Tuesday, January 4, 2011

Heart conditions (part 1: mitral valve disease)

Cardiology was not a favorite subject of mine in vet school. This was not due to the subject matter. I'm fascinated by the heart and find it be to more logical than some subjects (neurology, for example). Unfortunately, it was a 5+ inch stack of notes, only 2 exams, and a high level of stress. Now that I'm finished with school, I deal with the real aspects of cardiology and still find it fascinating (although challenging at times).

Animals, as people, develop heart conditions as they age. Unlike people, true myocardial infarction/heart attack is rare. It is much more common for dogs, as they age, to go into congestive heart failure due to pre-existing heart disease. MI is practically unheard of. I am going to talk about the heart disease that occurs in dogs and cats below. We'll start with conditions that develop as pets age rather than congenital conditions.

Understanding blood flow in the heart is relatively easy and necessary in the discussion of heart disease. The heart is an efficient pump. It takes blood from the body that has already been "used" (de-oxygenated) back into itself via the vena cavas. This blood flows into the right atrium, through the tricuspid valve, into the right ventricle. From there, it is pumped into the pulmonary artery (via the pulmonic valve) to the lungs. Here it receives a fresh load of oxygen. From the lungs, it goes into the left side of the heart via the pulmonary veins, into the left atrium, through the mitral valve into the left ventricle, and out the aortic valve/aorta to the rest of the body.

With that understanding, let us discuss diseases.

The most common that I see is probably mitral valve insufficiency (75-80% of heart disease in dogs). This typically occurs in small breed dogs as they age - Schnauzers, Shih-Tzus, Lhasas, Dachshunds, etc. Large breed dogs can be affected, but this is not as common.

The hallmark is an older dog abruptly developing a heart murmur. For some reason that we fail to understand, the body starts to produce thicker layers (fatty) within the leaflets of the valves that separate heart chambers. This is called myxomatous valvular disease (also endocardiosis and degenerative valvular disease). This is very rare in cats.

The mitral valve separates the left atrium from the left ventricle. As the fatty deposits build up, the leaflets of that valve pull apart, causing improper blood flow between the chambers (regurgitation of fluid from the left ventricle backwards into the left atrium). Blood should flow forward - from the lungs to the pulmonary veins to the left atrium to the left ventricle out through the aorta. When the gates between chambers become weak, blood becomes turbulent, flows improperly, and the patient will develop a heart murmur.

As this problem worsens, the left atrium will become enlarged and thin-walled. This happens due to back pressure of the improper blood flow and turbulence. As the atrium becomes thin, it becomes unable to do its job properly. Blood starts to back up, as it is not being pumped forward correctly. The blood backs up into the lungs causing changes in the pressure gradient there. Eventually, fluid starts to leak into places it shouldn't and pulmonary edema ("fluid in the lungs") develops. This is congestive heart failure.

So, what should you do if your older, small breed dog is diagnosed with a new heart murmur (this link will let you listen to a normal heart versus a heart murmur)?

First and foremost, your veterinarian should thoroughly and carefully listen to all areas of the heart - both the right and left sides of the chest. By just listening carefully, a veterinarian can often determine where the murmur originates and thus, what the most likely cause is. They can also determine if any degree of heart failure is present by listening for crackling and wheezing in the lungs.

Then xrays of the chest should be conducted. These will help evaluate heart size and for the presence of pulmonary edema and heart failure. A vertebral heart score should be determined. This is the only objective way to identify heart enlargement on xrays. Your veterinarian will measure the width and height of the heart, compare those numbers to a specific vertebrae, and come up with a number. Normal VHS in dogs is 10-10.5. Anything greater indicates an enlarged heart.

Next, your veterinarian should recommend an echocardiogram performed by either a cardiologist or a specialist with ultrasound training. This will help determine what is causing the murmur. Many owners balk at this point due to cost. It is understandable, but an echo is the only way to know for sure where the murmur originates. If owners cannot pursue an echo, the veterinarian should treat based on the most likely cause of the murmur. (The murmur itself is not a disease, it is a symptom, and as such, it is not specifically treated.)

If mitral valve disease is determined to be present, your veterinarian will likely recommend xrays to re-evaluate heart enlargement every 3-6 months. Your vet will also discuss monitoring for signs of impending heart failure such as exercise intolerance, weakness, fainting/collapse, and coughing or wheezing. Some veterinarians will start an ACE inhibitor at that time (enalapril/Enacard or benazepril/Lotensin). It is important to know that at this point, there is NO way to prevent progression of heart disease. In people, valvular replacement can be done, but this is not done (or only done extremely rarely) in animals. The cost is prohibitively expensive.

There is no way to tell how each individual dog will progress. Some will live years with a heart murmur and no heart failure, some will live months and then go into heart failure. It is individual to each dog. If your older, small dog has a heart murmur, your veterinarian should be monitoring closely at 3-6 month check ups for changes in lung sounds, severity of the heart murmur, and clinical signs.



Elizabeth said...

Do you ever read medical books about humans? I just finished Walk on Water, which is about a pediatric heart surgeon. It was a really great book.

I'm going through a medical phase. My next book is about the history of cancer. My husband thinks I'm crazy.

Shannon said...

I mean no disrespect when I say I'm too tired to read this right now it's been a long few days filled with helping a friend whose life is undergoing some dramatic changes. Regardless, I am really excited to read this in the morning after a good nights rest. One of my feline furkids has hypertrophic cardiomyopathy and I'm always interested in learning more. So thank you for posting it.

The Homeless Parrot said...

Elizabeth: Yes, I love human medical books and have read several. I can't recommend Atul Gawande's books enough - Better, Complications, and The Checklist Manifesto.

Is the history of cancer book called The Emperor of All Maladies? I saw it listed on a website I frequent and was very interested.

Shannon: No disrespect taken :)

Elizabeth said...

HP, yes that's the one. I found a link to Michelle Au's website on your page and I went through all her archives and made a list of the books she recommended for reading. The Emperor book is good so far.

Gawande's books are fantastic. His writing style is so pleasant to read.

Nicole said...

This is a topic near and dear to me. I lost my nine year old rhodesian ridgeback to a combination of heart disease and pneumonia in august of 08. He was a very healthy working dog who competed in agility, obedience, did carting and weight pulls and was an amazing dog.
It's a very painful thing to remember because I had been taking him to my regular vet almost weekly for 2 months before I could even get them to do x rays. It started because he was panting more and I just knew in my gut it was his heart. I was told by 2 vets at the practice they couldn't hear anything and he was fine. I finally drew blood work myselfwhich showed a very low t4 level, I believe 0.2, and he was put on soloxine. Despite telling them numerous times that he wasn't doing better it still took another month before x rays were done. When they showed a slightly enlarged heart and pneumonia he was put on lasix and amoxi. When I asked about doing an echo I was told no, he didn't need one, I also asked about a trans-tracheal wash to culture the pneumonia before starting anti biotics and was told he didn't need it. He died five days later.
I'd worked as veterinary assistant for 12 years at over 10 clinics, I trusted the doctors I worked with. This really showed me that as my pets only voice I need to listen to my gut and if I have to I'll drive across the state to the vet school for better care. I no longer work in the veterinary field. I just couldn't deal with it after this.

The Homeless Parrot said...

Nicole, that is very sad, and I am so sorry to hear that it made you leave the veterinary field. Unfortunately, veterinarians fail their clients and owners sometimes. We all do. I've failed people. Carrying those failures with me and promising to never let it happen again is one way I strive to make myself a better veterinarian.

I hope you have found some peace.

Nicki said...

having border collies, I never worried much about heart conditions in my own dogs. Now I have a Pom. Ugh.