Objectives & Background Previous studies suggest intravenous or nebulised magnesium sulphate may improve respiratory function in patients with acute asthma. We aimed to determine whether intravenous or nebulised MgSO₄ improve symptoms of breathlessness and reduce the need for hospital admission in adults with severe acute asthma.

Methods In a double-blind, placebo-controlled trial, we enrolled adults with severe acute asthma at emergency departments of 34 UK hospitals. We excluded patients with life-threatening features. Allocated participants received IV MgSO₄ (2 g in 20 min) or nebulised MgSO₄ (3×500 mg doses in 1 h)+standard therapy, or placebo+standard therapy. We assessed two primary outcome measures in all participants according to treatment group: the proportion of patients admitted to hospital within 7 days and breathlessness measured on a 100 mm VAS in the 2 h after initiation of treatment. We adjusted for multiple testing. Study registration no: ISRCTN04417063.

Results 1109 patients were recruited (Jul 2008 to Jun 2012). 261 (79%) of 332 patients allocated nebulised MgSO₄ were admitted to hospital, as were 285 (72%) of 394 patients allocated intravenous MgSO₄ and 281 (78%) of 358 controls. Breathlessness was assessed in 296 (89%) patients allocated nebulised MgSO₄, 357 (91%) patients allocated intravenous MgSO₄, and 323 (90%) controls. Rates of hospital admission did not differ between patients treated with either form of MgSO₄ compared with controls or each other. Change in VAS breathlessness did not differ between active treatments and control, but change in VAS was greater for patients in the intravenous MgSO₄ group than the nebulised MgSO₄ group (5.1 mm, 0.8 to 9.4; p=0.019). Intravenous or nebulised MgSO₄ did not significantly decrease rates of hospital admission and breathlessness compared with placebo: intravenous MgSO₄ was associated with an odds ratio of 0.73 (95% CI 0.51 to 1.04; p=0.083) for hospital admission and a change in VAS breathlessness of 2.6 mm (−1.6 to 6.8; p=0.231) compared with placebo; nebulised MgSO₄ was associated with an odds ratio of 0.96 (0.65 to 1.40; p=0.819) for hospital admission and a change in VAS breathlessness of −2.6 mm (−7.0 to 1.8; p=0.253) compared with placebo.

Conclusion These data suggest nebulised MgSO₄ has no role in the management of severe acute asthma in adults and at best suggest only a limited role for intravenous MgSO₄ in the ED.