Bundle Branch Blocks and Other Intraventricular Conduction Delays

This section will go over different types of intraventricular conduction delays, while focusing on bundle branch blocks.

Topics to be covered:

• An introduction on the anatomy of the His-Purkinje system, from the His bundle to the bundle branches to the post-divisional branches of the left bundle (i.e. the anterior and posterior fascicles). 

• Left bundle branch block (LBBB), or blocks in the left bundle branch before it splits into the anterior and posterior fascicles (i.e. "pre-divisional"). We will cover the pathophysiology behind this disease, common etiologies, as well as the ECG criteria required for diagnosis. We will cover the various degrees of LBBB (incomplete LBBB, intermittent LBBB, and complete LBBB), as well as atypical variants.

• We will cover post-divisional left bundle branch blocks, such as left anterior fascicular blocks and left posterior fascicular blocks. We will also cover the differential diagnoses for these findings.

• Right bundle branch block (RBBB). Again, we will cover the pathophysiology, common etiologies, as well as ECG criteria required for diagnosis. We will also cover the various degrees of RBBB, atypical variants, and differential diagnoses (such as Brugada syndrome, epsilon waves, preexcitation, and other non-specific intraventricular conduction delays).

• We will cover various manifestations of bilateral bundle branch blocks, which are conduction abnormalities affecting both bundle branches to some capacity, increasing the risk of complete infranodal AV block. This will include a discussion on bifascicular blocks and trifascicular blocks, in which a complete RBBB coexists with varying degrees of block in either the left anterior fascicle, left posterior fascicle, or both. We will also cover alternating bundle branch blocks, in which the block pattern switches from left to right bundle and vice-versa, and masquerading bundle branch blocks, in which there is the morphology of the block has the appearance of a RBBB in some leads while having the appearance of a LBBB in other leads.

• We will provide a discussion into functional bundle branch block, in which an otherwise healthy bundle branch may exhibit conduction delay due to changes in the heart rate. This will include an overview of the Ashman phenomenon, as well as mechanisms for tachycardia-dependent (phase 3) blocks and bradycardia-dependent (phase 4) blocks. We will also discuss mechanisms behind persistence of functional bundle branch blocks after correction of the heart rate.

• We will provide an overview of reasons why someone with a baseline bundle branch block may exhibit narrow QRS complexes, which would suggest paradoxical "improvement" of the block. This includes things like the gap phenomenon, supernormality, fusion PVC or preexcitation ipsilateral to the side of the blocked bundle branch, "balanced" bundle branch block, and so on.

• Finally, we will give an overview of non-specific intraventricular conduction delays, which happen when people have wide QRS complexes that don't fit the typical bundle branch block patterns. This can occur in the context of things like sluggish conduction due to toxic-metabolic effects, cardiomyopathy, previous cardiac surgeries, preexcitation, and so on.