They act mainly by inhibiting phosphodiesterase, which has various isozymes. They specifically inhibit the phosphodiesterase 3 enzyme responsible for breakdown of cAMP as:
a. increased levels of cAMP cause increase in intracellular Ca++ in heart, leading to increase in force of contraction.
b. has vasodilatory effect
a. Acute CCF
b. Treatment of acute attacks in patients suffering from chronic heart failure.
Vomiting, nausea, arrhythmias
Increase in liver enzymes, having adverse effects on liver cells, may cause liver damage.
These drugs are only used in emergency, when patient is non-responsive to other drugs. Milrinone is safer while Inamrinone is not.