Third-degree atrioventricular block

From Dog
3rd degree AV block. AV dissociation is present: there is no relation between p-waves and the (nodal) QRS complexes.[1]

Third-degree (complete) atrioventricular block is a heart disease of dogs characterized by a serious bundle branch block with ventricular rate slower than the atrial rate.

Unlike benign first-degree atrioventricular block, this condition can cause sudden death and is often associated with serious underlying heart pathology.

Causes include:

A breed predisposition has been noted in the Cocker Spaniel, Pug and Doberman.

Since there is no conduction between the atria and ventricle, ECG studies usually show a ventricular rate slower (< 40 beats per minute) than the atrial rate (> 60 beats per minute), equating to more P waves than QRS complexes and no relationship between the two. The P-P and R-R intervals are usually constant. The QRS complexes are usually wide and bizarre when the ventricle is supplying the pacemaker, and normal when the escape pacemaker is located in the lower AV junction (above the bifurcation of the bundle of His) in patients without bundle branch block.

Clinically affected dogs usually present with exercise intolerance, episodic weakness, syncope and occasionally with congestive heart failure.

This condition should be differentiated from second-degree atrioventricular block.

Treatment usually requires a temporary or permanent pacemaker, and drugs such as atenolol or propanolol are contraindicated.


  1. ECG-pedia
  2. Peddle GD et al (2008) Gerbode type defect and third degree atrioventricular block in association with bacterial endocarditis in a dog. J Vet Cardiol 10(2):133-139
  3. Jung S & Jandrey KE (2012) Hyperkalemia secondary to renal hypoperfusion in a dog with third-degree atrioventricular block. J Vet Emerg Crit Care (San Antonio) 22(4):483-487
  4. Stern JA et al (2012) Complete atrioventricular block secondary to cardiac lymphoma in a dog. J Vet Cardiol 14(4):537-539