Dr. Keller states that while she can give McKay something to keep him awake, that eventually he'd would die from pulmonary failure.
What the writers likely meant to have her say was "cardiopulmonary failure" as simple pulmonary failure (respiratory) would not be an issue from remaining awake for an extended period of time.
What the writers likely meant to have her say was "cardiopulmonary failure" as simple pulmonary failure (respiratory) would not be an issue from remaining awake for an extended period of time.