Supraventricular arrhythmia characterized by very rapid and chaotic, self-sustaining atrial electrical activity with an irregularly irregular ventricular response.
Species, Age, Sex
• Underlying cardiovascular pathology leading to:
• Frequent occurrence of atrial premature beats (eg, caused by electrolyte disturbances, structural lesions, viral disease, myocardial stretch [exercise])
• High vagal tone (leads to dispersion in refractoriness of the atrial myocardium)
History, Chief Complaint
• Obvious signs of performance loss in horses working at maximal speed: Pulling up during race with signs of respiratory distress, ataxia, or even collapse
• Less pronounced signs of decreased performance in jumping and dressage horses or horses working at lower levels
• May be an incidental finding in pleasure horses, breeding horses, or horses at rest
• Signs of heart failure in horses with severe underlying cardiac disease and secondary AF
Physical Exam Findings
• Irregular pulse with pulse deficits
• Signs of underlying cardiac disease
Etiology and Pathophysiology
• AF may be strongly suspected on auscultation: an irregularly irregular rhythm, a loud first heart sound, and absence of the atrial sound.
• Final diagnosis is made with an electrocardiogram (ECG) by the typical characteristics (Figures 1 and 2):
• Assess for signs of underlying cardiac disease such as a cardiac murmur, ventral edema, jugular pulsations, and tachycardia.
FIGURE 1 Atrial fibrillation (AF). During AF, P waves are absent and replaced by (coarse and fine) fibrillation waves (f waves). QRS complexes have a normal morphology, but RR intervals are irregularly irregular.