Ron Hines DVM PhD 1/11/08

This is the second of two articles on heart failure in dogs. The first, Canine Dilated Cardiomyopathy, described a common condition in larger breeds of dogs. Much of the two articles are repetitive because a failing heart must go through certain similar stages regardless of the initial cause. This article describes the most common heart abnormality occurring in small and toy breeds.

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. Each of the four chambers has an inlet and an exit valve to prevent back washing of blood. In smaller breeds of dogs it is these valves – particularly the mitral valve that divides the left atrium from the left ventricle - that often fails leading to an enlarged, inefficient heart.

The Mitral Valve:
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 to the lungs, where it receives oxygen. It then flows from the lungs through a series of veins into the left atrium where it is held briefly before going on to the left ventricle. Dividing the upper left atrium from the lower left ventricle is the mitral valve. The left ventricle contains the largest muscles of the heart so it can pump blood out through the arteries to all parts of the body. A three- cusped valve separates the left atrium from the left ventricle. When the left ventricle contracts, this valve must be in the closed position to force blood through the arteries (aorta) to supply the body. Because of the great force with which these left ventricular contractions occur, this valve wears out first in many older dogs. The majority of dogs over 8 years of age have this defect in varying degrees. At first, the leak is minor and produces no more than a high-pitched murmur (regurgitation) I can readily hear with my stethoscope. This murmur is most audible on the left side of the chest just posterior to the dog’s elbow. It is due to scaring and shortening of the cusps of the valve. Many dogs over the age of six develop these murmurs and most dogs over the age of ten have them. The disease is also called valvular endocardiosis, Chronic Mitral Valve Disease (CMVD) or Valvular Insufficiency. As the leakage of blood back into the atrium becomes more severe the efficiency of the heart as a pumping organ is reduced.

When leakage is severe, blood backs up through the entire vascular system but particularly so in the veins that drain the lungs. With abnormal pressure in these veins fluid leaks out of capillaries and into the functional portion of the lungs, the alveoli. This fluid, called pulmonary edema, produces the earliest signs of heart failure, a hacking coughing and an attempt to wretch up or gag fluid from the throat. Dogs in this stage of heart disease already lack the stamina to play and work as they once did.

As the condition progresses, and as pressure builds up in the heart, first the left side and then the right side of the heart begin to enlarge. This enlargement is not accompanied by any increase in pumping efficiency. The more the heart enlarges, the less the four valves of the heart are able to seal themselves during contraction.

With time, fluid backs up into the liver and leaks out into the abdomen causing a bloated, pear-shaped tummy. This fluid is called ascites. It is compounded by the release of several hormones that increase the total fluid capacity of the vascular system.

Symptoms of Heart Failure:
At this late stage of the disease all forms of heart failure have the same signs. 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. 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 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.

Congestive heart failure develops over many months and 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 can go into severe heart failure in what appears to be a matter of hours. Rapid, heavy breathing, blue tongue (cyanosis), excessive drooling, or collapse may be the first signs that anything is wrong.

By the time dogs develop ascites cyanosis and pulmonary edema they rarely live beyond a year or two. Many die sooner despite the use of many medications. The disease is known to run in families so dogs with this problem should not be bred.

Diagnosis of Congestive Heart Failure:
I can diagnose most cases of cardiologist 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. Heart rate is increased (tachycarida).

Early in congestive heart failure, one side of the heart is usually more affected than the other. When the left sided is primarily affected, shortness of breath and coughing during exertion are the first signs we notice. When the right side of the heart is primarily affected, we see ascites (fluid buildup in the abdomen) which causes a pot-bellied appearance. In both cases weight loss and exercise intolerance are common.

When a dog with abnormal heart sounds and these typical signs comes into my hospital, the first thing I suggest is that we run 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 are enlarged on x-rays The lungs of dogs in heart failure are abnormally dense due to fluid buildup within them.

Many veterinarians rely on electrocardiograms (EKG) to detect early heart abnormalities before x-ray diagnosis is possible. The portion of the EKG 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 cardiac enlargement 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:
Cardiac enlargement and heart failure responds best to a cocktail of medications. One of the oldest drugs used to treat this condition is digitalis (Digoxin, Cardoxin, Lanoxin) it has been used since the time of the Romans. It is used less in the USA today because of the serious side effects that sometimes occur. . 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, Fortekor, 0.125mg/ pound once a day). Benazepril is recommended when blood tests show an elevated BUN and creatinine, which signifies poor kidney function. Benazepril reduces the blood pressure and volume load on the heart in dogs with heart failure and leads to an extension of the life span of dogs with heart failure and also improves clinical signs, notably reduction in coughing, and improvement to the quality of life.

