Hypercapnia/Hypocapnia???

[scubasully]

I'm confused - I get that Hyper is too much CO2 and Hypo is not enough, but the book says that one is caused by hyperventilation and the other caused by rapid, shallow breathing. Isn't rapid shallow breathing the same is hyperventilation?

I took a practice test where Hypo and Hyper were both possible answers, and I got it wrong. The question was

Excessive breathing resistance in diving due to a maladjusted or improperly maintained regulator can result in...

I guessed Hypo, thinking if the reg wasn't working properly, that could cause stress and there for panicked breathing... but I was wrong. It's hyper. I checked the Encyclopedia, and I can't find anything about bad regs.

Help.

Thanks,
Maureen

 

[TSandM]

I don't know if I'm really supposed to post here, but this is really a medical question.

Hyperventilation, by definition, means running more air in and out of the lungs than you need to do, and results in DECREASED carbon dioxide tensions and an alkalotic blood pH. The symptoms are perioral tingling and tingling and even cramping in the palms of the hands. Hypoventilation is, by definition, moving less air through the alveoli than you need, and results in shortness of breath, anxiety, rapid heart rate, and eventually lethargy, somnolence and unconsciousness.

In scuba talk, the term "hyperventilation" is often used to describe a rapid, shallow breathing pattern which actually results in alveolar HYPOVENTILATION. In other words, although a lot of air is being moved in and out, little of it is actually getting down to the lungs, and the CO2 level in the blood rises.

In medical parlance, hyperventilation means LOW CO2, and we often make those patients breathe into a paper bag to try to bring the CO2 closer to normal and relieve the symptoms.

Increased resistance means increased work of breathing, which results in increased CO2, which will cause a rapid, shallow breathing pattern. So your answer to the question was wrong because the definitions used in scuba writing are wrong, not because your thinking was incorrect.

 

[scubasully]

Thanks. I just got off the phone with a doctor friend who said the same thing. I'll try to keep it straight for the exams!

 

[knotical]

Hi Maureen,

Another way to think of it is: shallow breathing does not empty our lungs very well, causing more dead air space and hypercapnia.

In my experience, "hyperventilation" means deep breathing, such as before a skin dive.

fwiw, the Encyclopedia (at least the old version) does talk about denser air at depth potentially leading to hypercapnia. I think the question is asking you to draw a similar conclusion for hard breathing regs.

k

I don't know if I'm really supposed to post here

TSandM,
Of course you should post here. The top of the page says:

Everything Dive Master Candidates and future Dive Cons always wanted to know but were afraid to ask.

IMO, your insight is always appreciated.

But your statement:

In scuba talk, the term "hyperventilation" is often used to describe a rapid, shallow breathing pattern

is strange to me. I've never seen or heard hyperventilation used that way. But then we run with different scuba crowds.

k

 

[wve]

The confusion often arises when people say "hyperventilation" when they really mean "tachypnea."

Hyperventilation occurs when the gases in the alveoli (tiny lung sacs) are exchanged more rapidly than normal. The most common cause of hyperventilation is tachypnea, which is the technical term for an increased rate of breathing.

Since tachypnea usually leads to hyperventilation, the two are often used synonymously.

But it is possible to have tachypnea with hypoventilation. This occurs in certain disease processes, such as emphysema, where there is increased "dead space." It can also occur in scuba if a diver takes rapid, shallow breaths which only move the air back-and-forth within the major airways, not getting the air all the way down into the alveoli where actual gas exchange occurs.

When there is too much gas exchange (hyperventilation), CO2 levels decrease and you have hypocapnea. When there is inadequate gas exchange (hypoventilation), you get hypercapnea.

Note that oxygen levels are not influenced by breathing rates as much as CO2 levels are. Obviously, breathing too slow can cause hypoxia (too low oxygen). But breathing too fast will not improve your oxygen levels (unless you are on a ventilator where we can manipulate things a bit more).

