A preterm infant (32 weeks gestation) born to a mother with prolonged rupture of membranes has an APGAR score of 3 at 1 minute (HR 50 bpm, no respiratory effort, limp, no response to stimulation, pale). Immediate resuscitation has been initiated with positive pressure ventilation and chest compressions. After 15 seconds of adequate ventilation and compressions, the heart rate remains 40 bpm. What is the most appropriate next step?