Ivabradine reduces heart rate by selectively blocking the If (funny current) channel. In which specific cell type and at what phase of the action potential does this occur?

• A Sinoatrial nodal cells during phase 4 (spontaneous diastolic depolarisation) ✓
• B AV nodal cells during phase 0 (rapid depolarisation)
• C Purkinje fibres during phase 2 (plateau)
• D Ventricular myocytes during phase 3 (rapid repolarisation)

Explanation

Ivabradine selectively blocks the hyperpolarisation-activated cyclic nucleotide-gated (HCN) channel carrying the If (funny) current in sinoatrial nodal pacemaker cells. This current activates during phase 4 (spontaneous diastolic depolarisation) and is the principal determinant of pacemaker rate. By prolonging phase 4, ivabradine slows the sinus rate without affecting myocardial contractility, blood pressure, or AV conduction.

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

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