How is it that a dog can survive a "gas" chamber?

The link

Quentin is a mix breed that was put in a chamber to be euthanized with other unwanted strays. Only he didn’t die, he greeted the opened chamber with tongue and tail wagging in the midst of half a dozen or so dead dogs.

How can that happen?

God endorses no-kill shelters.

Cyber-dog!

IIRC, they give dogs a drug to help them relax (important to be relaxed if you’re going to die soon). Evidently in this case, they think that his breathing and heart slowed down so much from the drug that he didn’t inhale a lethal dose of the bad stuff.

That’s just one theory, I’m sure there are others out there.

Then why did this one dog survive, and not others? If their explanation is correct, you’d think this would be a common occurrence.

There can be only One!