As a non-selective bronchodilator, magnesium sulfate (MgSO4) is effective when administered intravenously in the treatment of the patients with acute severe asthma not responding to conventional therapy (oxygen, nebulized salbutamol, and corticosteroids), which can resulted in earlier improvement in clinical signs and symptoms of asthma and PEF. However, the use of intravenous MgSO4 administered is not common in clinical practice, because it's prone to have adverse effects and side effects such as nausea, vomiting, facial flushing, hypotension, decreased tendon reflexes and so on with this treatment. The aerosolised route offers the advantage of lower dosage, a shorter time of drug being delivered to the airway smooth muscle and lower incidence of side effects when compared to the intravenous route. At present, there are only a few studies about the effects of nebulized MgSO4 in the treatment of acute asthma in children, and the conclusions are controversial.

Inclusion Criteria:

• known cases of controlled bronchial asthma
• ≧4 years of age
• the result was positive after acetylcholine provocation test in follow-up visit

Exclusion Criteria:

• had received corticosteroids (inhaled or systemic) or β2-agonists or theophyllines before the clinic day
• had fever (axillary temperature>38.5℃)
• history of chronic disease like bronchopulmonary dysplasia or cystic fibrosis
• history of renal insufficiency and known allergy to acetylcholine, albuterol or magnesium