The investigators propose a 6-centre randomized, double-blind, placebo-controlled trial. Two groups will be compared: IV Mg sulfate and IV (intravenous) 0.9% saline placebo. After initial therapy with the systemic Corticosteroids (CSs) routinely used for acute asthma management at a given site, 3 treatments with inhaled salbutamol and ipratropium (ipratropium use as per local practice), eligible patients with PRAM ≥5 will, under the care of the research nurse, receive a 30-minute IV infusion of 75 mg/kg of Mg sulfate (maximum 2.0 g) \[experimental group\] or an identical volume of 0.9% saline \[control group\]. Outcomes will be measured during the 180-minute observational period in the ED and at 72 hours post ED discharge.
