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. Signs usually result from significant accumulation of fluid in various tissues (such as the lungs, abdomen, or under the skin) or poor circulation. Heart disease can be present without ever leading to heart failure. Heart failure, however, can occur only if severe heart disease is present.
Types of Heart Failure in Horses
Heart failure can be divided into 4 functional classifications: systolic myocardial failure, impedance to cardiac inflow, pressure overload, and volume overload. Depending on the condition, several of these abnormalities may be present at once.
Systolic myocardial failure is a general reduction in the ability of the heart muscle to contract. This can be identified with echocardiography (an ultrasound of the heart). 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 genetics, trauma, infection, drugs or poisons, electric shock, heat stroke, or tumors. Some cases have no known cause.
Heart failure caused by impedance (obstruction) to cardiac inflow can result in a decrease in blood flow into and out of the heart. 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 longterm 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) and left-to-right shunts (for example, patent ductus arteriosus, ventricular septal defect).
The cardiovascular system maintains normal blood pressure and blood flow. In heart disease, the body uses compensatory mechanisms to attempt to normalize these functions and offset the negative effects of the disease on the body. Some compensatory mechanisms, such as constriction of the blood vessels and an increased heart rate, act for short periods of time to account for sudden cardiovascular changes (such as reduced blood volume caused by dehydration). For the longterm changes associated with heart disease, hormonal compensatory mechanisms activate to increase cardiac output, blood pressure, blood volume, and blood flow. For example, these hormones can trigger the body to conserve water and sodium, increase thirst, and enlarge the heart muscle. These remarkable mechanisms allow an animal to compensate for mild, then moderate, and then even severe disease, often for years. Eventually, however, progressive heart muscle failure and the longterm activation of these compensatory mechanisms can damage the heart muscle and other organs, leading to further heart failure.
Signs of Heart Failure in Horses
Signs associated with heart failure depend on the causes of the heart failure and the heart chamber that is affected. With left- 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 within the lungs (called pulmonary edema). The most common signs are difficulty breathing, increased respiratory rate, poor performance, and tiring easily (especially with exercise or work). Horses may also cough.
Right-side congestive heart failure results in increased pressure in the vessels delivering blood to the right atrium and in the body’s veins and capillaries. This may cause fluid to build up under the skin on the underside of the chest and abdomen and in the limbs. Fluid can also accumulate in the abdomen (called ascites). Jugular distension and exaggerated pulses are also common.
Treatment of Heart Failure in Horses
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 build-up in the body.
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 determine the appropriate medications for your horse. 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. 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. In addition to drugs, other types of treatment are sometimes recommended, such as surgical procedures to remove excess fluid buildup from the chest cavity or abdomen.