What is the most important indicator of successful positive pressure ventilation in a newborn?

The most important indicator of successful positive pressure ventilation in a newborn is the improvement in the infant's heart rate and oxygen saturation levels. Effective ventilation should lead to an increase in heart rate to above 100 beats per minute and an increase in oxygen saturation, typically above 90%. Additionally, clinical signs such as improved chest movement, color, and responsiveness also indicate successful ventilation.