What does ‘carbon dioxide retainer’ mean?
Being a carbon dioxide (CO2) retainer means that too little CO2 is removed from the blood by the lungs. The resulting condition is called hypercapnia.
Hypercapnia, also known as hypercarbia and CO2retention, is a condition of abnormally elevated carbon dioxide (CO2) levels in the blood. Carbon dioxide is a gaseous product of the body's metabolism and is normally expelled through the lungs 
People with COPD are more likely to have the complication of hypercapnia if they are taking supplemental oxygen as part of their treatment regime.
During metabolism (the chemical processes that occur as the body carries out its many functions) oxygen is used and CO2 is produced. The CO2 is largely removed through the lungs when you exhale (breathe out).
What are the possible effects of hypercapnia?
High CO2 levels in the blood can cause symptoms such as headaches, drowsiness, lethargy, confusion and if not diagnosed and treated can lead to coma and death.
How does oxygen cause hypercapnia?
In healthy people high CO2 levels stimulate the brain to breathe. A person with COPD may have higher levels of CO2 than a healthy individual, and over time become less sensitive to high CO2 levels triggering the breathing response. This person then relies more on low oxygen levels to stimulate breathing.
If a person is inhaling too much oxygen this further reduces the trigger to breathe, allowing higher CO2 levels to accumulate in the body.
Too much oxygen can lead to higher CO2 levels with resulting hypercapnia.
The importance of medical advice
It is absolutely vital that you see a qualified medical practitioner to assess your specific medical situation in relation to supplemental oxygen needs.
”… controlled oxygen therapy to maintain oxygen saturation at around 90% (88-92% is an acceptable range) will minimise the risk of hypercapnia .”
Supplemental oxygen therapy is not always the correct or appropriate treatment for breathlessness. Medical grade oxygen must always be prescribed by a doctor.
Similarly you should not alter your oxygen intake without checking with your doctor.
Tran, Dr Khoa. ‘Carbon Dioxide Retention in Patients with Chronic Obstructive Pulmonary Disease’ in LungNet News, November 2009 at Carbon Dioxide Retention in Patients with Chronic Obstructive Pulmonary Disease