Heart: atrial fibrillation


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  • Cause : primary in large-breed or giant-breed dogs. Secondary to atrial enlargement in small and medium-sized breeds.
  • Signs : may be non-specific or signs of underlying pathology.
  • Diagnosis : auscultation, electrocardiography.
  • Treatment : conversion back to sinus rhythm (primary AF) or ventricular rate control (secondary AF).
  • Prognosis : depends on cause - primary AF has good prognosis, secondary AF has poor prognosis.
    Print off the owner factsheet on ( Heart rhythm disturbance atrial fibrillation) to give to your client.


Clinical signs

  • Auscultation of the heart gives a typical sound - chaotic heart rhythm, sometimes described as 'tennis shoes in the dryer'. May be other auscultation findings, eg murmur or respiratory crackles.
  • Tachycardia (heart rate often >180 BPM), if secondary to heart disease.
    Large and giant breed dogs with primary atrial fibrillation usually have ventricular rate within normal range (80-100 BPM).
  • Marked pulse deficit or variable pulse intensity if tachycardic.
  • Signs associated with underlying cardiac disease:
    • Pulmonary edema.
    • Ascites.
  • Pallor due to poor peripheral perfusion.

Differential diagnosis

  • Atrial premature contractions (APCs) .
  • Supraventricular tachycardia with AV block.



  • Primary AF: may be good if ventricular rate is not too fast initially or can be controlled with mediciation. Dogs with primary AF may remain asymptomatic for many years after diagnosis.
  • Secondary AF: prognosis is often poor due to severe nature of underlying heart disease. In general, presence of AF is a negative prognostic sign in dogs with mitral valve disese or dilated cardiomyopathy.

Expected response to treatment

  • Slower ventricular heart rate.
  • It is rare for atrial fibrillation to convert to normal sinus rhythm but ventricular rate on ECG should slow.
  • Improved exercise tolerance/demeanor.
  • Improved ability to treat congestive heart failure .

Reasons for treatment failure

  • Incorrect diagnosis.
  • Severe underlying pathology.
  • Inadequate doses of drug given or unacceptable side-effects associated with medication.
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