Understanding the ACVIM Stages of Mitral Valve Disease (A to D)

Your vet has examined your dog, perhaps run a scan, and then said something like "she's a stage B2" or "he's in stage C now". You nodded, took the leaflet, and only later realised you had no real idea what that letter and number actually meant. If that is you, you are in the right place. The staging is not jargon for its own sake. It is the single most useful shorthand in canine heart medicine, and once you understand it, almost everything your vet recommends starts to make sense.

The system comes from the American College of Veterinary Internal Medicine, which is why it is called the ACVIM staging system. It was set out in a consensus statement written by a panel of veterinary cardiologists and updated in 2019 (Keene et al., 2019). It sorts dogs with degenerative mitral valve disease, the slow leaking-valve condition that is by far the most common heart problem in dogs, into four stages labelled A, B, C and D, with stage B split into B1 and B2. This article walks through each one in plain language, and explains why the stage, rather than the murmur or your dog's age, is what drives the plan. It assumes you know the basics from the anchor guide to mitral valve disease, and focuses purely on what the stages mean for you.

Why staging exists: matching the treatment to the moment

Mitral valve disease is not one fixed thing. It is a journey that, in most dogs, unfolds slowly over years. A valve that has only just begun to leak is a completely different situation from a valve leaking so badly that fluid is backing up into the lungs. The treatment that helps in the later situation can be useless, or even unhelpful, in the earlier one, and vice versa.

That is the whole point of staging. Rather than treating "heart disease" as a single label, your vet places your dog at a specific point on the journey and matches the response to that point. Early on, the right move is often to do very little except watch carefully. At a particular turning point, starting a medication early has been shown to buy meaningful time. Later still, the focus shifts to controlling fluid and keeping your dog comfortable. Staging is simply the map that tells everyone where your dog is standing, so the plan fits the moment.

Stage A: at risk, but with a healthy heart

Stage A is the most reassuring stage of all, because it is not really a diagnosis at all. A stage A dog has no heart disease. There is no murmur, no leak, no enlargement, nothing. What puts a dog in stage A is simply belonging to a breed known to be predisposed to mitral valve disease, with the Cavalier King Charles Spaniel being the standout example, along with many other small breeds such as dachshunds, miniature poodles and various terriers.

In practice, stage A is a flag for the future rather than a problem for today. It means your dog is worth listening to a little more carefully at each annual check, because the odds of developing the disease one day are higher than average. There is no treatment, no medication and nothing to restrict. Giving heart medication at this stage, before any disease exists, is not recommended (Keene et al., 2019). The job here is awareness, not action.

Stage B1: a murmur, but a heart that is still normal size

Stage B begins the moment there is actual disease: the valve has started to leak, and your vet can hear a murmur over the left side of the chest. Stage B is then divided into two, and the difference between B1 and B2 is genuinely important, because it decides whether your dog needs treatment.

A stage B1 dog has the leak and the murmur, but the heart has not yet enlarged in response. The chambers are still a normal size on imaging. Your dog has no symptoms and, to all appearances, is a perfectly well animal who simply happens to have a noise in their chest.

For a B1 dog, the standard of care is monitoring rather than medication. There is no evidence that starting heart drugs at this stage does any good, so the right plan is to keep an eye on things and re-check at intervals, watching for the day the heart begins to enlarge. This is why a dog can carry a murmur for years with no tablets at all, and that is entirely appropriate. If you are in this position, living with a dog in the pre-clinical stage covers exactly how to monitor at home and how often to re-check, so this article will not re-tread that ground.

The one thing worth flagging is that the line between B1 and B2 is drawn by heart size, and heart size cannot be judged by the stethoscope: a louder murmur does not reliably mean a bigger heart. Telling B1 from B2 needs imaging, usually an echocardiogram (a heart ultrasound scan) and sometimes chest x-rays, which brings us to the most consequential step in the whole system.

Stage B2: a murmur plus an enlarged heart, and the treatment turning point

Stage B2 is the stage every owner of a dog with a murmur should understand, because it is where doing nothing changes to doing something. A stage B2 dog has the leak and the murmur of B1, but now the heart has enlarged in response to years of leaking, the left atrium and left ventricle have stretched to a defined degree, and yet, crucially, the dog still has no symptoms. From the outside, a B2 dog usually looks and behaves completely normally. The change is entirely internal, visible only on imaging.

