Final answer:
When ventilation fails for an apneic patient, check for airway obstruction, clear it if present, and if necessary, call emergency services and prepare to perform CPR, focusing on chest compressions.
Step-by-step explanation:
If an initial attempt to ventilate an unresponsive apneic patient fails and a second attempt after repositioning the head is also unsuccessful, the next step should be to check for airway obstruction and attempt to clear it. If the patient is suspected of choking, encourage them to cough and deliver a few hard back slaps to help dislodge the blockage. If these interventions are ineffective, perform the Heimlich maneuver to remove the obstruction. If the Heimlich maneuver fails and the patient is still not breathing, it's necessary to call for emergency medical services and be prepared to perform CPR based on the current standards, which emphasize on chest compressions.