Bundle Branch Blocks (Johnston)

Question 1

Impulse traveling TOWARDS the electrode produces what kind of deflection

Answer 1

• positive (R wave)

- Area of stimulated muscle completely depends on what electrode (what lead you are looking from)

Question 2

Impulse traveling AWAY from the electrode produces what kind of deflection

Answer 2

• Negative (S wave)

- Area of stimulated muscle completely depends on what electrode (what lead you are looking from)

Question 3

In normal ventricular conduction, what is the origin of a small q wave in V5 and V6

Answer 3

• Normally, heart is activated at the LEFT side of the ventricular septum then the RV and then finally the Left Ventricle
• So if electrode is on left side of LV, then impulse will travel AWAY (Left TO right), causing a negative deflection (seen as negative q wave) first. After left side of septum and right ventricle is depolarized, the left ventricle is activated so impulse is now traveling TOWARDS the electrode (R to L) so you see a POSITIVE R wave
• If there is a bundle branch block on the left, the left side of septum cannot be activated first so the q wave is lost
• If electrode was placed on the opposite side (Right), would see opposite reaction (would see positive q wave)

Question 4

Sequence of normal ventricular muscle depolarization

Answer 4

• LEFT Septal surface activated 0.1 sec BEFORE RIGHT
• Septum activated from Left TO Right
• Right wall is thinner than L wall, therefore the impulse activates the epicardium of the RV before the LV
• Finally the LV epicardial surface is activated from the apex to the base

Question 5

Bundle branch blocks are characterized by

Answer 5

• wide QRS complexes–time lapse from beginning of QRS to peak of R wave is 3 small squares or more (but not always)
• Width of QRS represents the time it takes to activate the epicardial surface of heart
• Bundle branch block and hypertrophy, it takes longer time to activate epicardial surface leading to a wide QRS complex

Question 6

Intrinsic Deflection is

Answer 6

• time lapse from beginning of QRS to peak of R wave

- Time that lapses from beginning of QRS complex to peak of R wave is measured horizontally

Question 7

V1 vs V6 time when impulse reaches the epicardial surface of ventricle (width of QRS complex)

Answer 7

V1: 0.2 sec
V6: 0.04 sec
-time difference is because impulse reaches RIGHT side of the heart (V1) before LEFT side since the right ventricular wall is THINNER
-If it takes longer for the ID to start downward, it means impulse is late in reaching the epicardial surface

Question 8

Late ID is seen in

Answer 8

• Bundle branch block

- Hypertrophy/dilated

Question 9

Common features of BBB

Answer 9

• Wide QRS complex (0.12 sec or greater)

- ST segment-T waves slope off in opposite direction to QRS

Question 10

Which side of the septum is activated first in RBBB

Answer 10

-Left side because nothing is changed on the left side

Question 11

In RBBB why is there an S wave (AFTER R wave) in V6?

Answer 11

• Because impulse cannot get to the right ventricle (After septal activation, activation begins in LV free wall instead of right)
• Since V6 (and V5) is on the LEFT side of the LV, and in a RBBB impulse cannot reach the right ventricle (goes to left ventricle first-R wave because TOWARDS). Impulse eventually reaches the right (AWAY from electrode) leading to negative deflection (S wave)

Question 12

Hallmark of RBBB

Answer 12

• R-S-R’ morphology in V1

- ST segment of T wave go in OPPOSITE direction of the polarity of the QRS complex

Question 13

In lead V1, why is there a small r wave morphology in RBB?

Answer 13

-Whichever conduction fascicle is blocked, that ventricle will be activated last so if you have RBBB, then RV is activated last

Question 14

Hallmark of LBBB

Answer 14

-R-R’ morphology

Question 15

RBB seen in which leads?

Answer 15

-Leads I, V1 and V6

Question 16

Sequence of Ventricular Activation in LBB

Answer 16

• Septum activated from R side, nearly same time as RV is activated (septal q wave is lost in V5 and V6–hallmark!!!!)
• Strong Septal forces therefore negative deflection in V1 (deep QS in V1 and V2–another hallmark)
• Positive deflection V6–monophasic R

Question 17

LBB more apt to occur with

Answer 17

• HTN
• Ischemia
• Aortic stenosis (valvular disease)
• Cardiomyopathy

Question 18

LBBB with LAD characteristics

Answer 18

• More myocardial dysfunction
• More disease in conduction system
• Maybe higher mortality

Question 19

LBB with RAD characteristics

Answer 19

• Associated with Congestive cardiomyopathy

- Limb leads in BBB-I and AVL usually have features of V6

Question 20

T wave in BBB polarity is

Answer 20

• Opposite to QRS direction
• Due to BBB, there is disturbance in depot-repol
• There are secondary T wave changes
• If T wave polarity is in the same direction of QRS complex, it is called PRIMARY T wave change; usually due to ischemia!!

Question 21

In LBBB which ventricle is activated first? Which ventricle is activated last?

Answer 21

-R first, L last

Question 22

Recap: Hallmarks of LBBB

Answer 22

• Right side: deep QS complexes (leads V1 and V2)

- Left side: absence of septal Q waves (leads V5 and V6)

Question 23

Recap: LBBB vs RBBB

Answer 23

• LBBB: monophonic R wave in Lead I, absence of septal q wave in V6, deep QS in V1
• RBBB: S waves in Lead I and V6 and R-S-R’ on V1