A 70-year-old man with heart failure with reduced ejection fraction (EF 30 percent) is on maximally tolerated ACE inhibitor, beta-blocker, and mineralocorticoid receptor antagonist. His blood pressure is 110/70 mm Hg and creatinine is normal. He remains NYHA class II with occasional dyspnea on exertion. What additional drug class should be added to reduce mortality and hospitalizations?