A 76-year-old man with a history of ischaemic cardiomyopathy (left ventricular ejection fraction 30%) and NYHA class II symptoms is on maximally tolerated doses of an ACE inhibitor, a beta-blocker and a mineralocorticoid receptor antagonist. His estimated glomerular filtration rate is 55 mL/min/1.73 m² and potassium is 4.6 mmol/L. According to current Canadian heart failure guidance, which additional medication class should routinely be added to reduce morbidity and mortality?