DSAE STEMI Review, Mod 2: LBBB


Diagrammatic Illustration of LBBB

As before, it will be our goal to predict the waveshape of the select leads.  In this case we will again look at two leads:

  1. Lead V6
  2. Lead I

First, you will notice that the septum is no longer stimulated by the septal fascicle (since this relies on an intact left bundle), but relies on stimulation from the right bundle.  The septum is subsequently depolarized from right to left as shown in the figure above at time “t3“.

From the vantage point of electrode V6, it is clear that the first deflection is upward at time t3.  The second deflection turns downward at time t4R, and the third deflection turns upward at time t4R.  The resultant waveform in lead V6 is predicted as shown in the figure below.

Therefore, it is predicted that an RSR'(slur or notch) is found in lead V6.  Similarly as in the case of the RBBB, this RSR’ may be more apparent in lead V5 or even lead V4.  All three of these leads are predominantly the LEFT-most precordial leads and it is therefore easy to remember that the clue to the diagnosis of the LEFT Bundle Branch Block lies in the LEFT-most precordial leads:  V6, V5, V4.

Since the left ventricle is depolarized via the slow conduction pathway, it is again expected that the QRS duration will be prolonged.  More specifically, the duration of the QRS complex will be at least 0.12 sec….

|QRS| > 0.12 sec in the case of LBBB

To summarize, we have developed two criteria of the LBBB:

  1.  RSR’ in Lead V6, V5, V4
  2. |QRS| > 0.12 sec

Again, we find it useful to analyze another lead:  Lead I.

The predicted waveform of Lead I follows in the diagram below:

From this diagram above, you can make the following conclusions:

  1.  QRS is monophasic (all upright)
  2. QRS complex is notched or slurred (RSR’)

In summary, the diagnosis of LBBB is listed as follows …………………..


  1.  |QRS| ≥ 0.12 sec
  2. RSR’ (slur of notch) in lead V6, V5 or V4 (LEFT-most precordial leads)
  3. All upright Lead I, notched or slurred.

Again, it must be emphasized that all three criteria above must be met in order to make the diagnosis of classic LBBB with certainty.  Without all three criteria the diagnosis would be best described as suggestive of LBBB or atypical LBBB.  It is important to note that the QRS shape in Lead I and Lead V6 (V5, V4), often instead of taking an “M” or rabbit ear shape will instead only have a notch or even a slurring of the QRS complex.

The importance of Primary and Secondary T-wave changes of LBBB when searching for the STEMI…

In the normal uncomplicated LBBB (or RBBB), we have the so-called “secondary T-wave changes” which are expected.

SECONDARY T-wave CHANGES Definition:

The T-wave will be inverted if the terminal portion of the QRS complex is positive, and

the T-wave will be upright if the terminal portion of the QRS complex is negative.

If this rule is violated, then the changes are suggestive of myocardial ischemia and are called “primary T-wave” changes.

The point of this section is to make the provider aware that when you make the diagnosis of the LBBB (or RBBB), you must examine the T-wave in EVERY lead, and if there are “primary” changes in some of these leads, it must be suspected as myocardial ischemia until proven otherwise.  This is especially true if you compare an old ECG of the patient and it does not show “primary” T-wave changes.

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