Dr WFS Sellers and colleagues describe several cases that demonstrate convincingly a protective effect of intravenous magnesium sulphate against the tachycardia produced by intravenous salbutamol in patients with asthma. This effect was observed both when magnesium was given before and when given after the salbutamol. It was seen in critically ill asthmatic patients and in a volunteer with well-controlled asthma.

Intravenous magnesium sulphate increases atrial contraction time and refractory times. It is used to treat atrial tachyarrhythmias and has a negative chronotropic and dromotropic effect.
Intravenous magnesium sulphate prevents intravenous salbutamol tachycardia in asthma
Br J Anaesth. 2010 Dec;105(6):869-70