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Monday, 24 September 2012

LBBB full discussion

#ECGclass case 11

Below is the ECG of a 62yrs lady who complains of dizzy spells and palpitations.
Examination is entirely normal.
She is not on any medication and has no significant past medical history.

What does this ECG show?

Answer: Left Bundle Branch Block.

Note the widened QRS complex. 

P-waves are present (agree, not clearly seen, because of baseline interference). 
P-waves are usually seen best in V1, so if you are unsure, always check this lead closely. (That said, in the ECG above I think they are clearest in V3 and V4).

Note how all the ST segments, are deflected in an opposite direction the the associated QRS. This is one of the criteria for LBBB.

LBBB makes any further interpretation of the ECG impossible.

This is quite possibly incidental to her symptoms. It would be helpful to look back through her notes to see if any older ECG's confirm whether or not this is long standing LBBB. 
If it is a recent change, then investigations for a possible underlying cause may be necessary. 

Causes of LBBB:
  • Coronary and Hypertensive Heart Disease
  • Aortic Stenosis
  • Idiopathic degenerative disease
  • Congestive Heart Failure
  • Pericarditis or Myocarditis
  • Cardiomyopathy and Myocarditis
  • Cardiac Surgery
  • Anteroseptal MI
  • Syphilitic, Rheumatic and congenital disease
  • Cardiac Tumours


Normal Conduction pathways

Assuming the conduction impulse starts as normal in the Sino-atrial node, and travels through the atria to the Atrio-Ventricular node, it should arrived safely at the Bundle of His.  From there, it should travel down the Left and Right Bundle Branches and into the Purkinje fibres within the ventricles, without delay.
Any delay in conduction, in either the Left or Right Bundle Branch, will give rise to a widened QRS complex. (Normal width is 1.5 to 2.5 small squares, or 0.06 - 0.11 seconds).  
This is then known as Left Bundle Branch Block (LBBB), or Right Bundle Branch block (RBBB). A QRS complex of 3 small squares, or more, is considered broad.

In any Bundle branch block, the P waves and PR intervals should remain normal, and constant.
(I say this because another cause of QRS widening, is if the depolarisation occurs within the ventricular muscle itself. In these circumstances, a P-wave would not be present. Sometimes called a 'ventricular escape rhythm').

If LBBB and RBBB occur together, this is a complete heart block (same outcome as when the block is within the Bundle of His itself).

Any block/delay in the Left Bundle Branch is seen most clearly in the leads that face the left ventricle. Those leads are V5 and V6 and lead I.

Defintion of LBBB:
  • QRS >/= 120ms
  • No secondary R wave in lead v1                                
  • No Q waves in the lateral leads (I, aVL and v5+6)
  • The ST changes are deflected opposite (discordant) to the dominant QRS complex. 
  • The T wave changes are in the same direction as the ST segments (ie. Opposite to QRS)
  • ST elevation in leads v1-4
  • T wave inversion in leads I and aVL
Other criteria of LBBB:
  • Rhythm must be supraventricular (i.e. conduction arises from atria: P-waves seen, and normal)
  • RsR waveform should be present in Lead I.
  • RsR waveform is also seen in V6.  (Coupled with a QS or rS in V1 this gives rise to the "W-L-M" waveform formation)

RBBB - I will cover RBBB in more detail another time. But for now, just remember that the QRS will once be again be wide. As the leads looking directly at the Right Bundle branch, are leads V1 and V2, it is in these leads that the RsR pattern will be most prominent. i.e. "M -(R) -W" formation.
RBBB makes further ECG interpretation difficult, but as the Right bundle branch is supplying a smaller muscle mass than the left, the ECG changes are less pronounced than with LBBB. 
LBBB electrical changes are so profound, that any further interpretation of the ECG is impossible.

LBBB and chest pain

So what about LBBB and ACS / STEMI's?

Given the abnormal appearance of the ST segments in LBBB (elevated in V1-4), What happens if this patient presents with chest pain?

Imagine this lady returns to see you in an urgent appointment the following month. She is complaining of prolonged chest pain at rest, for several hours. She has no past history of, or risk factors for, IHD, and her symptoms sound very much like acid reflux. Examination is normal and she looks clinically stable.

If this lady presents with chest pain, how will you know if the ST elevation is significant? 

This is a difficult problem.  LBBB makes any ST segment analysis very tricky!

NICE Chest pain of recent onset guidelines (2010) state:

"Follow local protocols for people with a resting 12-lead ECG showing regional ST-segment elevation, or presumed new onset LBBB consistent with acute STEMI, until a firm diagnosis is made"

This seems very sensible! In view of this we may not have to deal with this dilemma in GP-land....unless of course we are in a very rural location, with secondary care services a long ambulance journey away. 

So for the benefit of rural GP's and Paramedics, we have the Scarbossa criteria to assess likelihood of STEMI. Scores are added up to a mximum of 10. where Scarbossa score = 10, the patient has 100% liklihood they are having an MI.

Chest Pain Assessment in LBBB using the Scarbossa Score

In Summary, scores  are attributed as follows:

In the ECG example above, her Scarbossa score = 0, making the likelihood of this being an MI very low (16%)....but not impossible!

Below are examples of a Scarbossa score 7,  and a Scarbossa score 10 .

Sacrbossa Score = 7
Probability of MI= 96%

Scarbossa Score = 10
Probability of MI = 100%

Hopefully, you can see how the above two ECG's with LBBB, vary for the example in case 11.
This is difficult, and advanced stuff. 
Unless you are very satisfied clinically, that this is not ischaemic pain, the err on the side of caution, and follow standard chest pain pathways. 

Tah Dah! 
There you go. All you ever need to know, and more, about LBBB.  
Thank you. :-)

1 comment:

  1. Thank you. Clear explanation with examples. I wish I have found this blog earlier...