Complex sentence: The high potassium concentration in the extracellular cardioplegic solution lowers the membrane resting potential of cardiac cells to -50mV, which inactivates sodium channels, and results in a diastolic arrest of cardiac activity.
Split into simpler sentences: The high potassium concentration in the extracellular cardioplegic solution lowers the membrane resting potential of cardiac cells. As the resting potential approaches -50mV, sodium channels are inactivated resulting in in a diastolic arrest of cardiac activity.