Dose of magnesium sulphate for severe acute asthma.

Several studies have been published, looking at the efficacy of intravenous (IV) magnesium sulphate for severe acute asthma (1-3). Theses studies have used doses of magnesium sulphate in the range of 1.2 to 2 g IV over 20 minutes. No benefits were observed in patients with mild to moderate asthma, though there is some evidence magnesium sulphate may be beneficial in severe asthma.

A 35-year-old woman was admitted to our accident and emergency department suffering from severe acute asthma. She became critically hypoxic and required tracheal intubation and ventilation there. She was treated with nebulised salbutamol, nebulised ipratropium bromide, hydrocortisone IV and an aminophylline intravenous infusion.

After 12 hours of ventilation on our intensive care unit using remifentanil and propofol for sedation, her bronchospasm had resolved and she was extubated. She immediately developed severe bronchospasm again with rapidly worsening hypoxia. She was given a 5 g bolus of magnesium sulphate IV over five minutes and her bronchospasm rapidly resolved.

Re-intubation of the trachea was avoided. During infusion of the magnesium sulphate, no bradycardia, no arrhythmias and no muscle weakness occurred. Serum magnesium level was measured 10 minutes after the magnesium sulphate bolus and the level was reported as 1.8 mmol/l.

The author has personal experience of using high doses of magnesium sulphate, to induce deliberate hypotension without patients experiencing any cardiac arrhythmias or muscle weakness, despite having serum magnesium levels greater than 4 mmol/ l (4). The British National Formulary lists the magnesium sulphate loading dose for eclampsia as 4 g IV over five to 15 minutes (5).

The author postulates that the dose of magnesium sulphate used in earlier trials in acute asthma was inadequate (1.2 to 2 g IV over 20 minutes) and that 5 g IV over five minutes would be more efficacious. Magnesium sulphate is a safe drug when used at this dose and has the added benefit of its effects being immediately reversible by injecting calcium intravenously (6). A prospective, randomised clinical trial using this higher dose of magnesium sulphate is planned.


(1.) Rowe BH, Bretzlaff JA, Bourdon C, Bota GW, Camargo CA Jr. Intravenous magnesium sulfate treatment for acute asthma in the emergency department: a systematic review of the literature. Ann Emerg Med 2000; 36:181-190.

(2.) Silverman RA, Osborn H, Runge J, Gallagher EJ, Chiang W, Feldman J et al. IV magnesium sulfate in the treatment of acute severe asthma: a multicenter randomized controlled trial. Chest 2002; 122:489-497.

(3.) Porter RS, Braitman LE, Geary U, Dalsey WC. Intravenous magnesium is ineffective in adult asthma, a randomized trial. Eur J Emerg Med 2001; 8:9-15.

(4.) Sanders GM, Sim KM. Is it feasible to use magnesium sulphate as a hypotensive agent in oral and maxillofacial surgery? Ann Acad Med Singapore 1999; 27:780-785.

(5.) British National Formulary. From Accessed April 2009

(6.) Mordes JP, Wacker WE. Excess magnesium. Pharmacol Rev 1979; 29:273-300.


Kent, United Kingdom

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