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In 1965, Peter Bennett studied the effect of breathing different gas mixtures
at 
10.8 ATA total pressure on cortical (brain) oxygen and carbon dioxide levels in 
spontaneously breathing anesthetized cats 1.  Baseline cortical CO2 was 53 mmHg 
and O2 was 77mmHg at 1 ATA air.  Below is a table of the results with different 
gas mixtures.  All changes represent changes from the baselines above.
    
    Gas                     CO2 % change        O2 % change
    1 ATA O2                    +5.6%           +86%
    2.34 ATA O2                 +7%             +214%
    8.67 ATA He/2.34 ATA O2     +5.1%           +297%
    8.67 ATA N2/2.34 ATA O2     +12.1%          +121%
    8.67 ATA Ar/2.34 ATA O2     +20.5%          +91%

So, as the density of gas increased, the cortical CO2 increased. 
Unfortunately, 
arterial levels of CO2 and O2 were not measured, so it is difficult to say that 
the increase is from increased systemic CO2 production.  Interestingly, as the 
density of gas mixture increased, the increase in cortical O2 was limited.  
Again, this is not specifically addressed in this study, but it could be due to 
decreased respiratory efficiency with the denser gases.  CO2 also increased 
slightly at 1 ATA O2.  The mechanism of this was not addressed, but could be 
related to decreased dexoyhemoglobin transport of CO2 from the tissue (normally 
an important mechanism by which CO2 is removed from the tissue is by binding to 
the hemoglobin that has given up it's oxygen to the tissue.  When PO2's are 
high, there is less desaturated hemoglobin.).  

Increased CO2 can clearly exacerbate hyperoxic seizures.  The mechanism is not 
clear, however.  One potential explanation could be due to increased 
catecholamines in the brain.  Production of hydrogen peroxide from 
catecholamines by monoamine oxidase seems to be a very important mechanism in 
hyperoxic seizures.  When CO2 increases, catecholamines increase (and this
could 
also be an explanation of why stress can cause seizures).  

FYI,
Eddie Brian

1. Bennett PB: Cortical CO2 and O2 at high pressures of argon, nitrogen,
helium, 
and oxygen. J Appl Physiol 1965;20:1249-1252

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