Heart Failure in Dogs
Heart failure is not a specific disease or diagnosis. It is a syndrome in which severe dysfunction results in failure of the cardiovascular system to maintain adequate blood circulation. There are limited and specific mechanisms by which heart disease can result in failure of the cardiovascular system. Therefore, there are limited and specific signs that can develop as a result of heart failure.
Heart failure can be divided into 4 functional classifications: systolic myocardial failure, impedance to cardiac inflow, pressure overload, and volume overload.
Systolic myocardial failure is a general reduction in the ability of the heart muscle to contract. This can be identified with echocardiography (ultrasonography). There is reduced wall motion during contraction of the ventricles. If the reduction is significant, normal blood flow cannot be maintained. It may be caused by trauma, infection, drugs or poisons, electric shock, heat stroke, or tumors. Some cases have no known cause.
Heart failure resulting from the impedance (obstruction) to cardiac inflow may result in a decrease in blood flow. This may be caused by external compression of the heart (for example, fluid in the sac surrounding the heart), diastolic dysfunction resulting in a stiff ventricle and reduced ventricular filling, or abnormalities to physical structures of the heart.
Heart failure caused by pressure overload occurs as a result of long-term increases in stress to the heart wall during contraction. This may result from the obstruction of blood flow from the heart or increased blood pressure throughout the body or in the arteries of the lungs.
Volume overload heart failure occurs as a result of any disease that increases volume of blood in the ventricle(s), thus increasing blood flow. Eventually, this can bring on signs of congestive heart failure. Diseases that result in volume overload myocardial failure include valve disease (for example, degenerative valve disease of the atrioventricular valves), left-to-right shunts (for example, patent ductus arteriosus, ventricular septal defect), or high-output states (such as those caused by hyperthyroidism or anemia).
The cardiovascular system maintains normal blood pressure and blood flow. In heart disease, the body uses specific mechanisms to attempt to normalize these functions and offset the negative effects the disease is having on the body. In an animal with dilated cardiomyopathy, for example, the blood flow is slowed by the heart’s reduced ability to contract. This leads to a reduced blood pressure because less blood is pumped with each beat. The body compensates by using the sympathetic nervous system to increase the ability of the heart muscle to contract and to increase heart rate. These responses increase cardiac output and blood pressure. Unfortunately, longterm use of the sympathetic nervous system in this way damages the heart muscle and other organs. The damage again reduces the ability of the heart muscle to contract and leads to a cascade of actions, including the release of various hormones, that result in another increase in blood volume and blood flow. Blood volume can increase as much as 30% in pets with severe congestive heart failure. Eventually, progressive heart muscle failure and longterm activation of these hormones in an effort to normalize blood flow result in continued heart muscle failure. Blood flow becomes further compromised with progressive signs of congestive heart failure.
Signs associated with heart failure depend on the causes of the heart failure and the heart chamber that is affected. With left-sided congestive heart failure, signs are associated with a backup of pressure in the vessels delivering blood to the left ventricle. Fluid in the lungs and congestion (coughing, difficulty breathing) are the most common signs. Many dogs with left-sided congestive heart failure faint due to lack of blood flow to the brain. They may also have a low heart rate and low blood pressure and may collapse.
Right-sided congestive heart failure results in increased pressure in the vessels delivering blood to the right ventricle and the body’s veins and capillaries. This may cause fluid to build up in the abdomen (ascites), the chest cavity, and the limbs.
Biventricular failure can arise when both the right and left ventricles are not working properly, such as in dogs with heart muscle failure resulting from dilated cardiomyopathy or poisoning. Signs attributable to both forms of congestive heart failure can be noted, although it is common for signs of one to outweigh the other.
It is important to treat heart failure in order to improve heart muscle performance, control arrhythmias and blood pressure, improve blood flow, and reduce the amount of blood filling the heart before contraction. All of these can further damage the heart and blood vessels if not controlled. It is also necessary to reduce the amount of fluid in the lungs, abdomen, or chest cavity.
There are many types of drugs available for treating heart failure. The specific drugs, dosage, and frequency used will vary depending on the cause and severity of the heart failure and other factors. Your veterinarian is best able to decide on the appropriate drugs for your pet. All drugs prescribed by a veterinarian must be given to the animal as directed. Otherwise, they may not be effective and may even cause serious complications or harm.
Diuretics are usually prescribed for managing fluid overload in animals. Digitalis and digoxin, part of a group of drugs known as positive inotropes, may be used to help the heart muscle contract. ACE inhibitors (ACE stands for angiotensin-converting enzyme) and vasodilators can widen blood vessels and thus lower blood pressure. Beta-adrenergic blocking drugs (also called beta-blockers) and calcium channel blockers are helpful in some cases of congestive heart failure.
Some types of heart failure are caused by a deficiency of a nutrient and can be treated by supplementation with the missing nutrient. However, this type of heart failure is rare.
A low-sodium diet is frequently recommended for dogs with severe congestive heart failure that does not respond well to conventional treatment. In dogs with mild to moderate congestive heart failure, severe sodium restriction is not needed, but diets high in salt should be avoided. Prescription diets tailored for these differing levels of sodium restriction are readily available, as are recipes for home-made salt restricted diets. Sodium-free snacks should also be given in place of regular treats. Salt should not be restricted in dogs with heart disease that have no sign of congestive heart failure, because this can result in early activation of certain hormones.
Dogs with severe left-sided congestive heart failure and fluid in the lungs (pulmonary edema) may not get enough oxygen. Oxygen can be given by way of an oxygen cage, tight-fitting mask, or nose tube.
Thoracentesis and abdominocentesis are surgical procedures in which a needle is inserted into the chest cavity or abdomen, respectively, to withdraw excess fluid. It may be used to treat dogs with congestive heart failure that have an accumulation of fluid in these areas. The procedure can lead to rapid improvement in signs, has no significant adverse effects, and can be performed on a regular basis, if needed.
Bronchodilator treatment is generally reserved for patients with longterm airway disease. It is not typically used to treat congestive heart failure. The exception to this is for dogs that faint as a result of a brief cardiac arrhythmia associated with heart disease such as degenerative valve disease.
Cough suppressants are generally not recommended in the treatment of congestive heart failure, because masking signs of cough can worsen the underlying fluid in the lungs. If, however, a dog diagnosed with severe heart disease is coughing, and heart enlargement on chest x-rays shows no fluid in the lungs, the coughing may be caused by the enlarged heart pressing on the airways. Cough suppressants may be helpful for these dogs.