The unique mechanism of ivabradine's heart rate reduction — distinct from beta-blockers — is through:

• A Selective and specific blockade of the HCN4 channel (If — 'funny current') in the sinoatrial node, slowing diastolic depolarisation without affecting myocardial contractility or blood pressure ✓
• B Blocking the delayed rectifier potassium current (IKr) in SAN, prolonging action potential duration
• C Inhibiting the sodium-calcium exchanger (NCX) in the SAN, reducing calcium-dependent automaticity
• D Antagonising adenosine A1 receptors in the SAN, blocking adenosine-mediated heart rate slowing

Explanation

Ivabradine selectively blocks the HCN4-encoded hyperpolarisation-activated cyclic nucleotide-gated channel (If or 'funny current') in the sinoatrial node pacemaker cells. This If channel carries an inward Na+/K+ current during diastolic depolarisation phase 4, driving the membrane potential toward threshold. By blocking If in a use-dependent and heart-rate-dependent manner (binding from within the open channel), ivabradine slows spontaneous depolarisation and reduces heart rate without affecting AV conduction, myocardial contractility, or blood pressure — making it safe in heart failure where negative inotropy from beta-blockers may be problematic.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.