What is the rationale for the use of angiotensin II receptor blockers (ARB) in heart failure?

ARBs produce more complete blockage of RAS (rennin angiotensin system) – only 30% of angiotensin II production is suppressed by ACE inhibitors. Angiotensin II can be produced by non ACE dependant pathways. Chymases and Cathepsin G can convert angiotensin I to II. There is an additional role for ARBs in patients with intolerance to ACE inhibitors, either due to cough or allergy.