A 28 year old man presents with polyuria, polydipsia and vomiting. He has type 1 diabetes diagnosed at age 14 but has not taken insulin for several days. On arrival, he is tachypnoeic with deep respirations, blood pressure 104/64 mm Hg and heart rate 110 beats per minute. Capillary glucose is 26 mmol/L and venous blood gas shows pH 7.15. What is the most important immediate first step in management?