I think one thing people often forget is that a spay (ovariohysterectomy) might be the only major abdominal surgery that a pet has in their lifetime. The vast majority of healthy young pets that are spayed will do just fine, but that depends on multiple factors.
-A healthy patient. So this would be a patient with no detectable abnormalities. If young animals have something abnormal on physical examination/bloodwork, that should be explored further. Patients should be carefully asculted to see if there are any heart murmurs- which could be no big deal, or could be serious.
-The DVMs skill with anesthesia. There are a wide variety of anesthesia protocols out there and some are more up to date/safer than others. The best ones use a balanced protocol with analgesics/sedative, protect the airway and have a TRAINED person monitoring the patient closely (ie. NOT the DVM doing the surgery).
-The DVMs skill with surgery. Mistakes and poor technique are out there…not tying knots appropriately leads can lead to bleeding, carless surgical technique can cause kidney problems, etc.
-The owner’s compliance with postop instructions. There are lots of people that don’t put the e-collar on the dog, let it run, lick the incision, ect. which all lead to post op complications.
In the end, everything can be done right by all involved and then shit just happens. Thankfully that only occurs rarely (in people it’s about 1 in 100,000, and it’s about 1 in 1,000 in healthy dogs/cats (no one is completely sure why the difference in mortality exists, but it’s there). IME “heart failure” is a really odd reason for a postop death - were they able to get a necropsy (autopsy)?