Canis ISSN: 2398-2942

Heart: atrial fibrillation

Synonym(s): AF

Contributor(s): Mark Oyama, Mark Rishniw


  • 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) Heart rhythm disturbance (atrial fibrillation) to give to your client.



  • Primary - no known cause. Seen in large and giant breeds, eg Irish Wolfhound Irish Wolfhound , Great Dane Great Dane , Bull Mastiff Bullmastiff , and may be partly due to the large size of the atria and high resting parasympathetic tone.
  • Secondary - atrial enlargement (usually left) secondary to cardiac disease such as degenerative mitral valve disease.
  • Cardiac neoplasia Pericardium: neoplasia (heartbase tumor) or mass lesions (rarely).
  • Digoxin toxicity Digoxin toxicity.


Atrial stretching
  • Possibly associated with early dilated cardiomyopathy in giant breeds.


  • Unorganized and chaotic atrial depolarizations with no coordinated atrial muscle contraction. Variable and random conduction of atrial depolarizations to the ventricles results in an irregular occurrence of ventricular depolarizations. The ventricular heart rate depends on the number of impulses that are conducted through the AV node.
  • Atrial depolarization rapid and disorganized (~600 depolarizations/min).
  • Every electrical impulse in the atria cannot be conducted through the AV node. This 'AV block' limits the number of atrial impulses reaching the ventricles.
  • Degree of vagal tone and sympathetic tone determines AV conduction - high vagal tone, low sympathetic tone produces a relatively slow ventricular rate; low vagal tone and high sympathetic tone in secondary AF results in a high ventricular rate.
  • With high ventricular rates, alterations in cardiac output from beat to beat → variable pulse intensity and occasional absent pulses → pulse deficit.
  • Loss of atrial contraction reduces cardiac output.
  • With high ventricular rates → poor filling and contraction → reduced cardiac output.
  • Persistently high ventricular rates can lead to loss of ventricular contractility → further reduction of cardiac output.


  • May be acute, eg develops during anesthesia or associated with toxemic episode Shock: septic.
  • Most secondary AF is chronic.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Gelzer A R & Kraus M S (2004) Management of atrial fibrillation. Vet Clin North Am Small Anim Pract 34 (5), 1127-1144 PubMed.
  • Calvert C A, Jacobs G J & Brown J (1997) Signalment, survival and prognostic factors in Doberman Pinschers with end-stage cardiomyopathy. JVIM 11 (6), 323-326 PubMed.
  • Tidholm A & Jonsson L (1997) A retrospective study of canine dilated cardiomyopathy (189 cases). JAAHA 33 (6), 544-550 PubMed.
  • Manohar M & Smetzer D L (1992) Atrial fibrillation. Comp Cont Ed Pract Vet 14 (10), 1327-1333 VetMedResource.