Normal Heart Anatomy

The heart has four chambers.  There is a right upper chamber and a left upper chamber.  Each chamber is called an atrium, thus there is a left atrium and a right atrium.  The lower chambers directly beneath each atrium and these are called ventricles.  There is a left ventricle and a right ventricle.  The blood that is coming back from the body to be reoxygenated ends up in the right atrium and is stored very briefly here.  The blood is then ejected into the right ventricle and then into the pulmonary artery.  From there the blood goes into the lungs to pick up oxygen.  The blood then returns to the heart via the pulmonary vein and ends up in the left atrium.  Again, it is held there very briefly before being pushed forward into the left ventricle and then into the aorta.  The left ventricle muscle is the largest muscle of the heart in order to eject the blood into the body.  From the aorta, the blood flows through the body to oxygenate the tissues before returning back to the right side of the heart.

Dilated Cardiomyopathy

Dilated cardiomyopathy  (DCM) means that the heart muscle, especially the thick muscled left ventricle, becomes much thinner than normal.  The muscle gets thinner and cannot push as much blood out through the aorta.  Also, the pressure of the blood inside the heart allows the thinned walls of the heart to stretch, resulting in a much larger left ventricle chamber.  The two characteristics indicative of dilated cardiomyopathy are 1) the heart wall is thinner than normal, and 2) the chambers (mostly the left ventricle) are larger than normal.  In the end, the heart ends up being much larger in diameter than normal but it cannot contract very forcefully to get the blood out to the body.  This will result in congestive heart failure.

Is dilated cardiomyopathy common?

Fortunately, dilated cardiomyopathy is not the most common cause of congestive heart failure in dogs.  Unfortunately, when it is seen, it is typically more common in large breed dogs rather than small breed dogs or in cats.  It is very rare in cats.  Any large breed dog can be affected, but the most common breeds we see dilated cardiomyopathy in are Boxers, Doberman Pinschers, and Great Danes.  If it is seen in small/medium breed dogs, it is usually in Cocker Spaniels and English Springer Spaniels.

Signs of congestive heart failure

When the heart does not pump the blood properly, blood backs up into the blood vessels of the lungs.  This results in an increased pressure within the vessels.  This in turn causes small amounts of fluid to leak out of the capillaries (smallest of the blood vessels) and into the air passageways.  This collection of fluid in the lungs will subsequently cause coughing and/or gagging episodes.  Coughing is the most common sign of congestive heart failure.  However, remember that numerous things can cause coughing in dogs.  If your dog has numerous coughing episodes, have your dog examined by a veterinarian.  Other signs of congestion heart failure can include exercise intolerance (tire easily with minimal exercise), inappetance and lethargy.

Congestive heart failure starts when the heart is not able to provide adequate amounts of preoxygenated blood to the tissues.  Without adequate oxygen, the cells in the body trigger a series of responses.  Different hormones are released by different organs in order to correct the problem.  These hormones are helpful short term, but over a period of time these hormones cause more problems.  These hormones help conserve fluid in an effort to increase blood volume and the output of blood and oxygen by the heart.  Again, short term (days to several months), this cycle of events will help, but on a daily long term basis, this cycle becomes detrimental to the heart and the body.

The increased fluid retention ends up causing even more fluid to leak out of the capillaries and into the lungs.  This in turn will cause coughing, gagging and lethargy.  Fluid may also leak out into the abdominal cavity and into body tissues below the skin surface.  Excess fluid in the lungs is called pulmonary edema.  Fluid below the skin is called peripheral edema.  Fluid in the abdominal cavity is called ascites (pronounced ah-si-teez).

Tests to Check for Dilated Cardiomyopathy and/or Congestive Heart Failure

Unfortunately, many times we diagnose dogs with dilated cardiomyopathy only because they present to the veterinarian in congestive heart failure.  Most dogs will not show any clinical signs of dilated cardiomyopathy until congestive heart failure is present.

However, there are some tests dog owners can utilize to check those breeds predisposed to dilated cardiomyopathy.  It is recommended that Boxers, Doberman Pinschers, and Great Danes have yearly electrocardiograms (ECG's) done.  An ECG is an assessment of the electrical activity of the heart.  We see the heart rhythm and pattern by looking at a tracing of the electrical activity on a piece of paper.  If a dog has an abnormal rhythm or odd "beats" on the strip of paper, that could be an indication of heart disease.  It does not necessarily indicate dilated cardiomyopathy, but it does indicate that the heart may have a problem and further tests should be run.  Keep in mind that a normal ECG does not rule out heart disease necessarily.

An echocardiogram is an ultrasound examination of the heart.  This test uses sound waves which bounce off the structures of the heart and are read on a TV-like monitor.  It gives the most accurate determination of the size of each heart chamber, measures the thickness of the heart walls, and tells us if the heart is contracting normally, as well as other vital information.  Ultrasound may not be available in all private practices because of the additional training needed to learn how to perform the examination and because of the cost of the equipment.  Your veterinarian may call in a specialist or send you to a specialist to have this test done.  An echocardiogram is a very sensitive test and the most definitive test in diagnosing dilated cardiomyopathy or any other heart disease.  If you own a breed predisposed to dilated cardiomyopathy, most veterinarians recommend having your dog ultrasounded to check for this disease.  It is a non-invasive test and if diagnosed early, you can possibly help prolong the time before congestive heart failure sets in.  Breeding dogs should also be ultrasounded in order to not pass on any potential genes that might account for this disease.  We do not know if it is a genetic disease, but affected dogs should not be bred.

