Beating atrial fibrillation in horses

Beating atrial fibrillation in horses A technique pioneered at the University of Guelph is proving effective for treating atrial fibrillation in horses. Atrial fibrillation (AF) is the most common abnormality of heart rhythm that actually affects the horse's performance. It may occur secondary to other heart disease. But it is not uncommon in horses with no physical abnormality of the heart ("lone AF"). Normally, the heart muscle contracts in a coordinated way, starting with the atria and spreading through the ventricles. In AF, the atria contract rapidly in an uncoordinated manner. The ventricles are not able to contract in response to every stimulus coming from the atria and a haphazard heart rate results.

Horses seem more susceptible than other species to atrial fibrillation. They have a high vagal tone - impulses from the vagus nerve that slow the heart rate, and "damp down" the heart's contractions. However vagal tone has other effects as well, that may make the heart prone to atrial fibrillation, such as shortening the refractory period of the atrial muscle cells. Another factor is the large mass of muscle in the atria, in which it is easy for the electrical impulses that control the contractions to go astray.

In some horses, AF may resolve without treatment. Racehorses, in particular, are liable to suffer what is called paroxysmal atrial fibrillation - in which the condition is present for a day or two and then resolves spontaneously.

But, if the condition persists, what are the options for treatment?

For years, the treatment has been quinidine sulphate. This damps down the electrical activity in the heart and in many cases allows the normal rhythm to be re-established.  But the effective dose is very close to the dose at which toxic effects - such as depression, diarrhoea and colic - appear.

In some countries, quinidine sulphate is becoming more difficult to obtain. It is also particularly unpalatable, and so repeated doses need to be given by nasogastric tube. A refinement to the technique is to use a formulation of quinidine that can be given intravenously.

The problems with quinidine led to the search for an alternative approach. Electrical cardioversion (passing an electric shock through the heart) is an effective treatment for humans with AF. The current may be passed through the chest wall (transthoracic cardioversion) or by electrodes passed into the heart itself (internal cardioversion).  Now, horses are not ideal candidates for transthoracic cardioversion, because of their size and the position of the lungs full of air between the electrodes. But what about the use of internal cardioversion in horses?

Dr Kimberly McGurrin and her colleagues at the University of Guelph in Canada have been getting good results.  In a recent report they recorded the successful treatment in 71 of 72 episodes of AF in 63 horses using this technique. They use two electrodes, passed through the right jugular vein and placed in the right atrium and the pulmonary artery. The left pulmonary artery is preferred as it requires lower strength shocks. The electrodes are initially placed with the horses standing, using ultrasound guidance. 

The shocks are administered with the horses anaesthetised. They are timed to coincide with the R-wave of the ECG to avoid interfering with the contraction of the ventricles.

A low strength shock is given for a start. If this is not effective they increase the strength of the shock - until the heart reverts to normal rhythm, or until it is not safe to continue.

The whole procedure takes about two hours. As well avoiding the toxic effects of quinidine, this technique has the advantage of quicker return to work. Horses are allowed to return to light work after a couple of days. After quinidine treatment they would have been rested for 7-10 days. Of course there are risks associated with general anaesthesia - but the good response rate, absence of adverse effects and quick return to work outweigh these.

They found that higher energy shocks were required for females than males. In their patients, they found that horses affected for a long time before treatment were no less likely to respond.

A similar technique is also in use at the University of Ghent, in Belgium. Speaking at the Conference on Equine Sports Medicine and Science in Utrecht in July 2008 Dr Gunther van Loon reported the successful treatment of 9 out of 10 horses using internal cardioversion.  They had one case that recurred shortly after treatment, and so now they always give an anti-arrhythmic drug (amiodarone) before carrying out the procedure to reduce the risk of recurrence. 

Transvenous electrical cardioversion appears to be an effective treatment for atrial fibrillation in horses - especially when other treatments have failed.

For more details see:

Transvenous Electrical cardioversion of equine atrial fibrillation: patient factors and clinical results in 72 treatment episodes.
MKJ McGurrin, PW Physick-Sheard, DG Kenney.
J Vet Intern Med (2008) 22, 609 - 615

Transvenous electrical cardioversion of atrial fibrillation in six horses using custom made cardioversion catheters.
D De Clerq, G van Loon, S Schauvliege, R Tavernier, K Baert, Siska Croubels, P De Backer, P Deprez.
The Veterinary Journal (2008) 177, 198 - 204

Reproduced with kind permission of Mark Andrews BVM&S CertEP MRCVS
© Copyright Mark Andrews  - Equine Science Update 2008

Photograph: M. Kimberly J. McGurrin, DVM, DVSc, DACVIM

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