An asthmatic patient presents with a severe attack of acute wheezing and breathless.Arterial blood gas analysis done after half an hour of treatment with oxygen, nebulised salbutamol and intravenous corticosteroid show pH 7.26, PaO2 of 60 mm Hg and PaCO2 of 60 mm Hg.The next step in treatment should be -

• A. Mechanical ventilation
• B. Intravenous bicarbonate
• C. Intravenous salbutamol
• D. Increase rate of oxygen delivery