Your nine-year-old Shih Tzu has a heart murmur. Grade 3, the vet says, then asks whether you have noticed him slowing down on walks. He has not, not really. The vet wants a cardiac ultrasound anyway, which means a referral, a specialist appointment in three weeks, and a bill somewhere between $400 and $700.
That appointment is not really about the murmur. A murmur means the valve is leaking. It does not mean the heart has enlarged, and that second detail decides whether your dog starts medication now or gets rechecked in six months. A team at Chungbuk National University in South Korea has built a model that estimates that second detail from tests most dogs have already had.
What the Korean team trained it on
The study, published in Frontiers in Veterinary Science on 11 September 2026, came from the university’s veterinary internal medicine lab and its computer scientists. The researchers pulled records from 387 dogs seen at the teaching hospital between January 2018 and December 2025. A hundred and thirty-five had stage B2 disease confirmed by echocardiography. The other 252 were healthy dogs or dogs at the milder stage B1.
They trained a gradient boosting algorithm on 80% of those records and tested it on the rest, then repeated that split 20 more times to see whether the result held. The inputs were ordinary: age, heart rate, murmur grade, a complete blood count, serum chemistry, a urinalysis, and two measurements taken off a chest X-ray. No ultrasound data went in.
The model reached 82.5% accuracy, with an area under the ROC curve of 0.922. The X-ray measurements carried most of the weight. A model using only those two numbers scored 0.838; adding blood work and exam findings pushed it to 0.922.
Guidelines suggest a dog can be presumed stage B2 when the vertebral heart scale on an X-ray reaches 11.5. The typical B2 dog in this dataset measured 11.2, below the line. Fixed cutoffs miss dogs, and closing that gap is the point of a model like this one.
Why stage B2 is the stage that matters
Myxomatous mitral valve disease is the most common acquired heart disease in dogs, and it turns up mostly in small breeds. Cardiologists stage it in a way owners can follow. B1 means a murmur with no measurable enlargement. B2 means a murmur plus an enlarged heart. Stages C and D mean the dog has gone into heart failure.
B2 is the only stage with strong trial evidence behind its treatment. The EPIC study, published in the Journal of Veterinary Internal Medicine in 2016, followed 360 dogs with enlarged hearts. Pimobendan stretched the median time to heart failure from 766 days to 1,228 days, roughly 15 extra months of a dog feeling like himself.
Murmur grade, it turns out, is a poor substitute for knowing the heart’s size. Research cited by the Korean team found no evidence of enlargement in about 56% of dogs with a moderate murmur and 29% of those with a loud one. You cannot hear how big a heart is.
Ultrasound still measures it, and the authors say their model does not replace one. It could help a general practice sort the dog who needs a specialist this month from the dog who can wait until the next recheck.
What this means for your dog
There is no version of this available in a clinic. The model was built from one hospital’s records and tested on a slice of the same records, with no outside validation. When the researchers retrained it 20 times on different splits, average sensitivity fell to about 0.69, so it missed a fair share of B2 dogs along the way. A tool that points a vet toward dogs who need a closer look is worth having. I would rather see a screening model that over-refers than one that tells an owner everything looks fine.
The practical moves stay unglamorous. Ask for the murmur grade, and write it down with the date, because a grade that climbs over two years tells your vet more than any single exam. If your dog has a murmur, ask whether the chest X-rays included the vertebral heart scale and vertebral left atrial size, since those are measurements someone has to take on purpose. Ask what stage your dog is in and what would move him to the next one. If only an echo can say, ask whether it is worth doing now or at the next recheck.
Do not wait for the signs everyone knows. Night coughing, tiring on the usual route, and fainting mean heart failure, which is past B2, and by then the proven window has closed. What heart murmur grades actually mean is worth reading before your next appointment. Skip the over-the-counter heart supplements. None has been shown to delay heart failure, and the drug with the evidence behind it, pimobendan, comes only from a veterinarian. Keeping a dog lean and active matters here too, as it does for high blood pressure in dogs and other quiet problems that build with age. If you want the basics on what a routine exam covers and what those lab numbers mean, the Dog Health 101 hub is a reasonable place to start.
None of this is a diagnosis, and none of it replaces your veterinarian. A murmur is a finding, not a verdict, and work like this exists to get more dogs into treatment earlier.