Final answer:
The next step after an unsuccessful initial attempt to ventilate an unconscious adult victim is to reposition the head and try to ventilate again. If this fails, check for and clear any visible obstructions with a finger sweep before proceeding to CPR if needed.
Step-by-step explanation:
When you arrive at the scene and find an unconscious adult victim where the initial attempt to ventilate proves unsuccessful, the next step should be to reposition the head and attempt to ventilate again. If ventilation does not work after repositioning the head, it may be necessary to perform a finger sweep to check for and remove any visible obstruction in the mouth that could be blocking the airway. If you do not see an object, do not perform a blind finger sweep as you could push the object further into the airway. Remember to follow the current guidelines for cardiopulmonary resuscitation (CPR) if the heart stops, focusing on high-quality chest compressions and ventilation as appropriate.