Euthanasia

Question
I was wondering if you guys could point me in the right direction.
What are the practices for euthanization of animals, when it is done for mercy/old age/ etc?

I just had my 18yr old cat put down by an emergency hospital in Calgary, and am wondering if it is common practice for an embolism to serve as the mercy, instead of by an over dose of anesthesia/cocktail of barbituates?
I won't lie, I am devastated by my cat being gone, but I'm horrified over the fact that my cat was screaming when she was being injected with air from a syringe. I brought her in to be fixed, or to be put down peacefully in her sleep. She went from being limp and so very near death, unresponsive, to... when the Vet stuck the first needle in, she grew aware, and the second needle full of air... she screamed. I can't sleep right now, as this is a reputable veterinary clinic/hospital where I am... and the sound of her screaming when the air was being pushed into her.... I just... I don't know. I feel like the monster right now.
Please, tell me where I go with this.

Answer
Dear Natasha,
I just spoke with my Calgary peer. She reminded me that while it is not at all normal to inject air, sometimes cats do scream even when we inject the drugs because a nerve may inadvertently be touched.  
We do suggest you contact the Alberta Veterinary Association - Suite 950-5555 Calgary Trail NW  Edmonton, AB T6H 5P9, Canada
(780) 489-5007 to let them know of this occurrence.

There may be some explanation, but I think you would be wise to help others by exploring this further, if your heart directs you.

Because I am from the United States, I do not know what is legal or ethical in Alberta. This does not sound right to me at all.  I did call a peer of mine in Calgary and I am awaiting her call. If you would email me at HealthyAnimals@aol.com, I can get back to you as soon as I hear from her. One comforting thought is that our lives are the sum total of all the days we lived, not just one experience. You loved and tended her for 18 years and that is the energy she takes into her next adventure and lifetime. She loved you and does not want you to feel guilt.
Christina chambreau