Mitral Valve Disease in Dogs

by Robert Reed, DVM, MSES, MPA

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Heart disease is one of the most common age-related illnesses we see in dogs. While there are many potential factors that can contribute to heart problems in any given individual, statistics indicate that 75 to 80% of heart failure in dogs stems from valvular disease. The term MVD (mitral valve disease) is used in a general sense to describe the condition, but it is more technically identified as myxomatous mitral valve degeneration (MMVD), and frequently involves both the mitral and tricuspid valves within the heart.

To better understand the meaning of and consequences from mitral or tricuspid valve degeneration, we need to take a closer look at the anatomy and normal function of the heart.

The primary job of the heart is to move blood through the body in a one-way system of circulation. The heart is divided into right and left halves. The right side receives oxygen-deficient blood from the body and sends it to the lungs for oxygenation. Blood then returns from the lungs to the left side and is subsequently pumped from the left heart to the rest of the body.

Each side of the heart has two chambers, an upper atrium and a lower ventricle. On each side, the atrium receives blood and pushes it into the ventricle through a valve. The valve on the left side is the mitral valve, and the one on the right is the tricuspid valve. As the heart contracts, these valves serve as one-way gates that allow blood to flow from the atrium to the ventricle but restrict the flow of blood back into the atrium as it gets pumped to the rest of the body. The closing of these valves creates part of the sound we associate with the heart beating.

Degeneration of a valve causes it to become deformed in a way that prevents it from closing completely. When that happens a thin stream of blood squirts back through the insufficiently closed valve in the wrong direction as the heart muscle contracts. The sound of that blood flowing turbulently through the valve is what we hear as a heart murmur (see What Is a Heart Murmur? On psge 70).

As degeneration continues the volume of wrong-way blood flow progresses and becomes increasingly problematic. Over time it creates internal pressure changes that lead to enlargement, or dilation, of the affected atrium. When that dilation becomes severe the heart loses its ability to contract effectively, which in turn reduces the flow of blood coming from the heart. As that happens, pressure builds up in the vessels that enter the atrium and causes them to leak fluid into whatever tissue or space surrounds them. If the changes affect the right side of the heart (from tricuspid valve degeneration), leakage occurs into the abdominal cavity, creating ascites (free fluid within the abdomen). If the changes impact the left side of the heart (from mitral valve degeneration), fluid leaks into the lungs. Fluid accumulation within the lungs is termed pulmonary edema and creates the symptoms we usually associate with congestive heart failure (see sidebar, Signs of Congestive Heart Failure, below).

We still don’t know what causes MMVD, but research strongly suggests it is an inherited condition. The condition hits mainly small- to medium-sized dogs, and certain breeds are particularly affected. It is well known in Cavalier King Charles spaniels but is also frequently seen in dachshunds, toy poodles, Chihuahuas, and other breeds. The unavailability of genetic tests and the late onset of the disease have frustrated efforts to reduce its incidence through breeding.

The condition is usually first detected through identification of a heart murmur. But, while the presence of a murmur indicates the existence of disease, it does not mean a dog is in heart failure. Some dogs with murmurs never develop heart failure, and others may not do so for years. The decision on when or if we begin treating it relies on interpretation of each individual dog’s needs and the amount of diagnostic information available. Until recently, most experts believed that therapy provided no benefit until a dog’s condition progressed to the point of heart failure, but studies conducted within the last few years have suggested that earlier treatment may be helpful in many cases. Medications used in early stages of disease tend to focus on improving the contractility of the heart, but as heart disease progresses a number of options may exist, including diuretics to reduced pulmonary edema and blood pressure reducers to ease the workload of the heart.

