Complex sentence: Initial attempts to haul him free failed because the rope broke several times, and when he lost consciousness as carbon dioxide from his own respiration built up in the tunnel, he was unable to assist further rescue attempts with a stronger rope.
Split into simpler sentences: Initial attempts to haul him free failed because the rope broke several times. When he lost consciousness as carbon dioxide from his own respiration built up in the base of the shaft, he was unable to assist further rescue attempts made with a stronger rope.