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At 04:10 PM 96-3-18 CST, you wrote: >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 >-- Brian, I have another theory on this paper. I know nothing of feline anesthesia, but the baseline pCO2 of 58 is probably typically high as you would expect anesthetized humans spontaneously breathing. The values of CO2 increase is comparable, 5%,7%,5% for 02 and He. The CO2 increase in the first three levels was slight compared to the density (1,2,10 ATM), but when the narcotic value of the mixture increase, the CO2 retention increased dramatically. Notice the difference in N2 and then to Ar which is the most narcotic. As you know, increasing depth of anesthesia increases CO2 retention in spontaneously breathing man and probably cats too. I have no idea what the MAC of 8 ATM of Argon is, but I think you could probably do some serious surgery at that level. There is no mention of respiratory minute volume, but I suspect that it was so depressed at the Ar level that the O2 was simply not being exchanged well. Is the anesthesia effect of narcotic gasses more important than density at normal diving depths? Maybe. Some other thoughts. Is the 5% increase with 1 Atm of O2 due to it's reputed narcotic effect? The only place where density jumps out at me is between groups 2 and 3 where O2 is the same and the He is added and CO2 retention falls. This may not be statistically significant however. Art P.S. Is this Bennet the one at DAN?
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