A patient with acute decompensated heart failure on chronic carvedilol therapy remains hypoperfused despite low urine output. In this setting milrinone may be preferred over dobutamine because milrinone:
- A Stimulates beta-1 receptors more potently than endogenous catecholamines
- B Blocks phosphodiesterase type 5 selectively in the pulmonary vasculature
- C Increases cAMP downstream of the beta receptor, so its action is preserved despite beta-receptor blockade ✓
- D Sensitizes troponin C to calcium without raising intracellular calcium
Correct answer: C. Increases cAMP downstream of the beta receptor, so its action is preserved despite beta-receptor blockade
Explanation
Milrinone inhibits phosphodiesterase type 3 in cardiac myocytes and vascular smooth muscle, preventing cAMP degradation. Because it acts distal to the beta-adrenoceptor, its positive inotropic and vasodilating effects persist in patients receiving chronic beta-blockade, where dobutamine's beta-1 agonist effect is blunted. This combination of inotropy and vasodilation is called an inodilator effect. Troponin sensitization describes levosimendan's mechanism, not milrinone's, and PDE5 inhibition relates to pulmonary hypertension agents such as sildenafil.
Reference: Katzung's Basic and Clinical Pharmacology, 16th ed.
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