Certified Registered Nurse Anesthetist (CRNA) Practice Exam

What enzyme does milrinone inhibit to exert its effects?

1. Phospholipase

2. Phosphodiesterase-3

3. Cyclic AMP

4. ATPase

The correct answer is: Phosphodiesterase-3

Milrinone exerts its effects primarily by inhibiting phosphodiesterase-3 (PDE-3). This enzyme is responsible for the breakdown of cyclic adenosine monophosphate (cAMP) within cardiac and vascular smooth muscle cells. By inhibiting PDE-3, milrinone increases the levels of cAMP, leading to enhanced intracellular calcium concentrations and subsequent positive inotropic (increased contractility) and vasodilatory effects. In the context of heart failure and cardiogenic shock, the use of milrinone can significantly improve cardiac output and reduce myocardial oxygen demand, making it a valuable tool in managing patients with these conditions. The inhibition of PDE-3, therefore, is crucial because it directly correlates with the drug's ability to modulate cardiac contractility and vascular tone effectively.