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I have been running 20 miles a week (three hour long runs, monday, thursday, and saturday) for a long long time. Last year in my annual physical I had a venous blood carbon dioxide level of 29. This year it is 33 (upper limit of normal is 31).

Is my blood carbon dioxide level high because my system has 'learned' to tolerate high levels of carbon dioxide? Am I going to drop dead in a week or two? Does anyone know?

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As far as I am aware, you don't get hypercapnia simply from being an endurance athlete, there is usually some other disease mechanism going on, or there is some sort of artificial/environmental cause. I would be interested in why you are having CO2 levels drawn/measured in the first place? Its a specific blood test, although possibly included in metabolic panels. – JohnP Sep 6 '13 at 22:40
I have to take annual physicals. @JohnP – user10739 Sep 7 '13 at 21:24
That says nothing. I take annual physicals as well, and I've never had a CO2 level drawn other than during some college classes. Is this just a normal test your doctor orders, or is there reason that you have an uncommon test performed yearly? – JohnP Sep 8 '13 at 14:25
Dear John- CO2 levels are just part of the chemistry panel at my hospital. They are included along with things like electrolytes, BUN, liver enzymes, and so forth. @JohnP – user10739 Sep 9 '13 at 20:03
CO2 chem panel is a measure of HCO3- in the blood. It can be artificially high if you are dehydrated or taking antacids or certain medicines. (Sorry, I missed the response and had thought you were getting the CO2 as part of a blood gas, not the chem panel. Blood gases are not routinely done.) – JohnP Jun 5 '14 at 15:09

Your system might have "learned" to tolerate acutely high levels of CO2 during training, but it is not normal during rest. As your cardiovascular fitness increases, you become more efficient at using O2 and excreting CO2, and as such your ventilation rate falls to maintain a normal blood PH.

There are many reasons for an increased CO2, many of which should be examined.

A ventilation perfusion mismatch can be caused by bronchitis, asthma, pulmonary edema, hepatopulmonary syndrome. In case you are a smoker, it can be due do emphysema or COPD. Infections such as TB. Almost any pathological process in the lungs can increase the CO2.

I'm assuming your other values are fine since you did not mention them. That includes the blood PH. Your ventilation rate can decrease to compensate for a high PH caused by kidney problems; compensated metabolic alkalosis. This has many reasons, with the most frequent one being diabetic nephropathy in case you are diabetic. You can also have overtraining syndrome, which increases cortisol, which in term damages the kidneys, but that is hard to diagnose.

I would suggest you go to your doctor to check your kidney function (urinalysis and bloodwork), and pulmonary status (spirometry etc.). In case they turn up negative, try lowering the intensity of your workouts for some time to decrease cortisol levels. It is obviously a chronic change, so there is no need to worry about dropping dead at practice, however, I would recommend that you stop practicing until you know the etiology so as not to stress your body.

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