Conduction Blocks

Question 1

What are the 2 broad categories of heart block?

Answer 1

1. Atrioventricular block.
   - Block in either the AV node or the His bundle.
2. Bundle branch block.
   - Block lower in the conduction system.

Question 2

Name 2 common causes of heart block.

Answer 2

1. Coronary artery disease
2. Cardiomyopathy
3. Fibrosis of conducting tissue

Question 3

What are the 3 forms of AV block?

Answer 3

1. First-degree AV block
2. Second-degree AV block
3. Third-degree Complete block

Question 4

Describe first degree AV block.

Answer 4

Fixed prolongation of the PR interval due to delayed conduction to the ventricles.

So, every atrial depolarisation is followed by conduction to the ventricles but with delay

Asymptomatic - so no treatment!

Question 5

How does first-degree AV block appear on ECG?

Answer 5

Prolonged PR interval (>0.22 s)

Question 6

List causes of first degree heart block.

Answer 6

• Increased vagal tone
• Athletic training
• Electrolyte disturbances e.g. hypo/hyperkalaemia
• Hypokalaemia
• Myocarditis (Lyme disease)
• Inferior MI
• AVN-blocking drugs:
  e.g. beta blockers (Bisoprolol), CCBs (Verapamil), Amiodarone and Digoxin

Question 7

Describe second degree AV block.

Answer 7

Occurs when some P waves conduct and other do not.

There are more P waves to QRS complexes because some atrial impulses fail to reach the ventricles and so you don’t get a QRS complex.

Question 8

Types of second degree AV block: describe Mobitz type I.

Answer 8

PR interval gradually increases until AV node fails and no QRS is seen.

Question 9

What is the mechanism of Mobitz I 2nd degree heart block?

Answer 9

Usually due to reversible conduction block at AVN - malfunctioning AVN cells progressively fatigue until they fail to conduct an impulse (dropped beat)

Question 10

List causes of Mobitz I 2nd degree heart block.

Answer 10

Drugs: beta blockers CCBs digoxin amiodarone
Increased vagal tone (e.g. athletes)
Inferior MI
Myocarditis
Cardiac surgery

Question 11

Describe the ECG trace in Mobitz I 2nd degree heart block (Wenckebach phenomenon).

Answer 11

Progressive lengthening of PR interval, followed by absent QRS (a non-conducted P wave), then PR interval returns to normal, then begins to get longer again when the cycle repeats.

PR interval is longest just before dropped beat, and shortest just after.

Question 12

Types of second degree AV block: describe Mobitz type II.

Answer 12

There is a sudden unpredictable loss of AV conduction and so loss of QRS.

PR interval is constant but every nth QRS complex is missing.

Question 13

What is the mechanism of Mobitz II 2nd degree heartblock?

Answer 13

Usually due to failure of conduction at His-Purkinje system

Generally due to structural damage to conducting system “all-or-nothing”

• No progressive fatigue like in Mobitz I, instead His-Purkinje cells suddenly and unexpectedly fail to conduct

Question 14

List causes of Mobitz II 2nd degree heart block.

Answer 14

Anterior MI (septal infarction with necrosis of bundle branches).
Idiopathic fibrosis of conducting system.
Cardiac surgery.
Inflammatory conditions (rheumatic fever, myocarditis, Lyme disease).
Autoimmune (SLE, systemic sclerosis).
Infiltrative myocardial disease (amyloidosis, haemochromatosis, sarcoidosis).
Hyperkalaemia.
Drugs: beta blockers, CCBs, digoxin, amiodarone.

Question 15

Describe the ECG trace in Mobitz type II 2nd degree heart block

Answer 15

Intermittent non-conducted P waves without progressive prolongation of PR interval.

P waves ‘march through’ at constant rate.

Wide QRS complex.

PR interval is constant and QRS interval is dropped.

Question 16

What is the difference between Mobitz type I and Mobitz type II heart block?

Answer 16

Type 1: the PR intervals get longer and longer until a QRS complex is dropped

Type 2: the PR intervals remain a constant length

Question 17

Describe third degree AV block.

Answer 17

All atrial activity fails to conduct to the ventricles.

P waves and QRS complexes therefore occur independently.

Question 18

What is the mechanism of complete heart block?

