Unlike skeletal muscle, cardiac muscle cannot be made to sustain tetanic contraction by high-frequency stimulation. What property of the ventricular action potential explains this?

• A Its plateau phase prolongs the absolute refractory period so it outlasts peak mechanical contraction ✓
• B Its rapid phase 0 upstroke prevents summation of successive depolarizations
• C Its dependence on extracellular calcium prevents repeated release of stored calcium
• D Its long duration exhausts ATP needed for cross-bridge cycling

Explanation

The plateau (phase 2) lengthens the ventricular action potential to 200 to 300 ms, so the absolute refractory period extends well into mechanical systole and the relative refractory period reaches early diastole. Because the muscle has begun to relax before it can be re-excited, temporal summation and tetanus are impossible. This protects ventricular filling. Skeletal muscle action potentials last only 1 to 2 ms, allowing fusion of contractions.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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