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• Delayed depolarisation of ventricular myocardium in patients with LBBB with following

• QRS complex >0.12s

• broad QS or rS pattern in the right precordial leads (V1,V2,V3)

• Monophasic R wave in lateral lead , absence of q wave in lateral leads.

Pathophysiology

• In LBBB the ventricles are activated from right to left leading to absence of r wave in V₁ & q wave in V₆ .This results in negative QRS in right chest leads and a positive wave in left chest leads with wide QRS.

• With LBBB, the QS wave in lead V₁ sometimes shows a small notch at its point, producing W shape. Similarly, the broad R wave in lead V₆ may show a notching at its peak, giving it a distinctive M shape.

Clinical Significance

• Advanced coronary artery disease

• Valvular heart disease

• Hypertensive heart disease

• Cardiomyopathy

Note that ECG shows QS pattern in right chest leads with discordance.

LBBB & MI

Characteristically in,LBBB the affected leads also feature discordance of the ST segment /T wave complex . When major QRS vector is directed downward the ST segment will be elevated & T wave will be prominently positive. Similarly, if major QRS vector is directed upwards, ST segment will be depressed & T wave will be inverted. Lose of this pattern is a clue to Acute MI.

Sgarbossa's criteria for STEMI in the presence of LBBB

• ST segment elevation ≥1mm concordant with QRS complex (score 5)

• ST segment depression ≥1mm in leads V1,V2 or V3(Score 3)

• ST segment elevation ≥5mm discordant to QRS complex (Score 2)

Score of ≥3 points established the diagnosis of acute MI with 90% specificity.

RBBB

RBBB in ECG is characterized by

• Lead V₁ shows an rSR' pattern with a wide R' wave.

• Lead V₆ shows a qRS pattern with a wide S wave.

• It can be either complete (QRS >0.12 sec)or incomplete (<0.12 sec)

RBBB: ECG shows rSR' pattern over the right side chest leads.

Pathophysiology

• The interventricular septum is activated first . The left side is stimulated first . On a normal ECG it represented as r wave in V₁ & a small septal q wave in V₆.

• The second phase of ventricular stimulation is the simultaneous depolarisation of the left and right ventricles. This results in a deep S wave in V₁ & tall R wave in V_{6 .} In RBBB there is increased time for stimulation of right ventricle resulting in widening of QRS complex.

• The third phase is due to delayed depolarisation of the right ventricle. The electrical voltages in third phase are directed to right leading to a positive wide deflection R'. the rightward spread produces a negative wave in V6.

Clinical Significance

• ASD with right to left shunt

• COPD

• Pulmonary artery hypertension

• Pulmonary stenosis

• Pulmonary Embolism

Updated on 8/9/2015

Reference : Goldberger's Clinical ECG