Carbon dioxide... In copd, if you are unable to effectively breathe, your carbon dioxide level would be high. If your oxygen level is low, one compensation is to increase your ventilation, if possible, resulting in low carbon dioxide levels but higher oxygen levels. This usually is what happens earlier in the disease process.
Low CO2... COPD is a chronic lung disease. If your carbon dioxide level is low, it means you are breathing deeper and/or faster and you are "blowing off" your carbon dioxide. It is often a sign that you are working harder to breathe. If your oxygen level is normal, it means you are able to get in enough oxygen. If your oxygen level is low, supplemental oxygen is prescribed.