Bundle branch block

Question 1

How many bundle branches in the cardiac electrical system? (formed when the bundle of his separates into its 2 main branches)

Answer 1

2 - right and left

in BBB one of these is no longer conducting electrical impulses normally

Question 2

the LEFT/ RIGHT bundle splits further

Answer 2

left - into anterior and posterior hemi-bundles

Question 3

escape rate in BBB?

Answer 3

40bpm

Question 4

what does the term ‘non-specific interventricular conduction delay’ refer to?

Answer 4

ECGs with broad QRS which doesn’t exhibit LBBB or RBBB morphology

Question 5

what are the 3 phases of activation in BBB?

Answer 5

1) septal depolarisation occurring from left > right, occurring alone
2) depolarisation of free wall of left and right ventricles, occurring together

Question 6

what are the ECG phases of activation in BBB?

Answer 6

increasing amplitude from V1 and 2 (RV) to V3 and 4 (IVS) and largest in V5 and 6 (LV)

Question 7

what is BBB?

Answer 7

a delay in conduction in either of the bundle branches

Question 8

how to determine which bundle is blocked?

Answer 8

look at V1 and V6

Question 9

is the QRS complex narrow or broad?

Answer 9

> 0.12s

Question 10

how does partial/ incomplete BBB differ on the ECG?

Answer 10

the pattern of LBBB or RBBB is present but the duration of the QRS does not exceed 0.12s

Question 11

in RBBB, how is the right ventricle stimulated? How is this seen on ECG?

Answer 11

by the impulse from the left ventricle.

Best seen in V1 - M shaped QRS (and W shaped QRS in V6)

Question 12

how to remember the ECG changes of left vs right BBB?

Answer 12

W i LL ia M - LLeft block
M a RR o W - RRight block
^ ^ ^
V1 Side V6

Question 13

Describe the phases of activation in RBBB?

Answer 13

• the septum depolarises from left to right as normal
• the left ventricle is depolarised as normal
• finally the right ventricle is depolarised late (wide) in an anterior movement. Resulting QRS is wide due to slow conduction through myocardial cells as opposed to the rapid conducting system

Question 14

ECG criteria for RBBB?

Answer 14

• QRS > 0.12s
• RSR complex in V1 (‘M-shaped’ QRS complex)
• delayed S wave in V4, 5 and 6 (>0.12s)

Question 15

T/F: IF abnormal Q waves are present (e.g. MI) they will usually be masked by BBB pattern

Answer 15

false - not usually masked

This is because there’s no alteration of the initial part of the complex RS (in V1) and abnormal Q waves can usually still be seen

Question 16

What are some of the causes of RBBB?

Answer 16

section not loading

Question 17

in LBBB how is the left ventricle activated?

Answer 17

by the right bundle

Question 18

which fasicles are damaged in LBBB?

Answer 18

results from injury to both LAHB and LPHB

Question 19

ECG changes for LBBB?

Answer 19

best seen in V6

• W shaped QRS in V1
• M shaped QRS in V6

Question 20

Describe the phases of activation in LBBB

Answer 20

• impulse passes to the left of the septum below the block at the same time as the paraseptal region
• activation of the RV follows (small magnitude)
• finally delayed activation of the LV which is slow due to conduction through normal myocardium

Question 21

ECG criteria for LBBB?

Answer 21

• prolonged QRS complex (>0.12s)
• wide, notched QRS (M shaped) in I, aVL, V5, V6
• wide, noticed QS complexes in V1 (due to spread of activation away from the electrode through septum + LV)
• small R wave in V2,3 due to activation of paraseptal region

Question 22

causes of LBBB?

Answer 22

• usually indicative of heart disease
• IHD
• MI
• cardiomyopathy
• HTN

Question 23

The presence of LBBB makes the diagnosis of what challenging?

Answer 23

MI