Background

Definition Asthma is characterised by dyspnoea, cough, chest tightness, wheezing, variable airflow obstruction, and airway hyperresponsiveness. The diurnal variation of peak expiratory flow rate (PEFR) is increased in people with asthma. Chronic asthma is defined as asthma requiring maintenance treat- ment, and will be dealt with in a separate “Extract from Clinical Evidence.” 1 2 Acute asthma is defined here as an exacerbation of underlying asthma requiring urgent or emergency treatment.

Incidence/prevalence Reported prevalence of asthma is increasing worldwide. About 10% of people have had an attack of asthma. 3 4

Aetiology/risk factors Most people with asthma are atopic; exposure to certain stimuli initiates inflammation and structural changes in airways, causing airway hyperresponsiveness and variable airflow obstruction, which in turn cause most asthma symptoms. Stimuli include environmental allergens, occupational sensitising agents, and respiratory viral infections. 5 6

Prognosis About 10-20% of people presenting to the emergency department with asthma are admitted to hospital. Of these, fewer than 10% receive mechanical ventilation, 7 8 although previous ventilation is associated with a 19-fold increased risk of ventilation for a subsequent episode.9 It is unusual for …