Atrial Fibrillation & Atrial Flutter

Introduction

Atrial fibrillation is the commonest 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 disorganised depolarisation waves are conducted through the AV node reaching the ventricles. As a result the ventricles have a rapid, irregular rate. It can be a primary disease, or secondary to underlying cardiac disease.

Atrial Flutter: is similar to atrial fibrillation, but the atrial rate is generally slower and is characterised by saw-toother flutter waves in the ECG baseline. The ventricular response is generally rapid but may be regular or irregular.

Atrial fibrillation can occur in all species where there is atrial hypertrophy secondary to other cardiac lesions. In horses it may be detected unexpectedly during an examination of an animal with no history of heart disease. The atrial contraction only contributes to around 15% of the ventricular filling, so signs of atrial fibrillation are only seen during vigorous exercise. In cattle it occurs most commonly in animals with abnormalities of the gastrointestinal tract. In small animals, it occurs most commonly with dilated cardiomyopathy in large breed dogs.

It can also occur in rabbits secondary to severe heart disease.

Clinical Signs

Horses

This may be an incidental finding on clinical examination, especially if the horse is not used for highly athletic activities. If the horse is raced, hunted or an eventer, 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 pulmonary haemorrhage, so this can be a clinical sign of the condition.

Cattle

Signs generally relate to the underlying disease process, which in cows can range from gastrointestinal diseases such as a left-displaced abomasum, to uterine torsion.

Small Animals

Patients with primary atrial fibrillation are usually asymptomatic. However, there may be signs of haemodynamic compromise due to the rapid heart rate and a loss of atrial contribution to ventricular filling; which would usually account for up to 20% of cardiac output.

Other signs are generally related to the underlying disease process or congestive heart failure. There may be a history of coughing, dyspnoea, tachypnoea, exercise intolerance, episodes of syncope.

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 variable pulse quality and a variable intensity of heart sounds.
An electrocargiogram (ECG) can be performed for a definitive diagnosis of atrial fibrillation. It will show a lack of P waves and instead 'F waves'. The ventricular rate is high, the interval between QRS complexes is irregular but the QRS complexes themselves appear normal.

Atrial flutter on ECG has the following features: the atrial rhythm is regular, at approximately 300-400 beats per minute. P waves are usually discrete P waves or a saw-toothed baseline. Ventricular rhythm and rate generally depend on the atrial rate and AV nodal conduction, but are generally regularly irregular and rapid. It can sometimes mimic atrial fibrillation.

Echocardiography and radiography may help characterise the type and severity of the heart disease, if present. Mild left atrial enlargement may accompany the haemodynamic alterations imposed by the arrythmia.

Treatment

Horse

In horses where the condition is found without other concurrent heart disease, treatment is with the drug quinidine sulphate. The necessity for this depends on the requirement of the horse to perform work, as horses can be retired or used as broodmares and can live a normal life with the condition.

Quinidine sulfate 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.

There is a greater success with conversion in young horses and when conversion is attempted shortly following the onset of the arrythmia. If the arrythmia has been present for more than 4 months, therapeutic success is much less common and there is a higher recurrence rate.

Horses can also develop atrial fibrillation secondary to cardiac disease, such as mitral valve insufficiency, tricuspid valve insufficiency, or any acquired or congenital disease leading to atrial hypertrophy. Horses will usually develop congestive heart failure and have a resting tachycardia. These underlying conditions should be diagnosed and the congestive heart failure treated with diuretics and inotropes. These horses will have a poor prognosis for return to function and treatment is mainly to slow progression of disease.

Cattle

Cattle are not usually treated with an antiarrythmic drug as the heart will revert to sinus rhythm following the correction of the underlying abdominal disorder.

Small Animals

In dogs, digoxin, is usually the first line of therapy in slowing conduction through the AV node. The goal is to keep the heart rate between 140 and 160 beats per minute. If the heart rate remains high, a calcium channel blocker such as diltiazem or a beta-blocker should be used as well.

High-dose oral quinidine or electrical cardioversion can be used in primary atrial fibrillation cases to convert the heart back to sinus rhythm. This requires experienced personnel, general anaesthesia and special equipment.

In cats, diltiazem or atenolol are usually the drugs of choice. Digoxin can also be added if the heart rate is still high or there is evidence of heart failure.

In rabbits, digoxin has been used anecdotally to slow the heart rate.

In all cases with concurrent severe heart disease and possibly congestive heart failure, efforts to control the disease by using diuretics such as frusemide can be helpful in improving clinical signs. Calcium channel blockers and beta-blockers, both negative inotropes, should be used carefully in animals with myocardial failure.

Prognosis

In horses, the likelihood of the rhythm 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. Prognosis is poor if atrial fibrillation is caused by an underlying heart condition.

In cattle, prognosis depends on the underlying gastrointestinal condition and the treatment options chosen by the farmer.

In small animals, cases with primary atrial fibrillation and normal ultrasound findings have a good prognosis. Animals with secondary atrial fibrillation associated with severe heart disease have a guarded to poor prognosis.

Atrial Fibrillation & Atrial Flutter Learning Resources
Vetstream To reach the Vetstream content, please select	Canis, Felis, Lapis or Equis
Flashcards Test your knowledge using flashcard type questions	Rabbit Medicine and Surgery Q&A 02 Equine Internal Medicine Q&A 12

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), Saunders.
Mazzaferro, E. (2011) Blackwell's five minute veterinary consult clinical companion: ECC Wiley-Blackwell

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