We are talking about 500 bpm. That's like 8 per second. In a huge heart.

I can't see how a big heart like that could pump enough blood to not faint immediately even for a human, let alone an animal several times bigger, and then, if it doesn't get pretty fast back to normal result in death.

I know the fibrillation part is an exageration, but sill, 500bpm?

Do you thing a human, let alone a bigger animal wouldn't instantly faint?

Edit: You can't compare a dog that is maybe 35-50 kg to a grown man who is 80 and then use this to talk about giraffes as you can't compare blood presure extremes to heart rate extremes.