Heart Failure in Horses
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. 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 caused by impedance (obstruction) to cardiac inflow can 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 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. Unfortunately, the longterm activation of these compensatory mechanisms can damage the heart muscle and other organs, leading to further heart failure.
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 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. Beta-adrenergic blocking drugs (also called beta-blockers) and calcium channel blockers are helpful in some cases of congestive heart failure. 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.