Atrial Fibrillation & Atrial Flutter

Introduction

Atrial fibrillation is the commenest pathological dysrhythmia. It is common in horses and cattle, but rare in cats and dogs.

Atrial Fibrillation: Occurs when many ectopic waves of depolarisation spread throughout the atria. While the atria fail to contract some of the disorganized depolarization waves are conducted through the AV node reaching the ventricles. As a result the ventricles have a rapid, irregular rate.

Atrial Flutter: A rare unstable arrhythmia consisting of rapid atrial rate and abnormal atrial depolarization. Atrial flutter is an arrhythmia that cycles between atrial fibrillation and sinus rhythm.

For atrial fibrillation to occur there needs to be atrial enlargement, or it can occur in normal fit horses. In small animals, atrial fibrillation is usually seen only in animals with concurrent cardiac disease. The atrial contraction only contributes to around 15% of the ventricular filling, so signs of atrial fibrillation are only seen during vigorous exercise.

Clinical Signs

This may be an incidental finding on clinical examination, especially if the horse is not used for highly atheletic activities. If the horse is raced, hunted or an eventer etc, then signs may occur during exercise and include exercise intolerance, reluctance to exercise or a poor performance during exercise. It can be associated with exercise induced pulomonary haemorrhage, so this can be a clinical sign of the condition.

Diagnosis

A full history and clinical examination may provide a presumptive diagnosis for this condition. Findings on clinical examination will include an irregularly irregular heart rhythm on auscultation with no fourth heart sound. The heart will sound 'chaotic'. There may also be a varibale pulse quality and a variable intensity of heart sounds.
An electrocargiogram (ECG) can be performed and will provide a definitve diagnosis. It will show a lack of P waves and instead 'F waves' (atrial fibrillation). There will be small f waves (atrial flutter). There will be an irregular R- R interval due to random depolarisation of the AV node. QRST complexes will all be normal and alike in appearance.

Treatment and Control

It is vital that all underlying causes of the condition are diagnosed and treated as this will treat the Atrial Fibrillation. In horses where the condition is often found in normal animals without concurrent heart disease, treatment is with the drug quinidine sulphate. This drug acts by prolonging the effective refractory period. The horse should be given quinidine sulphate concurrently with digoxin, which will have been started two days prior to commencing quinidine sulphate. Quinidine sulphate should be given every two hours by stomach tube until conversion to sinus rhythm, or until six doses have been given. There are some severe side effects which can occur with this treatment and they include ventricular tachycardia, colic, diarrhoea and hypotension.
Cattle with the condition should also be treated with the same protocol.

Prognosis

The likelihood of the horse remaining converted after the treatment depends on the duration of the condition before treatment; if it was diagnosed less than three months before treatment then it is more likely the treatment will be effective. Also, prognosis is poor if atrial fibrillation is caused by an underlying heart condition.

References

Boswood, A (2008) Cardiovascular System Study Guide, Royal Veterinary College.
Ettinger, S.J. and Feldman, E. C. (2000) Textbook of Veterinary Internal Medicine Diseases of the Dog and Cat Volume 2 (Fifth Edition), W.B. Saunders Company.
Knottenbelt, D.C. A Handbook of Equine Medicine for Final Year Students, University of Liverpool.
Merck & Co (2008) The Merck Veterinary Manual (Eighth Edition), Merial.
Pasquini, C, Pasquini, S, Woods, P (2005) Guide to Equine Clinics Volume 1: Equine Medicine (Third edition), SUDZ Publishing.
Reed, S.M, Bayly, W.M, Sellon, D.C. (2004) Equine Internal Medicine (Second Edition), Saunders.
Robinson, N.E., Sprayberry, K.A. (2009) Current Therapy in Equine Medicine (Sixth Edition), Saunders Elsevier.
Rose, R. J. and Hodgson, D. R. (2000) Manual of Equine Practice (Second Edition), Sauders.