214
Alabama inmate executed with nitrogen gas was ‘shaking violently’ for 22 minutes, witnesses say
(www.theguardian.com)
This is normal in low-oxygen, low-CO2 hypoxia.
The patient is not conscious during these spasms, and feels no pain.
EDIT: Nitrogen hypoxia is an instant and painless death experience, because you pass out almost without conscious awareness, the same as if you go under the gas they have at the dentist while counting down from 10 and never remember getting to 1. The difference is that the body's death is very slow, as the body's tissues are deprived of oxygen and slowly run out of gas and stall out. As long as the patient continues to not spike in CO2, they will remain unconscious and unaware through the entire process - as necessary and brutal as it is to - you know - stop a body.
EDIT2: Exercise reasonable doubt when reading the comments below which are calling into question whether the patient 'gasped for air' or doubt for some reason that a mask is adequate. 1) The patient held their breath from the nitrogen intake as long as they could. This is the same as you sitting on a dentists chair refusing to inhale the gas. You will quickly suffer the normal heightened discomfort until your brain takes executive control and just makes you to inhale. The patient in this case brought their will right up against that reflex, a miserable experience and one in which I can't imagine it was a comfortable mental space to be in. Then the inhale reflex kicked in and they nearly instantly lost consciousness. Over the next hour or so their body slowly ceased function, without tripping any of the really scary nervous system alarms. Patient never woke up. 2) A mask is preferred in systems like this because of the inherent safety risk involved for the administers with invisible, body-encompassing volumes of nitrogen. A mask is also totally up to the task, as anyone who's been put down at the dentist can attest to. These masks also have multiple, multiple added redundancy features which help protect the air seal, as well as post-review-diagnostics. They're safe. The main concern in this situation is administer training, in the proper handling of the body's seizures such that the mask isn't somehow pried off.