Final answer:
When a newborn's heart rate is undetectable and the baby is not vigorous, beginning chest compressions is the immediate course of action. This is in line with CPR guidelines for newborns to maintain blood circulation, especially to the brain.
Step-by-step explanation:
If the heart rate cannot be determined by stethoscope and the baby is not vigorous, the correct action would be to begin chest compressions.
The absence of a heart rate and lack of vigor in the newborn are indications that the baby is in critical condition and requires immediate medical intervention.
According to resuscitation guidelines, chest compressions are part of cardiopulmonary resuscitation (CPR), which is essential in such emergency situations to maintain blood flow, especially to the brain to prevent irreversible damage. The technique involves applying pressure with the flat portion of one hand on the sternum between the line at T4 and T9. Compressions are to be performed at a depth of at least 5 cm and a rate of 100 compressions per minute.
CPR should continue until the baby regains spontaneous heart activity or a healthcare professional takes over.