ARRHYTHMIAS IN HORSES (ABNORMAL RHYTHMS)

Cardiac arrhythmias occur commonly in horses. The cause and the treatment vary widely depending on the underlying etiology and type of arrhythmia. An ECG is necessary to confirm the diagnosis and choose the appropriate treatment

ATRIAL FIBRILLATION

Atrial fibrillation is the most common rhythm abnormality that occurs in horses. It rarely requires emergency treatment. Fortunately, most horses have no underlying cardiac disease but come to the CUHA for exercise intolerance. There are two ways to convert atrial fibrillation to normal sinus rhythm: medical therapy with administering quinidine or electrical cardioversion. The Cornell College of Veterinary Medicine is one of a few veterinary schools that are currently performing electrical cardioversion.

VENTRICULAR ARRHYTHMIAS

There are many causes for ventricular arrhythmias in the horse. Ventricular tachycardia is either caused be severe systemic disease (intestinal disease; colic, and infectious disease) or underling heart disease (see myocarditis). If the heart rate is very fast, this can be life threating and emergency treatment is required.

HEART BLOCK

AV (atrioventricular) block can be a normal variation in horses and rarely requires treatment. Many horses have an arrhythmia called second degree AV block. This arrhythmia is very common and often goes away with trotting or exercise. Normally the heart beat is initiated by the part of the heart called the sinus node. The impulse conducts through the atria and to the junctional point between the atrial and the ventricles. The junction is known as the atrioventricular node or AV node, in short. In second-degree AV block the electrical impulse from the top chamber of the heart (atria) does not conduct to the bottom chamber of the heart (ventricle) because the AV node malfunctions. It is very rare for a horse to have advanced AV heart block that would require pacemaker implantation.

MYOCARDITIS (Inflammation of the heart muscle)

Some horses with ventricular arrhythmias or heart block may have underlying inflammation of the heart known as myocarditis. In these situations monitoring of the arrhythmia and other tests (e.g. echocardiography, troponin I,) are required. Often the cause of the myocarditis cannot be identified, but this inflammatory process can often be treated with medication.

“LEAKY VALVE DISEASE” OF OLDER HORSES
DEGENERATIVE ATRIO-VENTRICULAR (MITRAL AND TRICUSPID REGURGITATION) VALVE DISEASE

Clinical Population: This very common disease usually affects older horses

Characteristics: The most commonly affected of the four heart valves is the mitral valve, which separates the left ventricle from the left atrium. Normally this valve closes when the heart contracts. Normally, this closure prevents blood from going back into the atrium so that the blood goes to the body. When the valve leaks, the blood goes backwards into the left atrium. The flow of blood going the wrong way is called "regurgitation." Therefore, this disease often is called mitral regurgitation. (If the valve between the right ventricle and right atrium also is affected the term tricuspid regurgitation is used.) Eventually the left atrium enlarges followed by fluid accumulation in the lungs because of too much volume and pressure. The valve leaks because it has "degenerated." The normal structural integrity of the leaflets of the valve is lost.

Symptoms: Many horses with this condition can still be used for performance or riding. Clinical signs include exercise intolerance, coughing, trouble breathing, increased breathing rate, collapse, or weakness.

Diagnosis: Virtually all horses with clinically important mitral and tricuspid regurgitation or aortic insufficiency will have a cardiac murmur heard through a stethoscope. Veterinarians can hear a murmur long (months to years) before clinical signs are noticed. Echocardiography, or ultrasound, is an important diagnostic tool to fully characterize the structure and function of the valves. This test permits the examination not only of the muscle and valves, but also of the blood flow in the case of doppler echocardiography. Therefore, the changes to the heart are measured and the amount of blood "going the wrong way" can be semi-quantified. Other tests may be ordered to ensure that other body systems are functioning adequately for the clearance of the drugs that will be used to treat the disease process.

Treatment: Horses with only very advanced stages of the disease require treatment.

Prognosis: The prognosis depends on the severity of the disease, which is determined by when it is diagnosed. Many horses identified early live for many years, while others with heart failure may only live a few months. The diagnostic tests not only identify the condition, but also tell use the stage of disease in each horse.

How should an affected horse be monitored?
The frequency and intensity of the monitoring depends on each patient, the stage of the disease, the response to treatment, and the status of other body systems.