A 45 year old male known case of diabetes presents with nausea , multiple episodes of vomiting and abdominal pain.He is stuporous , having blood glucoselevel of 402 mg% and on urine examination Ketone bodies are detected.He is having tachycardina along with ohostatic hypotension. On lab investigation,his potassium levels are 3 meq/L.The next best step in the management will be: