Question

8. Injections of potassium chloride are used in lethal injections to stop the heart. Which part of the cardiac action potential is most affected by increasing extracellular potassium levels?

1. The plateau phase is greatly reduced.                                       d. The repolarizing phase is prevented.        
2. Tetanus develops as calcium drops dramatically.                    e. All these are affected.
3. The initial depolarizing phase is blocked.

Answer 1

d. The repolarizing phase is prevented

Explanation: Injection of potassium chloride leads to hyperkalemi. This increases the extracellular potassium concentration. As a result the potassium gradient across the cell membrane is reduced. potassium tends to remain inside the cell even during the phase of repolarization leading to persistent depolarization of the cell. As repolarization is blockaed, the heart cell is unable to undergo another action potential and contraction leading to stopping of the heart.