On this page of our guide to BioShock Infinite you will find the third part of a detailed walkthrough of Chapter 35 - Operating Theatre. Rescue Elizabeth by sabotaging the generators.
The option to save Elizabeth will not be available after you reach the operating room for the first time. Booker must first sabotage the two generators that supplied energy to the siphon. Once you've returned to the operating room approach the girl. Help Elizabeth to free herself and then lace up her corset.