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Please answer the following 3 questions before beginning this case study. Answers will follow at the bottom of this blog post (Questions referenced from Page 14 of the Interactive 12-lead ECG Workbook).

Q1: True or False: The requirement |QRS| > or = 0.12 sec is common to both RBBB & LBBB?

A. True

B. False

Q2: In a case of RBBB (Right Bundle Branch Block), Lead I is ____ with a broad terminal S-wave.

A. Inverted

B. Upright

C. Flat

D. Biphasic

Q3: In an otherwise normal case of LBBB & RBBB, if the terminal portion of the QRS complex is positive, the T-wave will be inverted. This is described as ________ T-wave changes.

A. Primary

B. Secondary

C. Tertiary


We will analyze the following 12-lead ECG of a 65-year-old male who presents with chest pain and is cold, clammy & diaphoretic with an O2 saturation of 97%.

Interpretation @ End of Blog Post

This month’s blog post contains:

Examining RBBB From Lead V1

  1. Determine the time of the depolarization wavefront
  2. Determine the direction of the depolarization wavefront
  3. Make a judgement of the magnitude of the depolarization wavefront

These three determinations will predict the resultant waveform For Illustrations of the Conduction System of the RBBB (and LBBB):

Diagram #38: The Right Bundle Branch Block Electrical Waves from Lead V1

Please take note of the time sequence of depolarization of the ventricles as follows:

1st the Septum is depolarized.

2nd the Left Ventricle is depolarized.

3rd the Right Ventricle is depolarized.

Next notice the direction of these wavefronts for presentation of Depolarization Wave of Septum and the LV 

Examining RBBB from Lead I

Now let us develop the waveshape of Lead I associated with the RBBB.  Using the same approach as for Lead Vwe will conclude the following: (you are again reminded to concentrate on three aspects of the depolarization wavefronts: direction, timeframe and magnitude, from the perspective of Lead I).

1st ……The initial deflection will be downward- right ward ——- Septum 

nd …..The next deflection will be downward– left ward ——– Left Ventricle 

3rd ……The next deflection will be downward- right ward —- Right Ventricle 

This resultant waveform is shown schematically below.  The color coding is the same as for Lead V1.  Remember that the vector representation of Lead I is Left ward.  Therefore, a signal traveling left ward (the LV [green] in this situation) will result in a positive deflection on the ECG. 

Diagram #41: Predicted Biphasic Waveshape of Lead I in RBBB

Please observe that this QRS complex has a negative and a positive component, mathematically described as biphasic.  Regarding Lead I, our conclusion is as follows:   Lead I is biphasic with a broad terminal S-wave.

Triad for the Classic RBBB: 

1. QRS duration = 0.12 sec. (or exceeds)

2. Lead I biphasic with broad terminal S-Wave

3. Lead VV2 VRSR’ (sometimes only notch or slur)


Primary & Secondary T-wave Changes

If you find an LBBB or RBBB you must further analyze all the T-Waves to categorize them as either Primary or Secondary.

If Primary T-Waves are found, this is consistent with ischemia.

PEARL:  One of the greatest booby traps of electrocardiography is the LBBB.  The ST-Elevations, T-wave peaking and significant Q-waves are usually due to the distortion caused by the LBBB conduction i.e. these are usually pseudoinfarction patterns.  An exception to this conclusion holds if primary T-waves are present.  (When evaluating the ACS patient, look for the Modified Sgarbossa Criteria to diagnose the STEMI.)

NOTE:  Occasionally you will encounter a cardiogram that demonstrates a widened QRS > 0.10 seconds but otherwise does not meet the classic criteria of the LBBB or RBBB.  In order to acknowledge this finding we label the cardiogram as an IntraVentricular Conduction Delay (IVCD). This is a convenient label to demonstrate that the electrocardiographer recognized the widening of the QRS complex and also that it did not fit the classic criteria pattern of the RBBB or LBBB.

Sometimes it is appropriate to label the tracing as an atypical LBBB or atypical RBBB.


In the above 12-lead ECG, you will notice that Leads II, III & avF exhibit very subtle Primary T-wave changes (aka Concordance of the QRS & T). Although soft in presentation on this ECG, this patient was found to have elevated cardiac markers, was diagnosed with an ST Elevation MI (Inferior) and was sent to the nearest catheterization lab for treatment. As with all cases, it is recommended that in additional routine labs, serial ECGs, CXR and serial cardiac markers, comparison with an old ECG be done when possible as well as consultation with the cardiologist. (Case Presentation Referenced from Page 128 of the 12-lead ECG Workbook)

INTERPRETATION of Above Tracing:


Q1: True or False: The requirement |QRS| > or = 0.12 sec is common to both RBBB & LBBB?

A. True

B. False

Q2: In a case of RBBB (Right Bundle Branch Block), Lead I is ____ with a broad terminal S-wave.

A. Inverted

B. Upright

C. Flat

D. Biphasic

Q3: In an otherwise normal case of LBBB & RBBB, if the terminal portion of the QRS complex is positive, the T-wave will be inverted. This is described as ________ T-wave changes.

A. Primary

B. Secondary

C. Tertiary

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