The reason B2 matters so much is that this is the stage at which starting a medication called pimobendan, before any symptoms appear, has been shown to delay the onset of heart failure. A large international trial found that beginning pimobendan in dogs at stage B2 postponed the arrival of congestive heart failure by roughly 15 months on average, compared with waiting (Boswood et al., 2016). That is a substantial stretch of good-quality time, gained before the disease ever makes the dog feel ill. It is genuinely one of the most worthwhile interventions in canine cardiology.

Because that decision hinges entirely on whether a dog has crossed from B1 into B2, the precise measurements your vet uses to define B2, and the full story of the trial behind the recommendation, deserve their own deep dive. Stage B2 and the EPIC trial covers exactly that: what B2 means down to the measurements, why a scan rather than a murmur defines it, and why finding a murmur is a good reason to image. If your dog has a murmur and has not yet been scanned, that article makes the case for why the scan is worth doing. The headline to carry from here is simply this: B2 is the moment watchful waiting turns into early treatment, and it is found by imaging, not by listening.

Stage C: heart failure, now or in the past

Stage C is the point at which the disease stops being silent. A dog reaches stage C when the leaking valve has finally overwhelmed the heart's ability to cope, and fluid begins to back up into the lungs. This is congestive heart failure, and it is what the earlier stages were quietly building towards, and what the treatment at B2 is designed to delay.

The defining feature of stage C is that your dog has shown clinical signs of heart failure: most commonly a faster resting breathing rate, increased effort to breathe, a soft cough, tiredness or reduced stamina. A dog stays in stage C even once those signs are brought under control with medication, because the stage describes the fact that heart failure has happened at all, not whether the dog currently feels unwell. A well-managed stage C dog can feel much better than the label sounds.

Stage C is where the treatment expands. Alongside pimobendan, dogs typically start a diuretic, usually furosemide, to clear the excess fluid from the lungs, and often other medications too. The detail of that combined regimen belongs to the heart failure medication toolkit, and the practicalities of living with a diuretic are covered in managing furosemide at home, so I will not duplicate them here.

Two things matter most for a stage C owner day to day. The first is that the sudden onset of laboured, rapid breathing is an emergency, and knowing how to respond to it is essential reading: see recognising a heart failure crisis. The second is that your most valuable monitoring tool at home is now the resting respiratory rate, counted while your dog sleeps, because a rising rate is often the earliest sign that fluid is creeping back before your dog looks unwell. The resting respiratory rate guide explains how to count it properly, and the breathing rate tracker lets you log it and spot a trend.

Stage D: heart failure that no longer responds to standard treatment

Stage D is the most advanced stage, and it is reached when a dog's heart failure no longer responds adequately to standard doses of the usual medications. The fluid build-up becomes harder to control, breathing difficulty or other signs persist despite treatment, and the regimen has to be intensified, often with higher or more frequent diuretic doses and additional drugs, and sometimes with referral to a cardiology specialist.

It is worth being honest that stage D is the hardest chapter, and it is where quality-of-life conversations come to the fore. That said, it is not a fixed full stop, and the length of this stage varies considerably between dogs. Many dogs spend a good while at stage C before ever reaching stage D, and some never do. The aim throughout shifts firmly towards comfort and dignity: keeping your dog breathing easily, eating, and enjoying the things they love, for as long as that is genuinely achievable.

What changes at each step, and what to ask

The thread running through all four stages is that the stage, not the symptom and not the murmur, drives the plan. It tells you whether the right move is to watch (A and B1), to start treatment early to buy time (B2), to treat active heart failure (C), or to intensify and prioritise comfort (D). When your vet gives you a stage, it is worth asking three things: which stage is my dog in, what would move us to the next one, and what is the monitoring plan in the meantime.

One last reassurance. For most dogs, the move from one stage to the next is measured in months and years, not days, and a diagnosis in the early stages leaves a great deal of good life still to come. Knowing the stage simply lets you and your vet stay one careful step ahead of the disease. If your dog has just crossed into the pre-clinical danger zone, the next thing worth reading is Stage B2 and the EPIC trial, where the most important early decision in the whole journey lives.