Bundle branch block

If the depolarisation wave reaches the IV septum normally, then the PR interval will be normal. However, if there is abnormal conduction through either the right or left bundle branches (‘bundle branch block’) there will be a delay in the depolarisation of part of the ventricular muscle. The extra time taken for depolarisation of the whole of the ventricular muscle causes a widening of the QRS complex. Block of both bundle branches causes 3^rd degree heart block.

Bundle branch block on an ECG:

• Will be wide QRS complexes (>0.12s)
• Look in leads V₁ and V₆
• WilliaM (W in V₁ and M in V₆) is LBBB
• MarroW (M in V₁ and W in V₆) is RBBB

Causes of RBBB:

Congenital:

• ASD
• VSD

Pulmonary disease:

• Cor pulmonale
• PE

Myocardial disease:

• Acute MI
• Cardiomyopathy

Drugs/electrolytes:

• Hyperkalaemia
• Class Ia antiarrhythimics

Causes of LBBB:

LV outflow obstruction:

• Aortic stenosis
• HT

Coronary artery disease:

• Acute MI
• Severe coronary artery disease

Cardiomyopathy