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 Atria are not contracting in a coordinated manner.

 It is relatively common in horses due to their large atria and high degree of vagal tone.There is a higher incidence in Standardbreds, Draft horses and Warmbloods.

 Paroxysmal atrial fibrillation has been reported in Thoroughbreds during racing, and sinus rhythm returns spontaneous within 72 hours. In most cases however, atrial fibrillation does not spontaneously convert to sinus rhythm.

 No “P” waves are present. Fibrillation (f) waves are evident and they may have a frequency up to 500 beats/min, but few are conducted through the AV node to the ventricle.The f wave morphology can be course or fine.

 The RR interval is irregularly irregular and QRS complexes are normal if there is no underlying disease.

 Cardiac function can be relatively normal at rest or during mild to moderate exercise if the finding is incidental.

 Decreased cardiac function occurs if there is associated heart disease or at high levels of performance.