A 68-year-old man with ischaemic cardiomyopathy (left ventricular ejection fraction 30 percent) remains dyspnoeic on minimal exertion despite guideline-directed doses of an ACE inhibitor, a cardioselective beta blocker, and spironolactone. Blood pressure is 110/70 mmHg, and renal function is stable. According to Canadian heart failure guidance, which change is the most appropriate next step to optimise his medical therapy?