Please accept my condolences on the unexpected loss of your dog. There is little I can say to ease your grief.
As far as why this may I have occurred, anything I might say would be speculative and indeed it seems you have had a complete autopsy and pathology analysis which I am in no position to contradict. Such reports are generated by specialists in pathology with advanced training in autopsy, biopsy and pathologic analysis.
That said, the autopsy results suggest that because the organs were congested with blood, a bleeding disorder existed which led to the dog's death. Coumadin, a human anticlotting drug, is used for potential stroke patients and those with certain heart diseases that may led to stroke. Coumadin contains warfarin, and anti-clotting agent also used in certain rodent poisons. One way for a dog to get that is to eat a rodent bait someone left around somewhere. Another way is to eat an owners medications. Assuming neither of those things happened, the pathologist may have simply been looking for a reasonabe explanation of the findings. Could the findings be wrong? I cannot say,as I do not know the qualifications of the pathologist involved.
It is sadly and unfortunately true that pets die from suboptimally administered anesthesia protocols around the globe. This unfotunate circumstance has lessened over the last 30 years with better, safer drugs and with advancing science and training of veterinarians, but it still occurs too often, even in the US. Indeed even one time is of course too many.
You may also be interested in two other posts I responded to, just this week, on the same subject:
Regarding the events you describe here:
"The vet called at 11 am and said he would be beginning surgery in 30 - 45 minutes. At 12:15 he called to say my dog had died at the end of the surgery. The only explanation is that her heart stopped and they were unable to resuscitate. We can hardly get over this. We were not prepared that she might die. The vet said there was very little risk to the surgery."
......I can only say that it sounds like an anesthetic death. An ECG may or may not have picked up a reason to take a non-standard precaution or delay the procedure.
I just cant speculate and we can never know.
In closing please accept my condolences and those of all of us here at MedHelp and PDOC for this sad circumstance. I hope you will find comfort in knowing that at least he did not suffer.
Arnold L. Goldman DVM, MS
MedHelp & PDOC