Left bundle branch block

LBBB DDx & Considerations:

• There are several different causes of LBBB

• LBBB is usually seen in patients with structural heart disease (hypertrophy, dilatation, fibrosis, etc.)

  • Also seen in those with ischemic heart disease, valvular disease, and various cardiomyopathies

• May be caused by inflammatory changes, advanced hyperkalemia, digitalis toxicity, or primary diseases of the conduction system (Lenegre's or Lev's disease, where there is fibrosis or degeneration of the conduction system resulting in heart block)

• Patients may have a LBBB at baseline that may have no relevance to their presenting symptoms in acute settings

• In patients presenting with symptoms of acute ischemia, new or old LBBB may make the detection of acute MI challenging

• LBBB may have no adverse prognostic significance in those without evidence of structural heart disease, such as young patients with idiopathic LBBB

Conduction in LBBB: reasons for the ECG changes

• In normal conduction, the ventricular septum is activated from left to right.

  • This results in the appearance of small Q waves in the lateral leads (I ,aVL, V5-V6)

• Block of an entire bundle branch requires that its ventricle be activated by the other ventricle

  • This results in delayed activation and prolongation of the QRS complex (≥ 120 ms)

• Complete LBBB may be a result of conduction disturbance in the main LBB or both of its fascicles

  • The LV is no longer activated with the RV simultaneously (normal conduction occurs left to right) , it is activated after the RV through septal depolarization

  • In LBBB, the RV is still activated normally through the right bundle branch, from right to left

  • Delayed LV activation results in a small R wave followed by a large S wave or a large Q wave with no R wave in the right precordial leads (V1-V2)

  • Delayed LV activation eliminates the normal Q waves in the lateral leads (I ,aVL, V5-V6)

    • Unless the patient had a prior anteroseptal MI with Q waves

• LBBB results in mid-QRS notching or slurring in leads V1/V2 or the lateral leads (I ,aVL, V5-V6)

Conventional criteria for LBBB

• QRS ≥ 120 ms

• Leftward conduction delay (QS or rS in lead V1)

• Endocardial mapping research suggests a significant portion of those diagnosed with conventional criteria do not actually have activation consistent with LBBB, hence the more strict and specific criteria used by cardiologists for cardiac resynchronization therapy

"Strict" criteria for LBBB

• • QRS ≥ 130 ms in women, QRS ≥ 140 ms in men

  • QS or rS in lead V1

  • Mid-QRS notching/slurring in two of the following leads; V1/V2 or I, aVL, V5 or V6