Atrial fibrillation is the most common arrhythmia of clinical significance in horses. In many horses it is clinically silent but in horses undergoing strenuous exercise it is associated with reduced athletic performance and Exercise-induced pulmonary haemorrhage (EIPH).
Causes
There are two important causes of atrial fibrillation:
Being a horse can simply predispose to atrial fibrillation. They naturally have a large heart, and the bigger the horse, the bigger the atria. Horses also have high resting vagal tone, which also predisposes to fibrillation. This is called ‘lone’ atrial fibrillation. Vigorous exercise and/or excessive seating can trigger these episodes of AF. Horses with frequent supraventricular (atrial) premature beats may be predisposed to AF.
Mitral regurgitation describes leakage of blood across the mitral valve during systole (ventricular contraction) from the left ventricle back into the left atrium. This blood adds to the blood returning from lungs causing increased volume and pressure in the left atrium leading to dilation of the left atrium, predisposing the horse to atrial fibrillation. This is a much more serious condition as the horses may develop congestive heart failure.
Signs
In the absence of underlying structural heart disease, such as mitral valve regurgitation, most horses with ‘lone’ AF will have low, normal heart rates with an irregularly irregular heart rhythm – think tennis shoes in a very slow spinning dryer. It is not uncommon to confuse a low heart rate AF with the more common physiological second degree heart block. This is because horses will tend to cluster beats with long pauses between the clusters. Most racehorses will develop paroxysmal atrial fibrillation associated with fast work or racing and spontaneously revert to a normal rhythm 24 to 48 hours later without any treatment.
Atrial fibrillation can predispose horses to exercise-induced pulmonary haemorrhage, even at sub-maximal exertion.
A normal contracting atria is responsible for 15-20% of ventricular fill before contraction (ventricular systole). Consequently there is loss of cardiac output at peak exertion. In fact horses with atrial fibrillation will have higher heart rates at the same gait than a horse without AF. Normally the maximum heart rate of a horse at a gallop will be 220-240 beats per minute. It is not uncommon for horses with lone AF to have heart rates that approach or exceed 300 beats per minute. These rates can be dangerously high leading to the R on T phenomenon and ventricular arrhythmias. Consequently exercise tests are very important in horses continue to exercise at a high level.
Diagnosis
The arrhythmia is strongly suspected on the basis of auscultation – an irregularly irregular rhythm and an absence of the sound heard when the healthy atria contract (S4). A horse with ‘lone’ AF will have low to normal heart rate at rest, whereas horses with AF secondary to atrial dilation will typically have a normal to high resting rate and a systolic left-sided cardiac murmur. The diagnosis is confirmed using an electrocardiogram (ECG) (Figure 1). This is critical if the horse is clustering beats (Figures 2 and 3). New handheld technology has made collection of a surface ECG quite easy – the most affordable device is the Kardia (formerly AliveCor) plate which connects to your phone using Bluetooth. A more sophisticated device is the Eko Duo which is an amplified stethoscope and ECG recorder in one. An image from the Duo is below and includes the ECG and corresponding phonocardiogram (figure 4).
Treatment
Horses with AF secondary to valve disease and atrial dilation should never have attempted conversion. This is why an ultrasound of the heart is indicated before attempted conversion.
Horses with lone AF have a good chance of conversion if the arrhythmia has been present for 3 months of less. Longer than 3 months, and certainly longer than 6 months, the chances of conversion are more unlikely, and even if successful, there is a good chance of going back into AF.
Atrial fibrillation begets atrial fibrillation. This is not a condition where you cannot afford to wait too long, especially if the horse is expected to perform strenuous activities, such as racing.
The most common treatment involves chemical conversion with quinidine, with or without digoxin. Quinidine is toxic so the horse needs to be monitored closely during the chemical conversion. An alternative method is electrical cardioversion, which is preferred, but more expensive. There are only a handful of practices around the world that are set-up for this technique, but it is available locally at Murdoch University in Perth, Western Australia. It does require general anaesthesia, radiology, specialized equipment and training.
Can a horse live with atrial fibrillation
Absolutely. There are many horses with atrial fibrillation that lead normal lives and continue to be exercised and ridden safely. I know of several top line Dressage horses competing at an elite level with atrial fibrillation. It is strongly recommended to have an exercise test with an electrocardiogram. Horses with atrial fibrillation will have higher heart rates at a given gait than horses with a normal heart rhythm. If the heart rates are too high this could potentially lead to oxygen and energy depletion within the heart muscle. This could induce a more serious ventricular arrhythmia. The exercise should mimic the maximum level of activity for that individual horse.
Tags: Cardiology