Pat, I am very sorry to hear of Mutley's passing. Everyone who is a pet lover sends you a prayer. There is always a risk when putting a pet(or human) under anesthesia. I don't know much about Xylazine, except that it can cause decreased blood pressure, heart block, and low heart rate. As a nurse, I do know that they probably gave Mutley the Atropine in an attempt to reverse the effects of Xylazine. Atropine blocks the effects of drugs like Xylazine, and stimulates the heart to beat faster and raise blood pressure. It is also an antedote for certain types of nerve gas. Was the vet who did the necropsy independant of the one who administered the drugs? Also, did they take a GOOD LOOK at his heart? My dog has a congenital heart defect that many, many vets missed in the beginning. It was finally diagnosed when he was about 8 months old via ultrasound. Now, no vet in their right mind will attempt to give him anesthesia without doing an echocardiogram to check his heart function first, knowing that anesthesia may kill him. It is possible that Mutley had something wrong with his heart, and it got missed. I wish you peace. Susan