Mr Smith is 74 years old and has recently been diagnosed with heart failure. He attends the GP as he is now getting short of breath when doing minimal activity and requiring three pillows to sleep comfortably at night. He denies any chest pain, cough, fever, or weight loss. Current medication includes Ramipril, Bisoprolol, Atorvastatin and Aspirin.
Examination reveals pitting oedema to the mid-shins and bi-basal crackles. Systems examination is unremarkable, and observations are within normal limits.
Which one of the following medications should be added next?