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6. Cardiovascular System > Conduction Blocks > Flashcards

Conduction Blocks Flashcards

1
Q

What are the 2 broad categories of heart block?

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

Name 2 common causes of heart block.

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

What are the 3 forms of AV block?

A
  1. First-degree AV block
  2. Second-degree AV block
  3. Third-degree Complete block
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4
Q

Describe first degree AV block.

A

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!

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5
Q

How does first-degree AV block appear on ECG?

A

Prolonged PR interval (>0.22 s)

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6
Q

List causes of first degree heart block.

A
  • 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
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7
Q

Describe second degree AV block.

A

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.

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8
Q

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

A

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

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9
Q

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

A

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

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10
Q

List causes of Mobitz I 2nd degree heart block.

A

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

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11
Q

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

A

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.

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12
Q

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

A

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.

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13
Q

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

A

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
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14
Q

List causes of Mobitz II 2nd degree heart block.

A

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.

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15
Q

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

A

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.

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16
Q

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

A

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

Type 2: the PR intervals remain a constant length

17
Q

Describe third degree AV block.

A

All atrial activity fails to conduct to the ventricles.

P waves and QRS complexes therefore occur independently.

18
Q

What is the mechanism of complete heart block?

A

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)

19
Q

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

A

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

20
Q

List causes of complete heart block

A
  • 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
21
Q

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

A

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

22
Q

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

A

II and V1

23
Q

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

A

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
24
Q

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

A

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
25
Q

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

A
  • 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

26
Q

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

A

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.

27
Q

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

A
  1. Right Bundle Branch Block (RBBB)
  2. Left Bundle Branch Block (LBBB)
28
Q

What is the mechanism behind RBBB?

A

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.

29
Q

List 2 causes of RBBB.

A

PE, RVH, IHD, congenital heart disease, idiopathic

30
Q

Describe the ECG features seen in RBBB.

A

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

31
Q

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

A

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
32
Q

List 2 causes of LBBB.

A

IHD, LVH, aortic valve disease, post-op

33
Q

Describe the ECG features seen in LBBB.

A

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

34
Q

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

A

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
35
Q

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

A

RBBB or LBBB.

6. Cardiovascular System (24 decks)

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