In a patient with complete heart block, the ventricular escape rhythm originates from Purkinje fibers at ~30 bpm. The slow diastolic depolarization (phase 4) in these cells is driven primarily by which ionic mechanism?

• A Inward funny current (If) through HCN channels activated by hyperpolarization ✓
• B Inward ICaT current through T-type calcium channels
• C Decreased IK1 (inward rectifier) conductance as membrane potential falls
• D Increased INa (fast sodium current) leak conductance

Explanation

Phase 4 spontaneous depolarization in all automatic cardiac cells (SA node, AV node, His-Purkinje fibers) is initiated by the hyperpolarization-activated cyclic nucleotide–gated (HCN) channels that carry the inward funny current (If). These channels open when the membrane is hyperpolarized (below –50 to –60 mV) at the end of repolarization. Beta-adrenergic stimulation shifts the activation curve to less negative potentials (increased cAMP binding to HCN), accelerating automaticity; hence ivabradine (HCN blocker) slows heart rate.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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