Mitral Valve disease (MVD) is the most common heart disease we see in dogs, primarily affecting small breed dogs with some breeds such as Cavalier King Charles Spaniels, Miniature Poodles and Shih Tzus highly represented. In 2009 the American College of Veterinary Internal Medicine (ACVIM) created a staging protocol for the disease based on the equivalent in human medicine. This was updated in 2019 to take account of changes in diagnosis, treatment and our knowledge of the disease.
In this blog I’d like to go through the categorisation of stages and what differentiates them, particularly B1 and B2, and why that is important for us in practice as we will see these cases regularly.
The ACVIM have adopted a staging system for MVD which has 4 stages, with one subdivision. It starts at A and goes to D, with B being divided into B1 and B2. So, lets have a quick look at what the different stages mean and then why the difference between B1 and B2 is so important.
A – This encompasses dogs who are at a high risk of developing heart failure from MVD. They will have no obvious structural abnormality at the time of exam (such as a murmur). The common breeds who would fall into this category are Cavalier King Charles Spaniels, Miniature and Toy Poodles, Dachshunds and Shih Tzus. It is recommended that they have regular exams by their vet focusing initially on auscultation. Some breed societies organise screening events for their members.
B – In stage B, something has been found that suggests that the dog has MVD, most commonly a murmur heard during a clinical examination. The dog will have had an Xray and an echocardiogram to confirm that the murmur is caused by MVD. These will allow measurements to be made to assess the severity of the disease. Primarily you will be looking at Vertebral Heart Score (VHS), Left Atrial to Aortic diameter ratio and Left Ventricular size. Stage B is then further subdivided into 1 and 2.
B1 – As MVD progresses, there will be remodelling of the heart. Stage B1 encompasses dogs who have a definite diagnosis of MVD, but have not yet reached a set criteria of remodelling to be classified as B2. They may range from dogs with normal chest x-rays, LA and LV up to dogs who have some changes but not all required to re-classify them.
B2 – Increasing Mitral Regurgitation (MR) will cause increased remodelling of the heart with MVD. When this reaches a point sufficient enough to justify treatment, based on the results of clinical trials, the dog will be classified as being in stage B2. The criteria for this are as follows –
1. Murmur intensity greater or equal to 3/6
2. On echo the LA:Ao ratio (right sided short axis) is greater or equal to 1.6
3. Left Ventricular internal diameter in diastole, normalised for body weight (LVIDDN) is greater or equal to 1.7
4. Breed adjusted VHS is greater than 10.5
C – When dogs start to show clinical signs of heart failure, they are classified as stage C. The stage includes all dogs with MVD who have experienced at least one episode of clinical heart failure but are not refractory to treatment. Common clinical signs seen are tachypnea, restlessness, coughing or respiratory distress. There may also be a reluctance to exercise as often or as actively as they used to. Be aware though, quite often the breeds that often suffer from MVD are also susceptible to small airway disease. Therefore care has to be taken to rule this out as the cause of respiratory symptoms.
D – When the dog becomes refractory to standard treatment for heart failure they will be classified as Stage D. This usually refers to the standard dose of our common diuretics i.e. furosemide or torsemide. Any arrhythmias should be controlled first.
So, why is differentiating B1 and B2 important? The important words in the classification are ‘sufficient enough to recommend treatment before the onset of clinical signs based on the results of a clinical trial. Back in 2016, the results of what still is the largest drug trial in Veterinary medicine, were published – The EPIC trial.
It looked at the effects of Pimobendan in the preclinical stage of MVD. The outcome was that dogs who met the 4 criteria listed above should be started on Pimobendan as it could add up to 15 months on to the preclinical stage of the disease. That was wonderful news for our patients as it means we are adding quality time onto their lives. In fact, the results were so good that the trial was stopped early on ethical grounds so that the placebo dogs could be started on Pimobendan. There was no evidence that treatment prior to B2 made any difference to outcomes of the disease. Therefore, if we can demonstrate that our patients have moved over into B2, we can make a big difference to both their and their owners lives. Likewise, if they are still in B1, we know that starting them on medication is unnecessary thereby saving money on medication for our owners.