Left Posterior Fascicular Block (LPFB)

Left Posterior Fascicular Block (LPFB)

In left posterior fascicular block (previously left posterior hemiblock), impulses are conducted to the left ventricle via the left anterior fascicle, which inserts into the upper, lateral wall of the left ventricle along its endocardial surface.

  • On reaching the ventricle, the initial electrical vector is therefore directed upwards and leftwards (as excitation spreads outwards from endocardium to epicardium), causing small R waves in the lateral leads (I and aVL) and small Q waves in the inferior leads (II, III and aVF).
  • The major wave of depolarisation then spreads along the free LV wall in a downward and rightward direction, producing large positive voltages (tall R waves) in the inferior leads and large negative voltages (deep S waves) in the lateral leads.
  • This process takes up to 20 milliseconds longer than simultaneous conduction via both fascicles, resulting in a slight widening of the QRS.
  • The impulse reaches the inferior leads later than normal, resulting in a increased R wave peak time (= the time from onset of the QRS to the peak of the R wave) in aVF.

ECG Criteria for Left Posterior Fascicular Block (LPFB)

  • Right axis deviation (RAD) (> +90 degrees)
  • Small R waves with deep S waves (= ‘rS complexes‘) in leads I and aVL
  • Small Q waves with tall R waves (= ‘qR complexes‘) in leads II, III and aVF
  • QRS duration normal or slightly prolonged (80-110ms)
  • Prolonged R wave peak time in aVF
  • Increased QRS voltage in the limb leads
  • No evidence of right ventricular hypertrophy
  • No evidence of any other cause for right axis deviation

Example ECG

Left Posterior Fascicular Block (LPFB)

Left Posterior Fascicular Block (LPFB) lqR rS complexesLeft Posterior Fascicular Block (LPFB) qR rS complexes
  • rS complexes in leads I and aVL
  • qR complexes in II, III and aVF

Left Posterior Fascicular Block (LPFB) Right axis deviation
Right Axis Deviation (RAD)
  • Leads II, III and aVF are POSITIVE;
  • Leads I and aVL are NEGATIVE

Prolonged R-wave peak time LPFB

Prolonged R-wave peak time

  • R-wave peak time: Time from onset of the QRS to the peak of the R wave in aVF > 45 ms

Handy Tips

  • LPFB is much less common than LAFB, as the broad bundle of fibres that comprise the left posterior fascicle are relatively resistant to damage when compared with the slim single tract that makes up the left anterior fascicle.
  • It is extremely rare to see LPFB in isolation. It usually occurs along with RBBB in the context of a bifascicular block.
  • Do not be tempted to diagnose LPFB until you have ruled out more significant causes of right axis deviation: Example: acute pulmonary embolus; tricyclic overdose; lateral STEMI; and right ventricular hypertrophy.

ECG Examples of LPFB

Example 1
Left Posterior Fascicular Block (LPFB) 2

Example 2
Left Posterior Fascicular Block (LPFB) 3

Related Topics

LITFL Further Reading

Advanced Reading




Emergency Medicine Specialist MBChB FRCEM FACEM. Medical Education, Cardiology and Web Based Resources | @jjlarkin78 | LinkedIn |

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