Intervention consisted of 40 once daily 60-minute hyperbaric chamber compressions at 2.0 atmospheres absolute ( ATA ) at 1 of 3 randomly preassigned oxygen fractions , resulting in respective blinded groups with an oxygen-breathing exposure equivalent to ( 1 ) surface air ( sham ) , ( 2 ) 100 % oxygen at 1.5 ATA , or ( 3 ) 100 % oxygen at 2.0 ATA .