Chest radiographs (x-rays) are also used to look at the lungs and the overall size of the heart.  Radiographs are also non-invasive and provide much information on what is going on with the dog.

Blood tests do not give direct information on dilated cardiomyopathy.  However blood tests do allow veterinarians to assess the dog for other diseases and/or to see if the dog can go on medication if needed.

Treatment for dilated cardiomyopathy

Unfortunately, there is no treatment for dilated cardiomyopathy.  If your dog is diagnosed with it, there are no drugs to reverse the process.  The treatments given to dogs with dilated cardiomyopathy are aimed at treating congestive heart failure, if present, or if not present, to prolong the time it takes until congestive heart failure is seen.

Treatment for congestive heart failure

There are medications for the treatment of congestive heart failure (CHF).  These medications can help reduce fluid backup into the lungs and thus keep the dog more comfortable.  There are also medications to help slow down the rhythm of the heart in cases of heart arrhythmias.  Numerous drugs are available for the treatment of CHF depending on what the dog needs.  If the dog has a sudden onset of CHF, immediate administration of drugs is essential.  The following drugs may be used at various stage of treatment, however, initial stabilization of CHF usually utilizes the first two drugs below.

1. Diuretics  Diuretics are drugs that stimulate the kidneys to excrete excess fluid from the body.  The most common diuretic used is furosemide.  There are other diuretics used and will vary from case to case.
2. Nitroglycerin  This drugs is a venodilator.  It dilates (opens up) the veins throughout the body.  This will allow a decreased amount of blood to return to the heart, allowing some of the blood to "pool" in the veins (to be stored in the veins).  The reason we want this effect is because this takes some of the workload off the heart.  It is very effective to help alleviate pulmonary edema, but the effective only last a few days, thus it is used in emergency CHF situations only.
3. Digitalis  Digitalis improves heart function several ways.  It regulates excess hormones that have been released, strengthens each contraction of the heart, and most importantly, slows the heart rate down.  Digitalis is commonly used if heart arrhythmias are seen.
4. Enzyme Blockers  This class of drugs can directly block the compensation systems that has gotten out of control (see the above section on "signs of congestive heart failure").  The drugs are commonly called ACE Inhibitors.  ACE Inhibitors block the enzyme called angiotensin converting enzyme.  This enzyme is responsible for allowing the heart to compensate early on in heart disease.  Low term, this enzyme will continue to do what it is suppose to do, but this is detrimental to the body and heart.  It actually causes more problems long term.  By blocking this enzyme, we can stop the body's cycle of trying to compensate for heart disease and this will significantly improve many dogs (and cats) conditions.    The effects an ACE Inhibitor has on the cardiovascular system include decreased peripheral (throughout the body) resistance, decreased blood pressure, increased cardiac output, and increased exercise tolerance to name a few.  A commonly used ACE Inhibitor is enalapril.
5. Vasodilators  These drugs dilate the arteries and/or the veins in the body so that the heart does not have to generate as much pressure to eject blood through the aorta and out into the body.  These drugs have continuous effects can be used long term unlike nitroglycerin.
6. Carnitine  A few dogs, especially Boxers, have a deficiency of this amino acid.  The deficiency can cause cardiomyopathy.  By adding carnitine into the diet, the heart function will improve.  A deficiency must first be diagnosed based on measuring carnitine levels in the dog's blood.  Giving carnitine to a dog not deficient in it will not help the cardiomyopathy.
7. Taurine  Many years ago, many cats were seen with DCM.  The cause was due to lack of taurine in their diet.  Cats cannot make taurine from the food they eat, they must ingest it.  Once taurine was added to all cat diets, DCM has practically vanished in cats.  Very rarely will a cat still get DCM.  If a cat is diagnosed with DCM, taurine levels can be measured in the cat's blood.  If taurine deficiency is not documented on the blood sample, then the cat has idiopathic (no know cause) DCM most likely.  Recently, veterinarians have seen Cocker Spaniels with a form of cardiomyopathy that is due to taurine deficiency.  Giving taurine to these dogs can sometimes reverse the disease.  There is no reliable blood test to predict which dogs will respond, so a therapeutic trial is appropriate.
8. Diet and Exercise  Low sodium diets are usually recommended in animals with CHF.  Sodium causes water retention and we do not want extra water in animals with CHF.  Your veterinarian may also restrict your animals exercise because we don't want to stress the heart out anymore than it already is.

Prognosis in dogs with DCM

The long term survival of your dog (or cat) will vary from case to case.  Several factors that veterinarians look for, before telling owners what the prognosis is, are test results (such as the echocardiogram) and is a favorable response to medication seen several days after medication is started.

If a response does not occur within the first few days of treatment, the prognosis is much less favorable.  Even in dogs that do respond to the medication within the first few days of treatment have a guarded prognosis, living a few months to sometimes many months.  The overall long-term prognosis is not good, meaning that the majority of dogs do not survive years after diagnosis.  It can be difficult to generate an accurate estimate for life expectancy when a dog has heart disease because many variables impact survival.