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Left Bundle Branch Block

caution
Variant

The same pattern, QRS 110 to 119 ms

HR75bpm
RR800msQT366ms
Draws the normal ECG in grey behind every lead. Shortcut: N.

Incomplete left bundle branch block

The same activation as the complete block in a shorter QRS: the left bundle still conducts, only slowly, so the septum is still reached from the right and the left ventricle late, in a QRS of 110 to 119 ms.

On the trace

Try each step, then check it.
  1. Measure the QRS.
    Show 114 ms: wide, short of the 120 ms of a complete block.
  2. Look at V1 and V2.
    Show Still an entirely negative QS, 15.3 mm deep in V1: the septum is still activated from the right.
  3. Look at I, V5 and V6.
    Show A broad R with no septal q in front of it, as in the complete block.
  4. Check the R-peak time on the left.
    Show 72 ms in V5 and V6, still over 60 ms: the left ventricle is reached late.
  5. Look at the ST and T.
    Show Still turned away from the QRS, but milder: ST +0.6 mm in V1, T inverted 1.8 mm in V6.

Telling it apart

Incomplete LBBBComplete LBBB
QRS110 to 119 ms (114 here)120 ms or more (143)
Septal q in I, V5, V6AbsentAbsent
R-peak time in V5 and V6Over 60 ms (72 here)Over 60 ms (90)
ST-T discordanceMildMarked
Reading the ST for ischaemiaThe ordinary STEMI thresholdsSgarbossa and Smith

What it means

Incomplete LBBB is usually a sign of disease rather than a variant: it often goes with left ventricular hypertrophy, and it can be the stage before a complete block. The Sgarbossa criteria were built for the complete block, so here the lead panel keeps the ordinary STEMI thresholds.

Full articleLeft Bundle Branch BlockHow to recognise it, why it happens, what it means and what to do.