Ivabradine reduces heart rate without affecting myocardial contractility or blood pressure. Its mechanism is:
- A Selective β1-adrenoceptor blockade in the sinoatrial node
- B Blockade of L-type calcium channels, reducing phase 0 upstroke velocity
- C Selective inhibition of funny current (If) channels (HCN4) in the sinoatrial node, slowing phase 4 depolarisation ✓
- D Activation of inward rectifier K+ channels (IK1), hyperpolarising the SA node
Correct answer: C. Selective inhibition of funny current (If) channels (HCN4) in the sinoatrial node, slowing phase 4 depolarisation
Explanation
Ivabradine selectively blocks the hyperpolarisation-activated cyclic nucleotide-gated channel HCN4 (If or 'funny' channel) in the SA node, which is the primary current driving phase 4 spontaneous depolarisation (pacemaker potential). By slowing If influx, it reduces spontaneous depolarisation rate, lowering heart rate without negative inotropy or blood pressure effects. It is used in stable angina (rate reduction) and heart failure with reduced ejection fraction when β-blockers are insufficient or contraindicated.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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