Ron Hines DVM PhD 6/24/06

The heart of all mammals is made up of four chambers. The upper left and right chambers are called the atria (atrium) and the lower left and right chambers are called the ventricles.

Blood flows from the veins of the body into the right atrium. It is stored there briefly as it is pumped on into the right ventricle. The right ventricle pumps blood into the lungs, where it receives oxygen. It then flows from the lungs into the left atrium where it is held briefly before going on to the left ventricle. The left ventricle contains the largest muscle of the heart so it can pump blood out through the arteries to all parts of the body.
There are two types of cardiac failure or myopathy that occur in dogs. In one form called hypertrophic cardiomyopathy the walls of the chambers of the heart thicken, leading to a decrease in pumping efficiency. This form of cardiac failure is quite rare in dogs. In the second form of cardiomyopathy the chambers of the heart increase in size and the muscles that form the walls of the heart stretch thinner. This is called dilated cardiomyopathy.

It is rather common for a dog’s failing heart to enlarge and no longer pump sufficient blood throughout the body. This form of heart disease is rare in the toy and smaller breeds but common in dogs that weigh over fifty pounds. Small dogs suffer more from valvular failure, which leads to congestive heart failure with signs very similar to dilated cardiomyopathy. Male dogs are slightly more prone to this condition than female dogs. Pets are usually over five years of age when the condition is first noticed. Doberman pincers, boxers and cocker spaniels often acquire this condition but any larger breed can develop it. For reasons we do not understand, the incidence of cardiomyopathy in doberman pinchers is greater than that in all other breeds combined. Great Danes, Irish wolfhounds and Scottish deerhounds are also frequent victims of this disease. When it occurs in Boxers it is common for serious heart rhythm abnormalities to occur while enlargement of the heart stays minimal.

As the failing heart enlarges, the left side looses its ability to contract forcefully to pump blood through the body’s blood vessels. When this occurs, blood begins to pools in the right side of the heart, which supplies blood to the lungs for oxygenation and receives spent blood from the thorax and abdomen. The dog’s heart works hard to compensate for these changes but eventually your pet can no longer perform the activities it once did. This stage of the disease is called congestive heart failure.

In congestive heart failure, the heart is no longer able to provide blood with adequate oxygen to supply the body. Without adequate oxygen, the body's cells become desperate and trigger a series of responses. Various hormones are released by several organs in an attempt to correct the problem. These hormones conserve and retain fluids in an effort to increase blood volume and the output of blood. For several months, these compensatory responses help the situation. However, increased fluid retention eventually becomes harmful. More and more fluid leaks out of the capillaries, causing increased gagging and coughing, and reduced stamina. Fluid in the lungs is called pulmonary edema, fluid below the skin is called peripheral or limb edema, and fluid in the abdomen is called ascites. Peripheral or limb edema is much less common in dogs than in people with congestive heart failure.

Signs of Heart Failure:
Dogs in the later stages of congestive heart failure become much less active and tire easily. Their appetite usually falls of and they show signs of difficult respiration, panting and coughing while at rest. Their tummy enlarges and takes on a pear-shape as fluid accumulates in the liver and abdomen. Electrocardiograms taken of these dogs are always abnormal. When I examine these dogs, the color of the membranes of the mouth are grayish rather than healthy pink and blood vessels on the surface are abnormally congested with blood. These dogs often have a condition called a jugular pulse in which the beating of the heart can be seen in the large jugular veins of the neck. The sounds of heartbeat that I hear through my stethoscope are always abnormal to some degree.

By the time dogs become symptomatic with cardiomyopathy and congestive heart failure, they rarely live beyond a year. Many will die in six months. The disease is known to run in families so families with this problem should not be bred.

Doberman pinchers develop abnormal electrocardiograms up to four years before they develop clinical signs of heart failure. Many of these dogs die suddenly without warning. Owners often think these dogs have been poisoned. Others develop the cough and fluid retention characteristic of the heart failure along with muscle wasting and difficulty getting about.

In Boxers abnormal heart rhythms are often picked up as an incidental finding on routine health exams when no signs of illness are present. Later, the Boxers may have sudden incidents of collapse, fainting and weakness due to this irregular heart beat.

Dilated cardiomyopathy or congestive heart failure develops over many months or even years. Its effects on blood flow also develops slowly. As heart function declines, the body is able to compensate for several weeks or months. However, at some point, the body's ability to compensate is no longer effective. At this point, dogs go into severe heart failure in what appears to be a matter of hours. Rapid, heavy breathing, blue tongue, excessive drooling, or collapse may be the first signs that anything is wrong.

Diagnosis of Cardiomyopathy:
I can diagnose most cases of cardiomyopathy with my stethoscope. I listen on the left side of the chest just behind the elbow. Listening here allows me to hear abnormal heart rhythms as well as the abnormal sounds of blood whooshing through overly distended heart valves. Heart sounds in this condition tend to be muffled and the raspy noise of air passing through fluid-filled lungs is often audible.

