Measuring CO2 tolerance with CO2 sensors

[ADR]

Has anyone experimented with measuring CO2 tolerance levels with CO2 sensors? I've found some cheapish but accurate sensors and was planning to measure expired gas at the end of a breathhold and use the CO2 readings as a relative measure of CO2 tolerance. The expired air would need to be captured at the end of the exhale to ensure that I wasn't reading the dead air spaces. I have two questions on this:

1. Would this method work for measuring relative CO2 tolerance?
2. Would a sensor that measures CO2 in the ranges of 350PPM-10,000PPM (output - 0->50mV) be OK for this?

thanks in advance

 

[efattah]

I have done this with my CO2 monitor, but your sensor won't be enough.

You would need a sensor which can measure at least 10% CO2, and 10% = 100,000 ppm.

If you can get an accurate reading of expired CO2, then this, combined with an oximeter, can give a good indicator of your CO2 tolerance (actually it gives a mixed indication of your CO2 tolerance with respect to your body's acid/alkaline balance).

 

[ADR]

Thanks Eric,

As usual you are a wealth of info and only too happy to share it. I take from your "mixed" comment that your bodies abilty to buffer CO2 will impact this approach. Would blood or urine PH testing help add clarity in that regard and do you know if the relationship is linear or otherwise?

thanks again

 

[efattah]

Good call!

Given the following info:
- Urine pH
- Oral temp
- SaO2 & etCO2 at time of 1st contraction

You should be able to get a great indicator of your CO2 tolerance at the moment.

Remember that low SaO2 causes contractions by itself; the CO2 level amplifies it. So CO2 alone is not enough; likewise, at hyperbaric O2 levels, the high O2 delays the CO2 breathing reflex. This is why both O2 & CO2 must be known, or at the very least, O2 must be fixed.

For example, suppose:

Day 1
urine pH 7.6
oral temp = 36.8C
SaO2 @ 1st contraction = 90%
etCO2 @ 1st contraction = 7.0%

Day 2
urine pH 7.6
oral temp = 36.8C
SaO2 @ 1st contraction = 90%
etCO2 @ 1st contraction = 7.2%

Then, from that data you can conclude that CO2 tolerance on day 2 was better than day 1.

The effect of body temp would be hard to calculate, but again I would standardize it. The effect is due to the fact that the SaO2 vs. PO2 (hemoglobin dissociation curve) shifts based on blood temperature, which would confuse the results unless you kept your body temp the same across different experiments.

Your breath-hold pattern between tests should also be the same. As the spleen contracts, the higher hemoglobin will allow for greater CO2 buffering. So, if one day you test your CO2 on the 1st hold, and on the 2nd day, you test it on the 3rd hold, the splenic contraction will confound the results.

 

[DeepThought]

etCO2 = expired CO2?

 

[ADR]

Hi Eric,

Thanks for the technique and I may even use the sensors I have if I can come up with a reliable way to dilute the sample by a known amount prior to measuring the CO2. It's all taking me back to my evil tech diving gas mixing days.

It's interesting what you said about contractions from low SaO2 and it is something that has been muddling around in my head from last time you mentioned it to me (about a year ago) I dabbled with it a little with the IHT unit as I figured with the CO2 scrubber I could actually discover what SaO2 level would trigger a contraction for me but it was taking quite a while and I started to feel rather strange and "chickened out".....maybe I should do a few more tests, but I am a little worried about extended low O2 exposures with the IHT unit as there is little to no life preserving dive response occuring.....

regards

 

[naiad]

You would need a sensor which can measure at least 10% CO2, and 10% = 100,000 ppm.

That's very high! I had no idea that it was possible to reach those levels of CO2.

ADR, make sure there's someone with you if you are experimenting with low O2 exposures.

Lucia