MMVD is not curable but management can add significant quality and longevity to a dog’s life. Good communication between veterinarians and pet owners is crucial in shaping decisions about treating heart disease and heart failure. Information from advanced imaging studies like echocardiograms and assistance from specialists in veterinary cardiology are enormously helpful, but they are not always available or affordable and, even when they are, they do not replace the value of good observation. Also, there is no single recipe for treatment. The combination of medications used and the recommended frequency of examination varies between individual patients. Finally, since pet owners know their dogs better than anyone, and some signs, like increases in a patient’s resting respiratory rate (see sidebar, Monitoring the Resting Respiration Rate (RRR), on page 68), are more accurately evaluated at home, veterinarians must rely on them to notice and report subtle changes. Any successful treatment strategy will need to be a team effort.

VCA Rancho Mirage Animal Hospital located at 71-075 Highway 111, Rancho Mirage, CA. 760-346-6103. www.vcaranchomirage.com.

SIGNS OF CONGESTIVE HEART FAILURE

■ Changes in the breathing rate, difficulty breathing, or shortness of breath

■ Coughing

■ Changes in behavior, like reduced energy, or exercise intolerance

■ Restlessness, especially at night

■ Reduction in appetite

Monitoring The Resting Respiratory Rate (RRR)

An increase in the resting (or sleeping) breathing rate is often the earliest indicator that a pet is entering congestive heart failure and provides a useful monitoring tool for owners of dogs with heart murmurs. The normal respiratory rate for dogs is 35 (or fewer) breaths per minute, but since some individuals vary, it’s a good idea to measure and understand what is normal for your dog, then compare observations over time to learn at the earliest point when the rate increases.

  1. Become comfortable with how to count the breaths. Most people find it easiest to watch the chest rise and fall as their pet sleeps. One breath includes both upward and downward movements.
  2. Observe the breathing rate over 15 seconds.
  3. Multiply the number counted by 4 to get the number of breaths per minute.
  4. Record the rate every day for a week to establish the reference range for your pet.
  5. If you notice an increase in the rate, repeat the measurement several times over the next hour. If the change is real it will remain consistent.
  6. If the rate consistently exceeds 35 breaths per minute or if it increases more than 25% over your dog’s baseline rate, contact your veterinarian.

What is a Heart Murmur?

A heart murmur is just a noise that is usually only audible through a stethoscope. It’s a swishing sound heard over the normal thumping sound we associate with a beating heart. A murmur results from turbulence in the blood flowing through the heart. It is roughly analogous to the difference in sound between a smoothly flowing stream and water moving through rapids. There are three fundamental types:

  1. Innocent—those occurring without evidence of heart disease, usually temporary or intermittent. They are uncommon and most often occur in young puppies.
  2. Congenital—those occurring as a result of defects in the heart or its associated vessels that are present at birth or arise during early development. They include most of the murmurs found in young animals.
  3. Acquired—those that develop over time, with age. They are usually associated with heart valve disease and make up the vast majority of cases.

What does it mean?

While it is rarely a cause for alarm, hearing a heart murmur is often the first indication that a dog has any kind of heart disease, so it is not a completely benign finding. It is important to remember, however, that the presence of a murmur alone does not signal heart failure. Many dogs live a long time with heart murmurs before needing therapy, and some never require treatment. The significance of a murmur lies in its underlying cause.

What should you do?

The best way to predict the consequences of a murmur is to determine what caused it. Since most acquired murmurs result from damaged heart valves, it is also helpful to understand the degree of valvular change that has occurred. Veterinarians usually recommend X-rays of the chest (to get a picture of the overall heart size) and an echocardiogram, or ultrasound of the heart (to measure sizes of the individual heart chambers, to learn which valves are damaged and to what degree), as the best way to determine a pet’s prognosis and whether they need treatment. An echocardiogram is usually performed by a veterinary cardiologist and provides a helpful reference point in tracking the impact of a murmur over time.

Research strongly suggests MMVD is an inherited condition. It is well known in Cavalier King Charles spaniels but is also frequently seen in dachshunds, toy poodles, Chihuahuas, and other breeds.

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