There's no need for rescheduling, just crank up the nitrogen and keep going.
I mean, if you really really want to imagine a scenario where the execution fails and the victim is left in pain for an extended period, you can do that with any method. You could imagine a scenario where the subject is strapped to a nuclear bomb that still ends up surviving with misery and pain. But you have to go to extremely unlikely outcomes with something as utterly dead simple as "flood the lungs with nitrogen".