Final answer:
The most ominous sign of impending cardiopulmonary arrest in infants and children is bradycardia.
Step-by-step explanation:
The most ominous sign of impending cardiopulmonary arrest in infants and children is bradycardia. Bradycardia is a condition characterized by a slow heart rate, which can be a critical indicator of cardiac dysfunction. It occurs when the heart beats fewer than the normal number of times per minute.