Decoding Axis Deviation: Unraveling RBBB vs. LPFB on the ECG

How to differentiate whether right axis deviation is due to right bundle branch block or left posterior fasicular block?

Differentiating between right axis deviation due to a right bundle branch block (RBBB) or a left posterior fascicular block (LPFB) can be challenging but can involve several key points:

QRS Duration: RBBB often causes a widened QRS complex (>0.12 seconds) while LPFB typically doesn't cause as much widening.

Presence of R/S Ratio in Lead I: In RBBB, Lead I often shows an R/S ratio less than 1, whereas LPFB displays an R/S ratio greater than 1.

QRS Morphology in V1 and V6: In RBBB, V1 often shows an rSR' pattern (bunny ears) and V6 shows a wide, slurred S wave. In LPFB, V1 can show a qR pattern and V6 often has a small r wave followed by a deep S wave.

Prolonged R-wave peak time in avF: Time from onset of the QRS to the peak of the R wave in aVF > 45 ms suggests LPFB.

Associated Findings: RBBB is frequently associated with ST-T changes and a slurred S wave in lateral leads, while LPFB may present with a prominent S wave in leads V1 and V2.

Interpreting the ECG comprehensively while considering these features in conjunction can aid in distinguishing between the two conditions causing right axis deviation. However, it might still be challenging, and consulting a cardiologist or electrophysiologist might be necessary for a precise diagnosis.

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