What is Atrial Fibrillation?
Atrial fibrillation is a cause of an abnormal heart rhythm. It is one of the most common rhythm disturbances that occur in the horse, and is also seen in humans too. A horse’s heart is made up of 4 chambers, the atria and ventricles of the left and right sides of the heart. Blood flows into the atria from either the body (right side) or the lungs (the left side). Blood then flows from the atria into the respective ventricles. The atria then contract, pushing approximately an extra 10% of blood into the ventricles. The ventricles then contract pushing the blood out of the great vessels to supply the body. In atrial fibrillation the coordinated contraction of the atria does not occur. Consequently the extra portion of blood squeezed into the ventricle is missing and cardiac output is slightly reduced.
Figure 1: The structure of the horse’s heart.
Why does atrial fibrillation occur?
Horses appear to be predisposed to developing atrial fibrillation due to their high parasympathetic (nervous) tone and the size of the equine heart. Both of these can allow interruption in the conduction of the normal electrical pathways within the horse’s heart.
What are the signs of atrial fibrillation?
At high level exercise atrial fibrillation can limit performance. Therefore the horse may feel a bit flat or not be performing well at the same level as previously achieved. At less intense levels of exercise no effect may be noticed as sufficient cardiac output is maintained. An irregular heart rhythm detected during a routine veterinary examination may be the first and only abnormality noted.
How is atrial fibrillation diagnosed?
There are several causes of an irregular heart rhythm in the horse. Therefore an electrocardiographic (ECG) examination is needed to definitively diagnose the arrhythmia. Electrodes are attached to the skin of the horse and the reading is transmitted to a computer nearby. In atrial fibrillation this reveals specific abnormalities as shown in figure 2. The trace shows that the baseline is wavy or fibrillating and that the beats occur at irregular times. The ECG recording from a horse with a normal rhythm is shown in figure 3, this has a relatively flat baseline with beats occurring at regular intervals.
Figure 2: The ECG of a horse in atrial fibrillation
Figure 3: The ECG of a horse in normal sinus rhythm following successful treatment of atrial fibrillation
An echocardiogram (ultrasound of the heart) may also be recommended to rule out any underlying heart disease that may have led to enlargement of the heart and the development of atrial fibrillation.
Figure 4: Echocardiographic image of the horse’s heart
How is Atrial Fibrillation treated?
There are two main treatment options for horses in atrial fibrillation, the administration of a drug called quinidine sulphate or electrical cardioversion.
Quinidine sulphate is a drug which is administered via a nasogastric (stomach) tube. By slowing the electrical conduction within the heart it allows normal rhythm to be restored. Success rates for treating atrial fibrillation with quinidine can be very good, however the longer the arrhythmia has been present the lower the success rate is. Quinidine can also be associated with side-effects such as diarrhoea, colic, depression and swelling of the nasal passages.
Electrical cardioversion is a newer technique for the treatment of atrial fibrillation. This involves the insertion of electrodes into the horse’s heart prior to delivering an electric shock. This procedure is always performed under general anaesthesia and therefore is associated with inherent risks of anaesthesia in the horse.
Whilst many horses treated successfully for atrial fibrillation will remain in normal sinus rhythm, there is the possibility that the arrhythmia may reoccur over the following weeks to months.
Dr Rosie Naylor BVetMed MVetMed DipACVIM PhD MRCVS