Short answer; vets sort heart disease into stages, and the ones that matter most to you are B1 and B2 (your dog has a murmur but feels totally fine) and C (heart failure has actually shown up). The jump from B1 to B2 is the big one, because that's usually when treatment begins, often more than a year before your dog would ever look unwell.
Let me walk you through the whole ladder without the jargon headache.
A murmur on its own doesn't tell you much (I get into exactly why in What Does Your Dog's Heart Murmur Grade (1 to 6) Actually Mean?). Staging is how vets turn “there's something going on” into “here's precisely where your dog is, and here's what we do about it.” The system most vets use comes from the ACVIM (the American College of Veterinary Internal Medicine), and it runs A, B1, B2, C, D. Think of it as a map, not a countdown.
Your dog has no heart disease yet, but belongs to a breed that's prone to it: Cavaliers, Dachshunds, Chihuahuas, and other small breeds. There's no murmur and nothing to treat. The whole job here is simply regular check-ups, so anything new gets caught the moment it appears. If you've got an at-risk breed, Small Breed Heart Disease in Dogs: The Early Signs Most Owners Miss is worth a read.
Now there's a murmur, so the valve has started to leak, but imaging shows the heart hasn't changed size. Your dog feels completely normal, plays normally, eats normally. At B1 there's usually no medication needed. The plan is monitoring: rechecks, plus keeping a casual eye on things at home. Plenty of dogs sit happily in B1 for years.
This is the one to really understand. B2 means the murmur is still there and imaging now shows the heart has enlarged to a defined size. Your dog still feels fine (no coughing, no struggle), but the heart is under measurable strain. Here's why it matters: this is the stage where vets typically start heart medication, and studies suggest doing so can delay the onset of heart failure by around 15 months. That's more than a year of extra good time, bought before your dog ever looks sick.
Stage C means your dog has shown actual signs of congestive heart failure, usually coughing, faster or laboured breathing, or fluid backing up into the lungs. It sounds scary, and yes it's serious, but it's also very treatable. A combination of medications clears the fluid and supports the heart, and many dogs bounce right back to a comfortable, happy life at home. Some dogs are first diagnosed at C during a sudden crisis and do beautifully once they're stabilised.
This is end-stage disease, where the usual treatments aren't controlling the signs as well anymore and care shifts toward specialist-level adjustments and, above all, comfort. Far fewer dogs reach D, and the focus here is entirely on quality of life, keeping your dog feeling as good as possible for as long as possible.
Here's the reassuring bit. The earlier your dog is staged, the more time you have to work with, which is the entire argument for check-ups before anything looks wrong. And a big chunk of the plan is squarely in your hands at every stage:
Staging matters a little extra in this climate. A dog in B2 or C copes worse with heat, because a hot day already raises the heart's workload before your dog has taken a single step. Walk in the cool hours, keep water within reach everywhere, and don't let a “he just seems tired” summer slump go unmentioned at the next recheck.
The stages aren't a doomsday clock, they're a plan. A and B1 are “watch and support,” B2 is “start protecting the heart early,” C is “treat the heart failure, and most dogs feel much better,” and D is “comfort first.” Knowing your dog's stage turns a frightening phrase, heart disease, into a set of clear, doable next steps. And the single best thing you can do is keep those rechecks, because catching the B1-to-B2 jump on time is where the extra good years come from.
If you'd like help working out where your own dog sits, our vet team is a message away.