A 74-year-old woman with chronic kidney disease stage 4 (eGFR 22 mL/min/1.73 m²) and hypertension presents with confusion and muscle weakness. Her medications include ramipril and spironolactone. Blood tests show potassium 6.8 mmol/L, creatinine 260 µmol/L (baseline 200), bicarbonate 18 mmol/L and ECG with peaked T waves. What is the most appropriate immediate management?