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 bring on failure of the cardiovascular system. Therefore, there are limited and specific signs that can develop as a result of heart failure.
Types 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 nutritional (taurine) deficiency, genetics, 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 (hypertrophic cardiomyopathy), or abnormalities of physical structures of the heart.
Heart failure caused by pressure overload occurs as a result of longterm increases in stress to the heart wall during contraction. This leads to thickening of the wall of the affected ventricle. This may result from the obstruction of blood flow from the heart (such as pulmonic or aortic stenosis) or increased blood pressure throughout the body (systemic hypertension) or in the arteries of the lungs (pulmonary hypertension).
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 increase in blood volume leads to enlargement (dilation) of the affected ventricle's chamber.
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 negative effects the disease is having on the body. The nervous system can immediately increase an animal's heart rate and blood pressure. These effects fade within a few days, at which time a special hormone system activates and triggers the kidney to save water and sodium. This increases the volume of blood in the body, which increases cardiac output, blood pressure, and blood flow. These remarkable mechanisms allow for an animal to compensate for mild, then moderate, and then even severe disease, often for years. Unfortunately, longterm activation of these compensatory mechanisms can damage the heart muscle and other organs in the end stages of the disease, leading to more severe heart failure.
Signs of Heart Failure
Signs associated with heart failure depend on the causes of the heart failure and the heart chamber that is affected. With left-side congestive heart failure, signs are associated with a backup of pressure in the vessels delivering blood to the left ventricle. This causes fluid to accumulate in the lungs (called pulmonary edema) and in the chest cavity outside the lungs (pleural effusion). Coughing and difficulty breathing are the most common signs, although cats with heart failure are far less likely to cough than dogs with the disease. Increased breathing rate, loss of appetite, or reluctance to exercise may also be noted. The decreased volume of blood pumped out of the heart can cause the paws and ears to feel cold and can lower a cat's body temperature. Cats with left-side congestive heart failure often breathe faster than healthy cats. Your veterinarian may direct you to count the number of breaths your cat takes within a minute when it is sleeping or resting (but not purring). This sleeping respiratory rate can then be regularly monitored to identify early heart failure and response to medications.
Right-side congestive heart failure results in increased pressure in the body’s veins and capillaries. This can result in an accumulation of fluid in the abdomen, chest cavity, liver, or limbs. Right-side heart failure is uncommon in cats.
Biventricular failure can arise when both the right and left ventricles are not working, such as in cats with heart failure resulting from dilated cardiomyopathy or toxin (poison) exposure. Signs attributable to both forms of congestive heart failure can be noted, although commonly signs of one type of congestive heart failure will 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 causes and severity of the heart failure and other factors. Your veterinarian is best able to decide on the appropriate medications for your cat. All drugs prescribed by your veterinarian must be given as directed. Otherwise, they may not be effective and may even cause serious complications and harm.
Diuretics are usually prescribed to reduce fluid overload. Digoxin and pimobendan, 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 also helpful in some cases of congestive heart failure.
In addition to drugs, other types of treatment are sometimes recommended. These may include a low-sodium diet (prescription or commercial diets are available), oxygen therapy to raise the level of oxygen in the blood, taurine supplementation in cats with verified deficiencies, surgical procedures to remove excess fluid buildup from the chest cavity or abdomen, or anti-anxiety medications for cats that are having trouble breathing.