rebreathing technique is strongly advocated as a tool for investigating divers' CO2 sensitivity and its implications in underwater environments.

PMID: 7385448 [PubMed - indexed for MEDLINE]

The observation that Dr. Thomas raises that men tend to suck air faster than women has been blamed on a higher metabolic rate owing to men's greater muscle mass.

But it is also true that individual divers can "learn" to reduce their air consumption with practice to a degree. The idea that these individuals may do so by learning to ignore a given level of hypercapnia might be suggested by the above abstract.

I suspect that the level of air consumption depends on: 1) depth; 2) basal metabolic rate (mainly proportional to muscle mass); 3) level of exertion; 4) fitness level -- which relates to the efficiency of oxygen utilization; and 5) minute ventilation variations that are individual to the diver.

Do you two agree?

Anyway, the reason I was thinking about all this was that episode of mild subjective dyspnea that I mentioned previously. Although I am a novice diver with around 22 dives on my list, I had one of the lowest levels of air consumption in our group. Of 26 divers, I was usually among the last 5 or so to need to surface.

Since I used nitrox almost exclusively for the week of diving (total of 13 dives, with one day off in the middle), and since hypercapnea potentially can make Oxtox worse, I was speculating on the possibility of some mild pulmonary oxygen toxicity occurring due to the coexistence of some mild hypercapnia. I admit this is rather far-fetched, but I do have a vivid imagination.:doctor:

 

[Dr Deco]

Dear divingDoc:

I can imagine that CNS oxygen toxicity is increased in the presence of increased arterial levels of carbon dioxide (through modification of autoregulation in brain perfusion), but I doubt that pulmonary oxygen toxicity is similarly affected by increased CO2.

Dr Deco :doctor:

 

[Viv]

I enjoyed the references to the studies concerning CO2 sensitivity. Air consumption is one of my main "concerns". As Dr. Thomas stated, small, athletic females tend not to use very much air. As a 5ft 2 in tall, 118 lb female runner all of my buddies think that I should never run low on air. However, I am always the first in my group to need to surface. My boyfriend/buddy, who is also a DM, constantly tells me to slow my breathing rate down, but I can't. It's not because I am in a state of panic or feel tired. It's just that when I am breathing through my mouth on a regulator I have a strong urge to breath rapidly. Do you think it's the CO2?

 

[Piscean]

Viv,

I don't think that it is the CO2, I think that it is because you are subconciously uncomfortable breathing through your mouth. Try putting your mask on on the sofa, I bet you start to breath faster then too. I once went to a course at Pan Aqua in Brookfield, CT, (but they have a store on 43rd bet. 7th & 8th in NYC too), where they had a Yoga instructor teach us to do some stretching exercises, which of course help you to relax. It reduced nearly everyone's breathing rate by 25% or so. Try doing some breathing excercises. I think with practice you will manage.

Oh, and don't think about air consumption or breathing while you are under water. In my experience, that normally makes it worse. If you want to think about something, then think about relaxing every muscle in your body one after the other until they are all relaxed. Better still, just look at the fish.

Piscean.
P.S. If I am cold, my breathing rate increases by 20%. As a slim athletic lady it is likely that you don't have the same layer of insulation as some which could also contribute.

 

[The Iceni]

Originally posted by DivingDoc
Here is an abstract that suggests that divers may fall in the lower part of the normal range for CO2 sensitivity:

: Undersea Biomed Res 1980 Mar;7(1):61-74 Related Articles, Books, LinkOut

Ventilatory and occlusion-pressure responses to hypercapnia in divers and non-divers.

Sherman D, Eilender E, Shefer A, Kerem D.

The observation that Dr. Thomas raises that men tend to suck air faster than women has been blamed on a higher metabolic rate owing to men's greater muscle mass.

But it is also true that individual divers can "learn" to reduce their air consumption with practice to a degree. The idea that these individuals may do so by learning to ignore a given level of hypercapnia might be suggested by the above abstract.

I suspect that the level of air consumption depends on: 1) depth; 2) basal metabolic rate (mainly proportional to muscle mass); 3) level of exertion; 4) fitness level -- which relates to the efficiency of oxygen utilization; and 5) minute ventilation variations that are individual to the diver.

Do you two agree?

Anyway, the reason I was thinking about all this was that episode of mild subjective dyspnea that I mentioned previously. Although I am a novice diver with around 22 dives on my list, I had one of the lowest levels of air consumption in our group. Of 26 divers, I was usually among the last 5 or so to need to surface.
doctor:

There are a couple of physiological phenomena that have been overlooked.

1) Divers can breath hold for longer that normal subjects. I do not have a reference to quote but I have always thought that this is because, just as the body learnes to tolerate certain drugs (especially the opiates), the respiratory centre of the brain learns to tolerate a higher partial pressure of CO2 because it is regularly exposed to marginally higher levels of CO2. (This causes localised acidity.)

2) The Haldane effect. This states that (because it contains less oxygen-bound haemoglobin) venous blood can carry more carbon dioxide (as inactive carbaminohaemoglobin). One might therefore expect Nitrox divers to have marginally more free - and therefore active - CO2 to act as a respiratory stimulant. I believe this is also why CO2 worsens the risks of CNS oxygen toxicity- it competes with the oxygen bound to haemoglobin and releases it as free and active oxygen at the tissue level (The Bohr effect.)

Like Piscean, I believe psychology has an important role, which is why I mentioned the increasing popularity of full-face masks and the immersion reflex (Hering Bruer [spelling?]) but I also wholeheartedly agree with your four points, Divingdoc.

Viv is a runner, and presumably does more running than diving so her respiratory drive is geared to a higher ventilation rate to keep pace with the high levels of aerobic metabolism demanded of running. In simple terms it appears to me for her to be a "habit".

Perhaps some research is needed?

 

[DivingDoc]

Paul wrote:

2) The Haldane effect. This states that (because it contains less oxygen-bound haemoglobin) venous blood can carry more carbon dioxide (as inactive carbaminohaemoglobin). One might therefore expect Nitrox divers to have marginally more free - and therefore active - CO2 to act as a respiratory stimulant.

Not quite sure that I see why it follows that Nitrox divers would have more CO2 due to the Haldane effect. I would accept that Nitrox divers might have more CO2 due to a slight suppressive effect of the higher pO2 level.

Re: Viv--

I agree with Picean. Breathing rapidly would actually lower your level of CO2. This could actually make you dizzy, by causing the blood vessels in the brain to constrict. Try breathing rapidly on dry land while standing still and you will notice this effect in short order.

 

[blackwater]

I would agree on habit, and relaxing. I do some running, and have always worked to breath (keeping aerobic) while running, and had to change habits for diving. My first dive (after a many year break), I sucked through a 80 Cu Ft. tank in about 20 minutes, now less than a year later, I typically last almost an hour (over if it's a shallow dive). the key for me was to relax, breath slowly, relax, make sure I'm not overheated/stressed at the beginning of the dive (e.g. from a long hike, surface swim, or racing to get off the boat). relax, move slowly to conserve energy (and air).
GT
:anon: