Cardiac arrhythmia (cardiac dysrhythmia)
Definition: any irregularity in the normal, two-beat rhythm of the heart
Types: Atrial fibrillations are irregularities in the contractions of the atria, the upper two chambers of the heart, which pump blood into the lower two chambers, called ventricles. Ventricular arrhythmias originate in the lower two chambers of the heart, which pump blood outward to the lungs and body. Weakness or irregularities in the activity of the ventricles are serious and potentially life threatening.
Causes: Arrhythmias generally arise when the electrical signals that control the heartbeat are delayed or blocked entirely. Causes of arrhythmia include inflammation of the heart muscle (myocarditis) as well as systemic illnesses such as colic or infectious diseases. However, the cause of an arrhythmia may be unknown.
Signs: Often, there may be no indication of a problem, especially when the horse is at rest. During exercise, a horse may lose energy and slow down or stop; he may also faint from lack of blood flow to the brain. Sustained lack of blood flow to the body can lead to congestive heart failure, when the heart cannot pump efficiently enough to maintain the body’s basic needs; signs include coughing, weakness and the accumulation of fluid in the chest.
Treatment: Arrhythmias that cause no distress to the horse and disappear when he exercises are most likely harmless and require no specific treatment. Treating underlying myocarditis or other illnesses affecting the heart may resolve related arrhythmias. Treatment to “convert” the heartbeat into a normal rhythm may be attempted with electrical currents (electrical cardioconversion) or with the administration of drugs, such as atropine or quinidine, which stress the heart in the hope that it will ease back into a normal rhythm.