Which statement best describes the effect of hyperkalemia on cardiac resting membrane potential and conduction?

• A. Hyperkalemia depolarizes the resting membrane potential, reduces excitability, and slows conduction.
• B. Hyperkalemia hyperpolarizes the resting membrane potential and increases excitability.
• C. Hyperkalemia depolarizes the resting membrane potential, reduces excitability, slows conduction, and causes characteristic ECG changes.
• D. Hyperkalemia has no effect on conduction.

Answer: (C)

Hyperkalemia raises the outside potassium concentration, which reduces the potassium gradient across the cell membrane. That makes the resting membrane potential less negative (depolarized). A depolarized resting potential in cardiac cells inactivates some fast sodium channels, lowering the upstroke velocity of the action potential and slowing conduction through the heart. Clinically, this manifests as characteristic ECG changes such as tall, peaked T waves, PR prolongation, QRS widening, and can progress to a sine-wave pattern with severe hyperkalemia. So the depolarized resting potential, reduced excitability, slowed conduction, and the ECG changes together describe the effect most accurately.