I thought this was a travel myth?

[rechoboam]

+1
There's actually a fine line between being unconscious and being dead. Many of the Moscow fatalaties were from simply having a compromised airway. So the gas knocked them out, but then they suffocated as there was no airway management.

And for that matter how much gas would you need? A "C" size cylinder only holds 300 litres. Remembering you have to flood the entire compartment, not just a mask and a pair of lungs.

Yes. They used a morphine-like drug. In opiate overdose if the airway is maintained-kept open- and the lungs ventilated even a massive overdose can just be waited out. Or treated with an antidote. Ambulance paramedics could have managed that situation.