So sorry to hear this. As Annlinda said, and the following quote confirms:
"You need an understanding and experienced veterinary surgeon. Sibes are sensitive to some drugs, particularly anaesthetics, sedatives and tranquillisers. This is due to their relatively low metabolic rate and lack of body fat. Also the bulk of their fur can lead vets to overestimate their weight and so overdose them. Sibes should always be weighed accurately beforehand to avoid this."
Sadly this is too late for your little baby, and I doubt very much if the vet would admit to having made a mistake. Of course, it could have been a heart problem but even with a postmortem it is unlikely you will ever know.
Let us hope if this was the case, that the vet has learned a very valuable lesson.
For information greyhounds also have similar problems with anaesthetics, again due to very little excess body fat.
They also suffer easily from dehydration so in warm weather should not be allowed to exercise too much.
You could try asking for a written report from the vet.