Another ACE inhibitor used frequently in Great Britain, Europe and Canada is Vetmedin (pimobendan, Boehringer Ingelheim). Since its introduction in 2000 it has been shown to increase both quality of life and length of life in dogs suffering from congestive heart disease, above and beyond previous treatment regimes. Vetmedin opens up the blood vessels and eases resistance in the circulatory system dilating blood vessels, and improving the efficiency with which the heart functions as a pump. The combined effect is therefore to improve cardiovascular function and the blood flow to major organs. Vetmedin® is licensed and appropriate for both the major causes of heart failure in dogs. In one clinically controlled trial, dogs with congestive heart failure survived on average (median) 42 days without Vetmedin, 217 days when Vetmedin was part of the therapeutic regime.

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).

Other Drugs That May Be Effective

Vetmedin (pimobendan)

Vetmedin is the trade name for Pimobendan. It is in a clalss called positive inotrope and mixed vasodilators. It's effect is similar to ACE inhibitors such as enalapril but it works by a different mechanism. It works by increasing the amount of calcium available to the heart muscles and also lowers vascular resistance and blood pressure (increased blood vessel diameter or vasodilatation). In doing so, the pet’s heart needs to do less work.
Vetmedin increases blood calcium levels. The more calcium that is present, the stronger are the heart’s contractions. The Drug was originally used in human medicine about 10 years ago. At the time, it was thought that it might offer hope to people with heart conditions similar to your pet’s. It did increase the force of human heart contractions, but it sometimes made the hearts pump too forcefully. Dogs and cats do not develop coronary artery disease as humans do. But human hearts with blocked or occluded coronary arteries became starved for oxygen when on this drug. This led to heart attacks and angina (chest pain). Because dog and cat hearts differ from human hearts in this respect, the drug has been reintroduced in Europe and Canada for use heart failure in pets. The published dose of Vetmedin is 0.1-0.3mg/ pound body weight/day. It should be given about an hour before meals with half the dose given before breakfast and half before supper. I obtained this dose from Boehringer-ingelheim’s Spanish language website. Their Dutch website gives the minimum daily dose as 0.15mg/lb/day. An occasional dog will experience vomiting on this drug. If your veterinarian has trouble obtaining this medication or if it's expense is too great, it can be ordered from a Canadian Pharmacy.

Forticor (benazepril HCl, Lotensin, Novartis Animal Health, Canada, etc.)
Benazepril is also an angiotensin-converting enzyme (ACE) inhibitor drug. It is similar in it’s effect to enalapril (Enacard) and Vetmedin. Forticor is used to increase the efficiency with which failing hearts pump blood and is also used to treat high blood pressure in pets. In the liver, the drug is metabolized to benazeprilat, the active compound. It can be given with or without food. It is also effective in lowering blood pressure (hypertension) in cats with kidney failure.

Spirolactone (Aldactone)
When the maximum dose of furosemide diuretic (1-3 mg/lb every 8-12 hr in dogs and 0.5-1.5mg/ pound every 8-12 hrs in cats) is not sufficient to eliminate fluid buildup in the tummy (ascites) and lungs (pulmonary edema) of your pet, another diuretic, such as spirolactone, can be added to the medications your pet receives. You need to be patient - it takes 2-4 days for the full effect of the drug to be reached. Spirolactone can increase the toxicity and effect of digoxin so your pet’s digitoxin dose may need to be lowered. It can also cause an increase in serum potassium, which can be dangerous. To monitor potassium, a blood sample should be taken and measured for serum electrolytes and kidney function on the 3rd or 4th day, the 7th day and periodically there after.

Diltiazem is in a class of drugs called calcium channel blockers that increase the force of heart contractions. This drug can also cause many side effects. These side effects are most likely when given with diuretics such as furosemide or with Enalapril. Diltiazem can slow the heart rate so much that the dog does not receive enough oxygen.

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: 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.

Although I know of no scientific studies that document these effects, some of my clients have noticed improvement in their pets when they are given fish oil capsules and aspirin. Cold-water fish oil is a rich source of omega-3 fatty acids, Eicosapentaenoic acid (EPA) and Docosahexaenoic acid (DHA). These essential fatty acids seem to reduce inflamation throughout the body. Always use the gelatin capsulated form and never straight fish oil. Aspirin (1 mg/pound once a day) is given to human heart patients to retard corinary artery disease in which small plaques and clots block the flow of blood to the heart. Corinary artery disease in dogs is exceedingly rare, so I do not know why or how aspirin would help.

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