A 70-year-old woman with ischaemic cardiomyopathy (left ventricular ejection fraction 30 percent) remains symptomatic with exertional dyspnoea despite appropriate doses of an angiotensin receptor blocker and a beta-blocker. Her blood pressure is 110/70 mm Hg, creatinine 95 µmol/L and potassium 4.5 mmol/L. According to Canadian heart failure guidelines, what is the next disease-modifying agent that should be added?