When a dog with abnormal heart sounds and these typical signs comes into my hospital, the first thing I suggest is that we perform a chest X-ray to see how bad the problem really is. Hearts in cardiac failure have a very distinctive globular shape. The normal, chiseled cardiac silhouette is replaced by a much larger, rounded heart shadow. Early in the disease the left side of the heart may be more enlarged than the right but with time both the left and right side of the heart enlarge. In boxers, rhythm irregularities may be present before x-rays show abnormal findings. The lungs of dogs in heart failure are abnormally dense due to fluid buildup within them.

Many veterinarians rely on electrocardiograms to detect early heart abnormalities before x-ray diagnosis is possible. A fast, out-of-control fibrillation of the muscle of the atrium is present in seventy to eighty percent of giant dogs with cardiomyopathy. The portion of the paper tracing called the QRS complex lengthens and increases in height (amplitude) signifying left ventricular enlargement. Heart rate is faster than normal in the tracing and premature contractions of the ventricles give the tracing an abnormal rhythm.

Visualization of the heart in real time with an ultrasound machine also gives me a good indication of the efficiency of the heart in pumping blood. It gives me the most accurate measure of the size of each heart chamber as well as some indication of the degree of heart enlargement.

Blood serum chemistry and urine chemistry tests do not detect heart problems but they do let me know if problems in the liver or kidneys might affect the action of heart medications I will later use.

Many cases of cardiomyopathy are accompanied by heart rhythm abnormalities. These are caused by disturbances in the electrical impulses that control heart rate and rhythm and they can be life threatening. It is a good idea to learn to check your dog’s pulse at its wrist to detect this abnormality at home. You may even choose to buy a nurse’s stethoscope for more accuracy.

Treatment:
Cardiomyopathy responds best to a cocktail of medications. One of the oldest drugs used to treat this condition is digitalis (Digoxin, Cardoxin, Lanoxin). This medication belongs to a group called positive inotropic agents which increase the concentration of calcium in heart muscle cells. This increases the force of cardiac contractions and usually slows heart rate. I begin dogs under forty pounds on 0.0025 to 0.005 mg/pound body weight given twice a day. It takes about five days for the drug to reach stable blood levels and show its effect. Larger dogs are given 0.22mg/m2 of body surface. The drug must be used in caution in dogs with kidney or liver problems. If loss of appetite, vomiting, diarrhea or lethargy occur I lower the dose. One or two weeks after beginning digitalis, I order a blood digoxin test to see if therapeutic levels of the drug are present. Digoxin is eliminated from the dog’s body through the kidneys so dogs with kidney damage are less able to tolerate the drug. In these dogs the dose should be lowered or digitoxin should be used instead because it is metabolized and excreted through the liver.

Another helpful group of drugs for dogs in heart failure are diuretics. These drugs remove accumulation of fluids that occur in the lungs and abdomen of cardiac patients due to the sluggish flow of blood. The most common and best drug of this class is furosemide (Lasix). The dose of furosemide is 1-3mg/pound of body weight given two or three times a day. Dogs taking furosemide usually drink and urinate more frequently. While on this drug the dog should receive a potassium supplement such as Morton’s NuSalt.

Another important group of drugs used in treating dilated cardiomyopathy are blood vessel dilators called ACE inhibitors. These compounds decrease certain chemicals that tighten blood vessels so more blood flows smoothly through them allowing the heart to pump blood more efficiently. The most commonly used drug in this class is enalapril (Enacard, Vasotec, Lotensin, Prinavil, Zestril). I give dogs 0.25mg/pound body weight once or twice a day. Since this drug can cause kidney problems it is wise to have a BUN and creatinine serum level performed two weeks after starting the medication and then every three or four months. When side effects of enalapril occur, they are usually lack of appetite, vomiting and an increase in toxic waste products due to decreased blood flow through the kidney (azotemia).
Two other ACE inhibitors that are occasionally used are captopril (Capoten ® 0.5-1.0 mg three times a day) and benazepril (Lotensin 0.125mg/ pound once a day). Benazepril is recommended when blood tests show an elevated BUN and creatinine, which signifies poor kidney function.

Drugs used to treat heart beat abnormalities (ventricular arrhythmias) include mexiletine (Mexitil,2-4mg/pound three times a day) procainamide (5-10mg/lb every 6 hours) or its long acting form, Procan SR, (5-10mg/lb every 8 hours). Unfortunately this drug and a sister compound, quinidine, often cause depression and lack of appetite. Mexiletine is often given along with atenolol (0.25mg/pound once or twice a day).

Both Hills Prescription Diets (h/D) and Purina’s CNM (CV formula) make low sodium diets for use in dogs with heart disease. A recipe for a low sodium home-cooked diet is included in this series www.2ndchance.info/homemadediets.htm

There have been scientific studies that indicate that a few cases of cardiac failure, particularly in boxers, are due to a deficiency in the amino acid L-carnitine. If tests show an abnormally low carnitine level, supplementing the patient’s diet with this product can halt the disease (110mg/pound twice a day, maximum of 4 grams per day). Because of its expense, it is not usually given unless a deficiency can be documented. Another group of cocker spaniels have been found to develop heart failure when there are deficient in another amino acid, taurine. Since this amino acid is not toxic and inexpensive there is no harm in administrating it to heart patients (500mg twice a day).

In humans, Coenzyme Q supplementation has improved the strength of heart muscle contraction (30-90mg twice a day). We do not yet know if this compound helps dogs in cardiac failure.

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