Answer 18

There is complete absence of AV conduction - end point of second degree heart block.

Either progressive fatigue of AVN cells (mobitz I) or due to sudden onset of complete conduction throughout His-Purkinje system (mobitz II)

Question 19

How are ventricular contractions maintained in third degree complete AV block?

Answer 19

Ventricular contractions are being maintained by spontaneous escape rhythms from below site of block.

Question 20

List causes of complete heart block

Answer 20

• Inferior MI.
• AVN blocking drugs - CCBs, beta blockers, digoxin.
• Idiopathic degeneration of conducting system.
• Structural heart disease e.g. transposition of great vessels
• Ischaemic heart disease e.g. acute MI
• Hypertension
• Endocarditis or Lyme disease

Question 21

What is the clinical significance of complete heart block?
How would it be treated?

Answer 21

High risk of sudden cardiac death.
Urgent admission for cardiac monitoring, backup temporary pacing followed by permanent pacemaker insertion

Question 22

In what lead(s) is complete heart block best seen?

Answer 22

II and V1

Question 23

What does the narrow-complex escape rhythm mean in third degree AV block?

Answer 23

QRS complex < 0.12 s

• Implies block originates in the His bundle and thus the region of block lies more proximally in the AVN
• Recent-onset, narrow-complex AV block that has transient causes may responses to IV atropine
• Chronic narrow-complex AV block requires permanent pacemaker if it is symptomatic

Question 24

What does the broad-complex escape rhythm mean in third degree AV block?

Answer 24

QRS complex > 0.12 s

• Implies block originates BELOW the bundle of His and thus the region of block lies more distally in the His-Purkinje system
• Dizziness and blackouts often occur
• Permanent pacemaker implantation is recommended

Question 25

What is Bundle Branch Block (BBB)? Why does it occur?

Answer 25

* The His bundle gives rise to the right and left bundle branches. * The left branch subdivides into the anterior and posterior divisions of the left bundle. 1) Incomplete bundle branch block * Bundle branch conduction delay results in the slight widening of the QRS complex (up to 0.11 seconds) 2) Complete block of a bundle branch - This is associated with a wider QRS complex (> 0.12 s) - The shape of the QRS depends on whether the right or the left bundle is blocked

Question 26

What happens to the width of the QRS complex in bundle branch block?

Answer 26

It gets wider. Conduction is slower. Because there is a blockage in the bundle of his, so having to conduct through the ventricular septum, which is much slower.

Question 27

What are the 2 types of complete bundle branch block (BBB)?

Answer 27

1. Right Bundle Branch Block (RBBB) 2. Left Bundle Branch Block (LBBB)

Question 28

What is the mechanism behind RBBB?

Answer 28

Right bundle no longer conducts, meaning that the 2 ventricles dont't get impulses at the same time, and instead spread from left to right. Produces the late activation of the right ventricle.

Question 29

List 2 causes of RBBB.

Answer 29

PE, RVH, IHD, congenital heart disease, idiopathic

Question 30

Describe the ECG features seen in RBBB.

Answer 30

Secondary, tall, late R waves in V1. Slurred deep S wave in leads I, V5 and V6.

Question 31

RBBB: what would you see in lead V1 and V6?

Answer 31

MaRRoW. Looks like maRRow - maRRow - Right bundle branch block - MarroW: * M - QRS looks like an M in lead V1 * W - QRS looks like W in V5 & V6

Question 32

List 2 causes of LBBB.

Answer 32

IHD, LVH, aortic valve disease, post-op

Question 33

Describe the ECG features seen in LBBB.

Answer 33

Opposite to RBBB. Secondary, tall, late R waves in left ventricular leads (I, AVL, V4-V6). Slurred, deep S wave in V1 and V2.

Question 34

LBBB: what would you see in lead V1 and V6?

Answer 34

Since the left bundle branch conduction is normally responsible for the initial ventricular activation, LBBB also produce abnormal Q waves. WiLLiaM. Looks like wiLLiam - wiLLiam - Left bundle branch block - WilliaM: * W - QRS looks like a W in leads V1 & V2 * M - QRS looks like an M in leads V4-V6

Question 35

What kind of heart block is associated with wide QRS complexes with an abnormal pattern?

Answer 35

RBBB or LBBB.