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OBJECTIVE: To develop a set of comprehensive, standardized evidence-based guidelines for the assessment and treatment of acute asthma in adults in the emergency setting. OPTIONS: The use of medications was evaluated by class, dose, route, onset of action and optimal mode of delivery. The use of objective measurements and clinical features to assess response to therapy were evaluated in relation to the decision to admit or discharge the patient or arrange for follow-up care. OUTCOMES: Control of symptoms and disease reflected in hospital admission rates, frequency of treatment failures following discharge, resolution of symptoms and improvement of spirometric test results. EVIDENCE: Previous guidelines, articles retrieved through a search of MEDLINE, emergency medical abstracts and information from members of the expert panel were reviewed by members of the Canadian Association of Emergency Physicians (CAEP) and the Canadian Thoracic Society. Where evidence was not available, consensus was reached by the expert panel. The resulting guidelines were reviewed by members of the parent organizations. VALUES: The evidence-based methods and values of the Canadian Task Force on the Periodic Health Examination were used. BENEFITS, HARMS AND COSTS: As many as 80% of the approximate 400 deaths from asthma each year in Canada are felt to be preventable. The use of guidelines, aggressive emergency management and consistent use of available options at discharge are expected to decrease the rates of unnecessary hospital admissions and return visits to emergency departments because of treatment failures. Substantial decreases in costs are expected from the use of less expensive drugs, or drug delivery systems, fewer hospital admissions and earlier return to full activity after discharge. RECOMMENDATIONS: Beta2-agonists are the first-line therapy for the management of acute asthma in the emergency department (grade A recommendation). Bronchodilators should be administered by the inhaled route and titrated using objective and clinical measures of airflow limitation (grade A). Metered-dose inhalers are preferred to wet nebulizers, and a chamber (spacer device) is recommended for severe asthma (grade A). Anticholinergic therapy should be added to beta 2 agonist therapy in severe and life-threatening cases and may be considered in cases of mild to moderate asthma (grade A). Aminophylline is not recommended for use in the first 4 hours of therapy (grade A). Ketamine and succinylcholine are recommended for rapid sequence intubation in life-threatening cases (grade B). Adrenaline (administered subcutaneously or intravenously), salbutamol (administered intravenously) and anesthetics (inhaled) are recommended as alternatives to conventional therapy in unresponsive life-threatening cases (grade B). Severity of airflow limitation should be determined according to the forced expiratory volume at 1 second or the peak expiratory flow rate, or both, before and after treatment and at discharge (grade A). Consideration for discharge should be based on both spirometric test results and assessment of clinical risk factors for relapse (grade A). All patients should be considered candidates for systemic corticosteroid therapy at discharge (grade A). Those requiring corticosteroid therapy should be given 30 to 60 mg of prednisone orally (or equivalent) per day for 7 to 14 days; no tapering is required (grade A). Inhaled corticosteroids are an integral component of therapy and should be prescribed for all patients receiving oral corticosteroid therapy at discharge (grade A). Patients should be given a discharge treatment plan and clear instructions for follow-up care (grade C). VALIDATION: The guidelines share the same principles of those from the British Thoracic Society and the National Institutes of Health. Two specific validation initiatives have been undertaken: (a) several Canadian centres have been involved in the collection of comprehensive administrative data to assess compliance and outcome measures and (b) a survey of Canadian emergency physicians conducted to gather baseline informaton of treatment patterns, was conducted before development of the guidelines and will be repeated to re-evaluate emergency management of asthma.