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BUNDLE BRANCH BLOCKS - RIGHT & LEFT, COMPLETE VS INCOMPLETE

Category: Cardiology

Topic: Bundle Branch Blocks and Ventricular Rhythms

Level: Paramedic

Next Unit: Premature Ventricular Complexes

13 minute read

Bundle Branch Blocks (BBB)

A bundle branch block (BBB) is a block of the right or the left bundle branches that propagate depolarizations from the Sino-Atrial node to and through the ventricles. The signal is conducted first through the healthy branch and then it is distributed to the damaged side. This skewing of one ventricle taking longer than the other widens the QRS-complex abnormally.

ABERRATION: defined as a markedly different manner of conduction from the normal conduction of the cardiac muscle and is usually applicable to right bundle branch block (RBBB) and left bundle branch block (LBBB).

COMPLETE BUNDLE BRANCH BLOCKS are full obstructions along the pathway that electrical impulses travel to get to the ventricles. A BBB can be either a

  • complete right BBB (RBBB) or a
  • complete left BBB (LBBB).

INCOMPLETE BUNDLE BRANCH BLOCKS are partial obstructions or delays along the pathway that electrical impulses travel to get to the ventricles and can be either right-sided or left-sided.

 

ECG Changes

You can distinguish between Left Bundle Branch Block and Right Bundle Branch Block simply by looking at the QRS morphology in V1 and V6.

  • If the QRS looks like W in V1 and M in V6 it is LBBB. (WiLLiam)
  • If the QRS looks like M in V1 and W in V6 it is RBBB. (MoRRow)

 

Right Bundle Branch Block (RBBB)

If the QRS looks like W in V1 and M in V6 it is LBBB. ( WiLLiam ).

Right bundle branch blocks are either delays or partial obstructions (incomplete bundle branch blocks) or full obstructions along the pathway that electrical impulses travel to get the to right ventricle, and are evidenced by wider than normal QRS segments in normal ECG’s, and require 12-lead acquisition for further differentiation.

Oftentimes, the biggest clue of a bundle branch block is found in lead V1 using the turn signal method.

In a RBBB the QRS in V1 will be deflected “up” from the J-point; remember “up” for a right turn signal.

During a right bundle branch block, activation of the right ventricle is delayed as depolarization has to spread across the septum from the left ventricle. Right bundle branch blocks do not affect the left ventricle so the early part of the QRS is unaffected.

Secondary R wave production is present in the right precordial leads (V1-V3) and a wide often slurred S wave is present in the lateral leads (I, V5-V6).

Delays in the activation of the right ventricle will also cause secondary repolarization abnormalities, showing with ST depression and T wave inversion in the right precordial leads (V1-V3).

 

Left Bundle Branch Block (LBBB)

If the QRS looks like M in V1 and W in V6 it is RBBB. (MoRRow ).

Left bundle branch blocks are either delays or partial obstructions (incomplete bundle branch blocks) or full obstructions along the pathway that electrical impulses travel to get the to left ventricle, and are evidenced by wider than normal QRS segments in normal ECG’s, and require 12-lead acquisition for further differentiation.

Oftentimes, the biggest clue of a bundle branch block is found in lead V1 using the turn signal method.

In a LBBB the QRS in V1 will be deflected “down” from the J-point; remember “down” for a left turn signal.

Reverse R → L Ventricular Depolarization: During a left bundle branch block, the normal direction of septal depolarization is reversed: instead of from left to right (L  R), it is from right to left (R L) causing a sequential depolarization instead of simultaneous depolarization.

Left bundle branch blocks have

  • dominant S waves in V1,
  • broad monophasic R waves in lateral leads (I, aVL, V5-V6),
  • absence of Q waves in lateral leads I, V5-V6 (small Q waves are still allowed in aVL); and
  • prolonged R wave peak time of greater than 60 ms in left precordial leads (V5-V6).

New onset left bundle branch blocks associated with chest pain are often treated as myocardial infarctions.

ECG DIAGNOSIS

The criteria to diagnose a left bundle branch block on the electrocardiogram:

  1. The heart rhythm must be supraventricular in origin.
  2. The QRS duration must be > 120 ms.
  3. There should be a QS or rS complex in lead V1.
  4. There should be a RsR' wave in lead V6.
  5. The T wave should be deflected opposite the terminal deflection of the QRS complex.

This is known as appropriate T wave discordance with bundle branch block. A concordant T wave may suggest ischemia or myocardial infarction.