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== Introduction ==

== Introduction ==

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

Atrial fibrillation is the commonest pathological dysrhythmia.  

'''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 Fibrillation (AF):''' 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 there is an irregular ventricular response. It can occur in absence of structural heart disease (primary/lone AF), or secondary to underlying cardiac disease. In animals with underlying heart disease, the ventricular response rate is usually elevated due to sympathetic predominance. In animals with primary 'lone' AF, ventricular response rate may be normal or only mildly elevated due to parasympathetic influence on the AV node. 

'''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 Flutter:''' is similar to atrial fibrillation, but has sudden onset and termination and is therefore a transient arrhythmia. It may be seen as a precursor to atrial fibrillation.  Ventricular response rate is variable, depending on the degree of physiological AV block.

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'''.  

Atrial fibrillation can occur in all species when there is '''atrial dilation''' secondary to other cardiac lesions.

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

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.

==Clinical Signs==

==Clinical Signs==