COPD accounts for up to 10% of all acute admissions, which makes it a priority area for all hospitals concerned with effective management of acute medical admissions. More than one in ten patients with COPD admitted to hospital are dead within 90 days of admission, and over 1 in 3 is readmitted to hospital during that time.  Studies on mortality after hospitalisation for an acute exacerbation of COPD have shown a one-year mortality ranging from 22% to 43%, and a 2-year mortality of 36 to 49%.

Exacerbations of COPD are caused by interactions between host factors, bacteria, viruses, and changes in air quality to produce increased inflammation in the lower airway.

We still tend to use the definition from 1987 when Anthonisen defined exacerbations specifically in terms of increased dyspnoea, sputum production, and sputum purulence: signs indicative of an infectious aetiology. If patients present with 2 out of 3 of these symptoms then the patient is defined as exacerbating. There are problems with this definition as many patients with COPD do not produce sputum but still exacerbate. Another common definition of an acute exacerbation is:

`a sustained worsening of the patient's condition, from the stable state and beyond -normal day-to-day variations, that is acute in onset and necessitates a change in regular medication in a patient with underlying COPD`.

For the patient an exacerbation is an alarming and often frightening experience. They can be of sudden onset and lead to a significant worsening of health status.

COPD exacerbations are categorized as mild, moderate and severe according to the level of health care contact: