A 58-year-old man with HFrEF (LVEF 30%) remains symptomatic despite maximally tolerated guideline-directed therapy. He is in sinus rhythm with a resting heart rate of 82 bpm and blood pressure 118/72 mmHg. A drug is added to reduce his heart rate further. What is its mechanism of action?
- A Non-selective blockade of beta-1 and beta-2 adrenergic receptors
- B Selective inhibition of the funny current (If) in the sinoatrial node ✓
- C Inhibition of phosphodiesterase type 3 increasing cAMP
- D Blockade of slow calcium channels in nodal tissue
Correct answer: B. Selective inhibition of the funny current (If) in the sinoatrial node
Explanation
Ivabradine selectively inhibits the hyperpolarization-activated cyclic nucleotide-gated funny current (If) in the sinoatrial node, lowering heart rate without affecting blood pressure or contractility. It is indicated in sinus rhythm with resting rate 70 bpm or more despite maximal beta-blockade. Phosphodiesterase-3 inhibitors such as milrinone are positive inotropes used acutely in decompensation, and they raise rather than lower heart rate. Calcium channel blockers are avoided in HFrEF.
Reference: Katzung's Basic and Clinical Pharmacology, 16th ed.
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