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Heart: right and left bundle branch block

ISSN 2398-2950

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  • The right and left bundle branches arise as bifurcations from the bundle of His, just below the level of the atrioventricular node. 
  • Right bundle branch block (RBBB) results from delayed or inhibited conduction along the right bundle branch or any of its more distal main ramifications. 
  • Left bundle branch block (LBBB) results from similar abnormalities to the main left bundle or its more distal components. 
  • Bundle branch blocks slow the conduction within the areas of the ventricular myocardium they supply and give rise to specific electrocardiographic findings. 
  • Because relative to the right bundle branch, the left bundle branch is better "developed" (ie it is thicker, ramifies more, and therefore conducts faster), it is less likely to be blocked when no organic heart disease is present. Bundle branch blocks, therefore, can be either a sign of significant underlying heart disease (LBBB) or found as an incidental finding (RBBB), potentially resulting from transient, self-limiting "fatigue" with no organic right-sided heart disease.

ECG characteristics

  • RBBB is characterized by deep, wide, and often notched S waves in ECG leads with a caudal positive electrode. 
  • The QRS complex will therefore be predominantly negative in leads I, II, III, and aVF.  For similar reasons, LBBB is characterized by wide and positive QRS complexes in those same leads (I, II, III, and aVF). The QRS complex is widened in both RBBB and LBBB due to the slow conduction in the affected side of the ventricular myocardium. 
  • The QRS duration is often >0.08 sec (4 squares when the paper speed is 50 mm/sec). Both LBBB and left ventricular hypertrophy can result in prolonged QRS duration; however, the QRS duration resultant to myocardial hypertrophy typically remains within 0.06-0.08 sec. 
  • The wide and bizarre appearance of the QRS complexes can be confused with ventricular tachycardia Ventricular tachycardia, however, both RBBB and LBBB are forms of sinus rhythm (ie the sinus node is still controlling the heart rate), and therefore typically exhibit a P wave in front of each and every wide and bizarre QRS, with a constant (albeit not necessarily normal in duration) PR interval. 
  • To avoid misdiagnosis, it is important for clinicians to recognize the consistent P wave that is present for each of the QRS complexes when LBBB or RBBB is present.

Clinical significance

  • The clinical significance of the two types of bundle branch blocks is markedly different. 
  • RBBB is generally regarded as a benign finding. Congenital RBBB has been reported and does not appear to significantly affect cardiac function or longevity. 
  • The superficial location of the right bundle within the right heart is thought to predispose the bundle to various injuries. For instance, following right heart catheterization for angiograms or balloon valvuloplasty, it is not uncommon for patients to develop RBBB following such invasive interventions. In these cases too, RBBB may spontaneously resolve over time. In general, the author considers RBBB to be of low clinical significance. 
  • In contrast, LBBB is typically seen in cats with severe underlying heart disease (ie dilated cardiomyopathy Heart: dilated cardiomyopathy (DCM) or mitral valve disease). Most cats with LBBB will have a heart murmur Cardiac sounds: overview, gallop, or other physical examination findings consistent with heart disease. The ECG diagnosis ECG: overview of LBBB should prompt further work-up including chest radiographs Radiography: thorax and echocardiography Ultrasonography: heart.


  • No specific treatment is indicated for either RBBB or LBBB, neither is it available. 
  • RBBB is typically a benign finding. 
  • LBBB is not treated but serves as an indicator of significant underlying heart disease.

Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Rausch W P, Keene B K (2002) ECG of the month. Sinus tachycardia with left bundle branch block. JAVMA 220 (7), 980-981 PubMed.

Other sources of information

  • Goodwin J K (2001) Electrocardiography. In: Manual of Canine and Feline Cardiology. 3rd edn. Tilley L P, Goodwin J K (eds), pp 43-70.