Bundle branch block

From Dog

Bundle branch blocks are a relatively uncommon heart disease of dogs characterized by QRS abnormalities.

The cardiac conduction system of the dog heart is comprised of the sinoatrial node, the Bundle of His and the right and left bundle branches.

Disorders of this conduction system include right and left bundle branch block and occurs due to failure of the sino-atrial impulse to propagate through the His bundle or bundle branches. It usually results in electromechanical delay and consequent electric interventricular dyssynchrony[1][2].

In most dogs, bundle branch blocks are seen associated with organic disease of the His bundle or bundle branches due to sick sinus syndrome, an ischemic area, scar, inflammation, infection with necrosis, neoplasia or granuloma.

Causes include:

- hereditary predisposition in the English Bulldog[3] and Pug due to congenital stenosis of the Bundle of His[4]
- Tetralogy of Fallot[5]
- Arrhythmogenic right ventricular cardiomyopathy in Boxers[6]
- Ventricular septal defects[7]
- Trypanosoma cruzi[9]

Changes in the ECG can suggest enlargement or conduction disturbances of a particular chamber and mimic supraventricular tachycardia and ventricular tachycardia[10].

Widening of the P wave has been associated with left atrial enlargement, and the atrial repolarization is obscured or may be evident as a baseline shift in the opposite direction of the P wave.

If the QRS complex is wider than 0.12 seconds this is mostly caused by a delay in the conduction tissue of one of the bundle branches:

  • Left bundle branch block

The block can be intermittent or constant and the ECG shows the QRS complex as prolonged. Supraventricular [arrhythmia]]s and premature ventricular complexes may be evident in Boxer dogs[11].

  • Right bundle block
  • Intraventricular conduction delay

A right or left axis rotation can be caused by a:

  • Left anterior fascicular block
  • Left posterior fascicular block

Sometimes this conduction delay is rate-dependent: the bundle branch block occurs only at higher heart rates and disappears at slower heart rates.

The ECG shows right axis deviation and wide QRS complex, as well as large, wide S waves in leads I, II, II and aVF.

Most affected dogs in both right and left branch bundle block are asymptomatic unless existing heart disease is present or trifascicular block has occurred in which case, syncope may be evident[12]. In these patients, ventricular pacemakers may be required[13].

In most canine patients, the ECG findings are usually incidental, particularly under general anesthesia[14].

A differential diagnosis would include first-degree atrioventricular block.


  1. Russell K et al (2011) Mechanism of prolonged electromechanical delay in late activated myocardium during left bundle branch block. Am J Physiol Heart Circ Physiol 301(6):H2334-H2343
  2. van Deursen CJ et al (2012) Vectorcardiography as a tool for easy optimization of cardiac resynchronization therapy in canine left bundle branch block hearts. Circ Arrhythm Electrophysiol 5(3):544-552
  3. Santilli RA et al (2011) Ventricular tachycardia in English bulldogs with localised right ventricular outflow tract enlargement. J Small Anim Pract 52(11):574-580
  4. James TN et al (1975) De subitaneis mortibus. XV. Hereditary stenosis of the His bundle in Pug dogs. Circulation 52(6):1152-1160
  5. Chiu SN et al (2011) The role of mechanical-electrical interaction in ventricular arrhythmia: evidence from a novel animal model for repaired tetralogy of Fallot. Pediatr Res 70(3):247-252
  6. Hariu CD et al (2012) Arrhythmogenic right ventricular cardiomyopathy in boxers. Compend Contin Educ Vet 32(12):E1-E7
  7. Fujii Y et al (2004) Transcatheter closure of congenital ventricular septal defects in 3 dogs with a detachable coil. J Vet Intern Med 18(6):911-914
  8. Choi R et al (2009) Ebstein's anomaly with an atrial septal defect in a jindo dog. Can Vet J 50(4):405-410
  9. Rodríguez-Morales O et al (2012) Plasmid DNA immunization with Trypanosoma cruzi genes induces cardiac and clinical protection against Chagas disease in the canine model. Vet Res 43(1):79
  10. Santilli RA et al (2012) Orthodromic atrioventricular reciprocating tachycardia conducted with intraventricular conduction disturbance mimicking ventricular tachycardia in an English Bulldog. J Vet Cardiol 14(2):363-370
  11. Baumwart RD et al (2005) Clinical, echocardiographic, and electrocardiographic abnormalities in Boxers with cardiomyopathy and left ventricular systolic dysfunction: 48 cases (1985-2003). J Am Vet Med Assoc 226(7):1102-1104
  12. Baron Toaldo M et al (2011) ECG of the month. Trifascicular block causing syncope. J Am Vet Med Assoc 239(4):438-440
  13. van Deursen C et al (2009) Left ventricular endocardial pacing improves resynchronization therapy in canine left bundle-branch hearts. Circ Arrhythm Electrophysiol 2(5):580-587
  14. Tou SE et al (2011) ECG of the month. Intermittent tachycardia-dependent left bundle branch block in a dog during anesthesia. J Am Vet Med Assoc 239(1):55-57