Canis ISSN: 2398-2942

Heart: 3rd degree atrioventricular block

Synonym(s): Complete heart block

Contributor(s): Barrett Bulmer, Mark Rishniw, Simon Swift


  • Cause: complete failure of conduction from the atria to the ventricles.
  • Signs: causes weakness, syncope and (rarely), congestive heart failure.
  • Diagnosis: electrocardiograph shows no relationship of P waves to QRS complexes. Ventricles develop their own pacemaker independent of the atrial pacemaker.
  • Treatment: medical treatment usually ineffective. Requires pacemaker implantation.
  • Prognosis: guarded with medical therapy. Can be good following successful pacemaker implantation.



Predisposing factors

  • Exercise or excitement can precipitate syncope or sudden death.


  • Lesion in conducting pathways prevents transmission of the impulse from the pacemaker region in the atria to the ventricles. Hence the atria beat at their own (usually rapid) rate, and the ventricles beat at an independent slower rate.
  • The lesion can occur in the AV node or junction, bundle of HIS, or as a progression of bundle branch block when all three are affected.
  • Ventricular pacemaker can arise from the lower AV node or bundles, producing a junctional escape with normal QRS complexes at approximately 40-60 per min. Alternatively, the Purkinje cells may form the pacemaker, producing ventricular escape complexes with abnormal QRS complexes at a rate of 15-40 per min.
  • Sudden death or syncope may result from sudden asystole, or the development of tachyarrhythmias leading to circulatory arrest (rare).
  • Atrial contraction on a closed AV valve cause cannon 'a' jugular waves.


  • Without treatment clinical signs persist and sudden death or a decision for euthanasia may occur.
  • A recent report identifed 12 out of 63 dogs with high grade second degree or third degree AV block that did not receive pacemakers, died suddenly within the first 30 days of diagnosis.
  • Following pacemaker implantation the clinical signs usually resolve. The survival time is variable but often dogs can live a normal life span.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Bulmer B J, Sisson D D, Oyama M A et al (2006) Physiologic VDD versus non-physiologic VVI pacing in canine third degree atrioventricular block. J Vet Intern Med 20, 257-271 PubMed.
  • Schrope D P, Kelch W J (2006) Signalment, clinical signs, and prognostic indicators associated with high-grade second- or third-degree atrioventricular block in dogs: 124 cases (January 1, 1997-December 31, 1997). JAVMA 228, 1710-1717 PubMed.
  • Wess G, Thomas W P, Berger D M et al (2006) Applications, and outcomes of transvenous pacemaker implantation in 105 dogs (1997-2002). J Vet Intern Med 20 (4), 877-884 PubMed.
  • Oyama M A, Sisson D D, Lehmkuhl L B (2001) Practices and outcome of artificial cardiac pacing in 154 dogs. J Vet Intern Med 15, 229-239 PubMed.

Other sources of information

  • Ettinger S (2005)Electrocardiography and Cardiac Arrhythmias.In:Textbook of Veterinary Internal Medicine.6th edn. Eds Ettinger & Feldman. pp 1065-1066 and 1075-1076.
  • Martin M (1998)Arrhythmias.In:Manual of Small Animal Cardiorespiratory Medicine and Surgery.Eds V Luis-Fuentes & S Swift. CheltenhamBSAVA.
  • Darke P G Get al(1996)Color Atlas of Veterinary Cardiology.pp 154-159.
  • Tilley L P (1992)Essentials of Canine and Feline Electrocardiography.3rd edn. pp 175-178.