A 56-year-old man with biopsy-proven cirrhosis secondary to hepatitis C presents with increasing abdominal distension. Examination shows shifting dullness and bilateral pitting oedema but no encephalopathy. Serum creatinine is 90 µmol per litre, sodium 136 mmol per litre. Ultrasound confirms moderate ascites. According to liver disease management guidelines, what is the most appropriate initial pharmacologic regimen for his ascites?