A 52-year-old woman with cirrhosis (Child-Pugh C) and portal hypertension is admitted with a serum lactate of 6.8 mmol/L, pH 7.25, and HCO₃⁻ of 12 mEq/L. Her blood glucose is 68 mg/dL, and ammonia is elevated at 120 µmol/L. Hepatic synthetic function is severely impaired (INR 2.8, albumin 2.1 g/dL). What is the most appropriate immediate next step in management?