How come it's necessary to limit o2 flow rate with people with copd?

Copd: More oxygen detrimental to a COPD patient as more co2 accumulates resulting in lethargy and worsening respiratory failure.

It varies.: It isn't usually necessary to limit o2 flow in copd. In some cases of respiratory failure the co2 climbs and can cause co2 narcosis. Elevated o2 can worsen this situation and lead to respiratory failure. The drive to breathe is usually from co2, but this can be blunted in COPD causing a low o2 to be more dominant in driving breathing. This is controversial however.

Oxygen therapy...: Oxygen is a drug and there is a danger if too much oxygen is used. Too much supplemental oxygen can result in more lung damage. Some people with COPD will have an increase in their carbon dioxide level if given increased oxygen. So, oxygen therapy must be carefully titrated so the right amount of oxygen is supplemented.