Interest in electrolyte disturbance in patients with respiratory failure has so far been focused on serum potassium especially linked to therapy with β2-agonists. Recently, hypomagnesaemia, hypophosphatemia, and hypocalcaemia have also been reported after administration of β-agonists in normal subjects and in asthmatic patients as well. Hypokalemia, hypomagnesemia, and hypocalcaemia, are well known causes of cardiac arrhythmias. In addition, hypophosphatemia can worsen respiratory failure in severely ill patients through impairment of respiratory muscle performance.