Aldosterone antagonists (spironolactone, eplerenone) also known as “potassium sparing diuretics” block the action of aldosterone inhibiting the reuptake of sodium and water. Normally, when sodium reabsorbed it is exchanged with potassium which is then excreted. Since aldosterone inhibition decreases sodium reabsorption, it also decreases potassium excretion resulting in higher serum potassium levels.
Spironolactone is indicated (class IIa, level of evidence B) in systolic heart failure with recent or current New York Heart Association functional class IV symptoms, preserved renal function and a normal potassium concentration.
Spironolactone was investigated in the RALES trial and a mortality benefit was shown in New York Heart Association functional class III and IV patients. Significant hyperkalemia did contribute to sudden cardiac death.
Note that unfractionated heparin also has aldosterone antagonist effects.