Children were randomly allocated ( 1:1 ) to receive nebulised salbutamol and ipratropium bromide with either 25 mL of isotonic MgSO ( 4 ) ( 250 mmol/L ; 151 mg per dose ; MgSO ( 4 ) group ) or 25 mL of isotonic saline ( placebo group ) on three occasions at 20-min intervals .