Left Bundle Branch Block

Left bundle branch block (LBBB) occurs when the left bundle no longer conducts, and the signal must pass to the left ventricle via myocyte-to-myocyte conduction. This pattern of conduction is slower than via the specialized conduction system, and results in a wide QRS complex (>120 ms). Conduction disturbances, like bundle branch blocks, result from structural abnormalities of the His-Purkinje system caused by necrosis, fibrosis, calcification, infiltrative disease, electrolyte disturbances, or impaired vascular supply. When conduction is impaired to both left ventricular terminals (the left anterior and posterior fascicles), the result is a LBBB. The criteria are:

QRS >120 ms
Dominant S wave in V1
Broad monophasic or notched R wave in lateral leads (I, aVL, V5, V6)
+/- left axis deviation

Depiction of depolarization in left bundle branch block. The septum first depolarizes from right to left (blue arrow), followed by near simultaneous depolarization of the right and left ventricles (pink arrows). The resultant vector leads to a negative deflection in V1 and a positive deflection in V6. SA, sinoatrial node; AV, atrioventricular node; RB, right bundle; LB, left bundle; CB, common bundle; RV, right ventricle; LV, left ventricle; red dash represents a block of the left bundle.

Examples

Left Bundle Branch Block

The left bundle branch block coupled with the presence of a first degree atrioventricular block suggests trifascicular disease. This patient may be a candidate for cardiac resynchronization therapy.

Left Bundle Branch Block

Left bundle branch block pattern with a wide and downgoing QRS complex in V1, and broad, notched R waves in the lateral leads (I, aVL, V5, V6).

Left Bundle Branch Block

The PR interval is borderline at 200 msec. When a left bundle branch block is accompanied with a first degree atrioventricular block, this suggests trifascicular disease. This patient subsequently went into complete heart block and lost consciousness.

Left Bundle Branch Block

Left bundle branch block pattern with a wide and downgoing QRS complex in V1, and broad, monophasic R waves in the lateral leads (I, aVL, V5, V6). Note the expected discordant ST changes (i.e., ST deviations in the opposite direction of the QRS complexes).

Left Bundle Branch Block

Classic left bundle branch block pattern with a wide and downgoing QRS complex in V1, notched, upgoing QRS complex in I, aVL, and V6, and left axis deviation.

Left Bundle Branch Block

When associated with a first degree atrioventricular block, a left bundle branch block can be misinterpreted as ventricular tachycardia, or, in this case, an accelerated idioventricular rhythm. The P wave is buried by the preceding T wave, masking the atrial activity. This patient was in sinus tachycardia (P waves are most clearly seen in V1).

Left Bundle Branch Block

While not classically referred to as a "trifascicular block," a left bundle branch block necessarily indicates failure of two fascicles - the left anterior and posterior fascicles - and, when associated with first-degree atrioventricular block (i.e., prolonged PR interval), likely represents trifascicular disease. This patient presented with syncope and subsequently received a permanent pacemaker after telemetry revealed intermittent high-grade atrioventricular block while inpatient.

Left Bundle Branch Block

Atrial fibrillation with rapid ventricular response and a left bundle branch block. The left bundle branch block resolved after rate control. In most cases, rate-related bundle branch blocks occur due to a prolonged refractory period of a diseased bundle. When a critical heart rate is exceeded, the diseased bundle fails. Rate-related bundle branch blocks can be especially challenging to diagnose when the rate is regular and fast (e.g., supraventricular tachycardia), creating a regular, wide complex tachycardia that appears like ventricular tachycardia.

Left Bundle Branch Block

Left bundle branch block with sinus bradycardia.

Left Bundle Branch Block

Classic left bundle branch block pattern with a wide and downgoing QRS complex in V1, a notched and upgoing QRS complex V6, and left axis deviation. There is also a first degree atrioventricular block (i.e., prolonged PR interval), which might represent a diseased right bundle branch block (i.e., trifascicular disease).

Sgarbossa Criteria

Concordant ST-segment elevation in I, aVL in setting of left bundle branch block is specific for acute myocardial infarction. Left heart catheterization revealed diffuse disease and percutaneous intervention was performed to the left anterior descending and left circumflex arteries.

References

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