Final answer:
If a newborn's heart rate remains under 60 bpm despite adequate ventilation and chest compressions, epinephrine administration and establishing vascular access is the next appropriate step in management.
Step-by-step explanation:
When resuscitation of a newborn is underway, and adequate ventilation with an endotracheal tube plus chest compressions for a heart rate under 60 bpm has been established, but there is no increase in heart rate after 60 seconds, the next step in management is to administer medications. According to Advanced Neonatal Life Support guidelines, the administration of epinephrine is usually considered at this point. This situation indicates a newborn emergency, and further medical interventions are necessary to stabilize the infant. Additionally, if not already done, establishing vascular access is essential for medication administration and possible fluid resuscitation. If the heart rate remains persistently low, it is important to reassess actions, such as the quality of chest compressions, airway management, and to ensure that there is no pneumothorax or other reversible cause of the low heart rate.