The plateau phase (phase 2) of the cardiac ventricular action potential is primarily maintained by which ionic current balance?

• A Inward fast Na⁺ current (INa) balanced by outward transient outward K⁺ current (Ito)
• B Inward funny current (If) balanced by inward Na⁺/Ca²⁺ exchanger current
• C Inward L-type Ca²⁺ current (ICaL) balanced by outward slow delayed rectifier K⁺ current (IKs) ✓
• D Inward T-type Ca²⁺ current balanced by outward rapid delayed rectifier K⁺ current (IKr)

Explanation

Phase 2 plateau results from near-equilibrium between sustained inward L-type Ca²⁺ current (ICaL) and outward slow delayed rectifier K⁺ current (IKs), plus the slow inward Na⁺ current. This prolonged plateau is unique to cardiac muscle and prevents tetany. The fast Na⁺ current (INa) is responsible for phase 0 (rapid depolarization) and is largely inactivated by phase 2. If (HCN channel current) is relevant to SA node automaticity, not the plateau.

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

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