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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, 2013 at 22:40
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    I have to take annual physicals. @JohnP
    – user10739
    Sep 7, 2013 at 21:24
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    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, 2013 at 20:03
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    I run the same number of days a week with Saturday as my long run. I had high cholesterol, decided to stop taking liptor and change my diet to control the problem. I get blood tests every other quarter during the year. Carbon dioxide has always been part of the normal lipid testing. My last 3 test have been 29, 28, 30, not sure why it's high, my doctor knows I'm a runner and not worried about it.
    – Rick
    Nov 27, 2017 at 2:31
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    Similar results here. I have high C02 (avg of 29.6 over 5 measurements over 5 years) I swim 30 min 2X per week, run 40 min 2X per week, do a 15 min HIITS with 100 m Sprints or on stairmaster (hard 30 sec x 5) , and a few tabadas and lift weights. Dont smoke, dont drink and eat super healthy. The only thing I've found on the internet is that C02 tends to be much higher after exercise. Do you have low RBC? Mine is 4.4-4.6 sciencedirect.com/science/article/pii/S187704281100509X
    – moonshot
    Feb 7, 2020 at 19:27

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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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