After 30 seconds of positive-pressure ventilation, a newborn's heart rate is 50 beats/min and its face and trunk are cyanotic. The EMT should:

In the scenario described, the newborn is exhibiting a heart rate of 50 beats per minute, which indicates bradycardia, and the presence of cyanosis in the face and trunk suggests inadequate oxygenation. In newborns, a heart rate below 60 beats per minute during resuscitation is a critical sign that requires immediate intervention.

Beginning chest compressions is the appropriate response because, according to neonatal resuscitation guidelines, if the heart rate remains below 60 beats per minute despite adequate ventilation, chest compressions are indicated. This approach helps to mechanically pump blood and enhance circulation, effectively delivering oxygen to vital organs. The combination of compressions and positive-pressure ventilation is essential in restoring a normal heart rate and improving the newborn's overall condition.

In this case, providing blow-by oxygen or increasing the ventilation rate alone would not be adequate, as the heart rate needs more direct intervention. Resuctioning the mouth and nose is also unnecessary at this stage since the primary issue is not airway obstruction but rather inadequate ventilation leading to bradycardia and cyanosis. Therefore, initiating chest compressions is crucial to improving the newborn's heart rate and oxygenation status.