Objective The objective is to compare the efficacy of spironolactone and eplerenone on clinical outcome in patients with heart failure and a reduced ejection fraction.

Method The study is a crossover cluster randomized trial. Each heart failure clinic in Denmark will be allocated to four periods (clusters): two periods with spironolactone and two periods with eplerenone as first drug. The planned total participation time for each department is 4 years and we estimate that data from 7200 patients will be accrued in this period. Endpoints will be assessed through Danish National Registries.

Inclusion Criteria:

• all patients registered in the Danish Heart Failure Registry who has a baseline left ventricular ejection fraction <40% and who has filled in a prescription for an aldosterone antagonist

Exclusion Criteria:

• patients who had filled in a prescription for an aldosterone antagonist prior ro registration in the Danish Heart Failure Registry

Data is located on research servers with Statistics Denmark. Individual participant data may not be exported from the servers.