You can't take steady breaths in a vacuum to keep your heart rate down; you can't do most of the physiological controls that slow your body's use of oxygen. (And this isn't hard vacuum, not quite yet, but it's pretty close. That air was thin to start with, and it's rushing out fast.)

DOORS CLOSING: 71%

All you can do is wait, and see who wins this race.

(River's not the best at waiting, under circumstances like these.)