A 67-year-old man with chronic kidney disease stage 4 presents with muscle weakness. His medications include lisinopril and spironolactone. Laboratory results show potassium 6.4 mEq/L, creatinine 2.8 mg/dL, and ECG with peaked T waves. He is haemodynamically stable. Which is the most appropriate initial therapy to reduce the risk of arrhythmia?