Fascicular Blocks

The left bundle branch is divided into the left anterior fascicle and the left posterior fascicle. It is important to note their anatomy to understand the EKG changes. The left posterior fascicle travels down the infero-septal wall. The left anterior fascicle is found more leftward than the left posterior fascicle. Diagnosing left anterior or posterior fascicular blocks requires one to look at the limb leads. Because there is a block, the QRS interval becomes prolonged, though not to the degree of a full bundle branch block (usually between 100-120ms).

Left Anterior Fascicular Block (LAFB)

The electrical activity in a left anterior fascicular block (aka left anterior hemiblock) will begin in the right bundle branch and left posterior fascicle, producing initial downward and rightward depolarization, followed by leftward depolarization. This is more common than a left posterior fascicular block as the left anterior fascicle is a small, single strand and the posterior fascicle is a broad bundle of fibers (and, therefore, more resistant to damage). The initial rightward depolarization will produce small A waves in the leftward leads (I, aVL) and small R waves in the inferior leads. As the depolarization becomes more leftward and upward, this produces large R waves in the leftward leads and large S waves in the rightward leads. In other words, the leftward leads will have a qR pattern and the inferior leads will have a rS pattern.

It is important to remember that the left ventricle has a larger area, so will have greater amplitude effects (hence why the R>Q in leftward leads). Additionally, it causes left axis deviation. Furthermore, it tends to cause increased voltage in the limb leads. This may mean that the patient meets criteria for LVH in aVL/I by the R wave >11mm, but there will be no LV strain pattern. Furthermore, it also causes a late transition in the precordial leads, mimicking poor R wave progression.

Left Posterior Fascicular Block (LPFB)

Left posterior fascicular block occurs much less commonly than LAFB as it is a larger fascicle with dual blood supply. If this occurs in isolation, this is likely indicative of multivessel CAD and a poor prognosis. The opposite EKG changes occurs in a LPFB. Because the initial impulse travels down the left anterior fascicle, the EKG will show small R waves in the lateral leads and small Q waves in the inferior leads initially. When the impulse then travels rightward and downward, tall S waves are produced in the lateral leads and tall R waves are produced in the inferior leads. In other words, the inferior leads will have a qR pattern and the lateral leads will have a rS pattern.

LPFB also causes right axis deviation (RAD); however, one must rule out other causes of RAD to diagnose a LPFB (ie RVH). Again, the limb lead voltages may be increased in the inferior leads.

Bifascicular Block

A bifascicular block is the combination of a RBBB and either a LAFB or LPFB. Note that a combination of LAFB and LPFB is...LBBB. Therefore, conduction to the ventricles is via the single remaining fascicle. Although this sounds concerning, the risk of degeneration to a complete heart block is 1% per year. The EKG will show RBBB plus either right or left axis deviation (for LPFB or LAFB, respectively). A RBBB and LAFB is the most common (for the aforementioned reason). As mentioned in the OMI section, a new bifascicular block in the setting of acute OMI indicates a high mortality rate and proximal LAD occlusion.

Bifascicular block. There is a RBBB and left axis deviation (LAFB)

Trifascicular Block

A trifascicular block produces a complete heart block and is a progression of a bifascicular block. It is noted by complete heart block with bifascicular block features. Incomplete or impending trifascicular block is a bifascicular block plus either 1* or 2* AV block. Although this sounds bad (impending), the risk of progression is quite low acutely. Sometimes, these patients will have an intermittent 3* AV block, so patients with these EKG findings in the setting of syncope should be admitted for monitoring.

Incomplete trifascicular block with RBBB+LAFB+1* AV block