Confusing? It can be. That's why doctors spend the first couple of years in medical school just learning all the darned terminology. So here is a summary of the terms:

Hyperventilation = too much gas exchange
Hypoventilation = not enough gas exchange

Hypercapnea = too much CO2
Hypocapnea = not enough CO2

Hypoxia = not enough oxygen

Tachypnea = breathing too fast

This all agrees with what TSandM said.

 

[scubasully]

Apparently "excessive" hyperventilation is different than regular hyperventilation in PADI-speak, which I don't speak well at all! In the review for the physio exam, my instructor talked about the difference - basically, HYPERvent = HYPOcap and excessive HYPERvent = HYPERcap.

I took the physio exam today and I got everything right except for the question on this, which is a real bummer as I started this thread 2 days ago to prevent getting this wrong! And I asked the instructor a million questions about it.

The question asked what happens when you excessively hyperventilate, and I chose hypercapnia. WRONG. I was shocked, and then he had me reread the question out loud, and DUH...it actually asked this in regards to apnea.... rats. Oops! Anyway, RDP is next, then Equipment, and then Skills & Enviro. I'm chugging along.

 

[Crowley]

I'd like to chime in and clarify the OP thing here - and TSandM you know I love reading your posts but I don't think there's anything wrong with the scuba literature in general although I do appreciate some divers use the term hyperventilation wrongly talking about breathing.

First off - the question: A improperly blah blah blah reg causes what? Correct answer = hypercapnia i.e. an excess of carbon dioxide. You are unable to efficiently eliminate the CO2 from your lungs and therefore your CO2 level rises leading to panic etc. The reason being the excessive breathing resistance from the regulator means you have to breathe harder which creates turbulence in the regulator (and your trachaea) which affects the flow of gases in and out of your lungs. You end up rebreathing a lot of the same air.

The breathing reflex is triggered by a build up of CO2, not a lack of O2. When you hyperventilate (deep, controlled breathing, not excessively rapid) prior to skin diving, you are eliminating more CO2 from your system than normal, effectively fooling your brain into believing you don't need to breathe for a while so you can hold your breath longer.

If you hyperventilate excessively you eliminate too much CO2 suppressing the urge to breath too far. Whilst submerged during a skin dive you are still metabolising the oxygen in your system but because you've fooled your brain into thinking you don't need to breathe right now, you use too much oxygen underwater so that during your ascent, the partial pressure of oxygen in your system decreases below the critical level required to maintain life (around .15 - .16 atm) and so you suffer from hypoxia (too little O2) and pass out - this is known as shallow water blackout.

The literature is simplified a little, but is not incorrect - certainly not in terms of the encyclopaedia of recreational diving.

Hope that helps,

happy breathing,

C.

 

[wve]

The question asked what happens when you excessively hyperventilate, and I chose hypercapnia. WRONG.

To build on Crowley's explanation:

In the subconscious "automatic mode," human breathing is controlled by blood CO2 levels. When CO2 gets high, you breathe faster and deeper in order to "blow it out" of your system. When CO2 gets too low, you breathe slower and shallower so that CO2 builds back up.

But humans can also go into "manual mode" whereby they can consciously override the natural breathing control system. You can force CO2 levels down by hyperventilating (breathing faster and deeper). Or you can allow CO2 levels to build up by holding your breath.

Eventually the life-sustaining "auto mode" will take back control, and that is why you can only hold your breath for so long. But as Crowley explains, competitive breath-holders intentionally "blow off" CO2 so that they'll have a longer interval before the CO2 levels build back up and stimulate the irresistible urge to breathe.*

If you can remember how the system works, then you should be able to keep things straight.

While exceptions can and do occur in the medical world, among healthy individuals the rule is:

Breathing fast and deep "blows off CO2."
Breathing shallow and slow lets CO2 accumulate.

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*As Crowley also explains, the risk of breath-holding under water is that the brain can run out of oxygen and cause unconsciousness before one gets back to the surface where it is safe for the auto-mode to restart breathing. The result is drowning.