Ivabradine is used in chronic heart failure and stable angina for heart rate reduction. Which unique mechanism differentiates it from beta-blockers?
- A Inhibits If ('funny' current) in SA node by blocking HCN4 channels, reducing heart rate without affecting contractility or conduction ✓
- B Blocks L-type calcium channels in SA node preferentially
- C Activates muscarinic M2 receptors to slow the SA node
- D Inhibits HCN4 channels only when heart rate exceeds 200 beats/min
Correct answer: A. Inhibits If ('funny' current) in SA node by blocking HCN4 channels, reducing heart rate without affecting contractility or conduction
Explanation
Ivabradine selectively and specifically inhibits the hyperpolarization-activated cyclic nucleotide-gated channel 4 (HCN4), which carries the funny current (If) in pacemaker cells of the SA node. If is responsible for the slow spontaneous diastolic depolarization (phase 4). Ivabradine's use-dependence means it enters and blocks the channel only when open (during hyperpolarization), so effect is heart-rate dependent. Unlike beta-blockers, ivabradine does not affect inotropy, AV conduction, or blood pressure, making it suitable for low-BP heart failure patients.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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