Final answer:
The best initial intervention to increase the heart rate of the bradycardic and cyanotic newborn would be to ensure a patent airway and start artificial ventilation.
Step-by-step explanation:
The best initial intervention for a newborn with a heart rate of 50 beats per minute and exhibiting cyanosis following the five-minute APGAR assessment would be to deliver adequate artificial ventilation after ensuring a definitive airway. This approach addresses the critical APGAR criteria of heart rate and respiration, which are essential for the stabilization of the newborn. In this scenario, interventions such as administering epinephrine or a vasopressor agent intravenously, or performing "fast and hard" chest compressions, would follow if initial resuscitation efforts with ventilation do not improve the infant's condition.