Left Bundle Branch Block

A left bundle branch block is produced by a slower left bundle. As with RBBB, the QRS is prolonged (>120ms); however, it not only changes the end of the QRS but the beginning as well. Normally, septal depolarization from left to right causes small Q waves in the lateral leads; however, in LBBB, the septal depolarization is reversed and there are no Q waves in the lateral leads. Additionally, because depolarization occurs primarily from right to left, the lateral leads are primarily a large R wave and the rightward leads are predominantly a large and broad S wave. An incomplete LBBB will cause the above changes, but the QRS will be between 100-120ms. In patients with CHF, wider QRS complexes are associated with poorer ejection fractions (and are an indication for cardiac resynchronization therapy).

These changes cause loss of the normal R wave progression and also tend to cause left axis deviation. Similarly to RBBB, discordant repolarization changes are also produced. This will manifest as ST elevation in the rightward leads and ST depression in the leftward leads.

Examples

LBBB and Ischemia

A new LBBB in the setting of chest pain used to be considered a 'STEMI equivalent'. Although it may still be considered one, it is almost never an OMI without meeting Sgarbossa or Smith-modified Sgarbossa criteria (<5%). Patients meet OMI criteria with one of these criteria in only 1 lead. The summation of these criteria is determining if the ST changes are concordant or excessively discordant as non-excessively discordant ST changes are expected with a LBBB. Concordance is when the ST segment is going in the same direction as the majority of the QRS complex; discordance is the opposite. The original Sgarbossa criteria had 3 elements:

concordant ST elevation >1mm in leads with a positive QRS complex
concordant ST depression >1mm in V1-V3 (leads with a negative QRS complex)
excessively discordant ST elevation >5mm in leads with a negative QRS complex

The specificity of these decreases when going down the above numbers (excessive discordance had the lowest specificity).

The Smith-modified Sgarbossa criteria altered the definition of excessive discordance, changing it to amplitude of ST elevation being >25% of the amplitude of the S wave. This was changed as the ST deviations will have greater amplitude when associated with QRS complexes of greater amplitude. Therefore, defining excessive discordance by a pure measurement cutoff will not take that into effective and provide false positives for patients with large QRS amplitudes (because of associated large ST elevation that is actually appropriately discordant). The modified version takes that into account and, therefore, improves specificity. Additionally, it improves sensitivity for those leads with very small QRS amplitudes as the ST deviation may not be expected to exceed 5mm. Overall, the Smith-modified Sgarbossa criteria are around 75% sensitive (similar to that of EKG without LBBB) and 99% specificity. Although they have been developed for LBBB, they can be applied to any wide complex rhythm (including PVC's).

Ischemia Examples

Concordant ST elevations in the inferior leads

Inferior OMI meeting excessive discordance criteria (by Smith-modified, not original Sgarbossa, criteria)