SM 135a - Bradycardia

Question 1

How can you distinguish 2nd degree AV block type I and type II?

Answer 1

• 2nd degree type I
  
  • Delay gets progressively longer
  • Dropped beats
• 2nd degree type II
  
  • Fixed delay
  • More dropped beats than type I

Question 2

What is more dangerous: a heart block at the AV node or a heart block at the His-Purkinje system?

Answer 2

A block at the His-Purkinje system is more dangerous; It indicates a significant structural abnormality

(2nd degree type II and 3rd degree AV blocks occur at the level of the His-Purkinje system)

Question 3

List 4 major causes of disorders of impulse initiation (intrinsic SA node dysfunction)

Answer 3

Sinus bradycardia

Chronotropic incopetence

Tachy-brady syndrome

Sinus arrest

Question 4

What are some possible symptoms of bradycardia?

Answer 4

Syncope

Presyncope (dizziness)

Poor exercise tolerance/fatigue

Congestive heart failure

No symptoms (possible presentation)

Question 5

Describe the ECG manifestations of 1st degree heart block

Answer 5

1st degree heart block = AV block

• Beats are not dropped, but there is a delay between AV and ventriclular depolarization
  
  • PR interval >200 ms

Question 6

As heart rate increases, the dominant pacemaker within the SA node shifts ____________

Answer 6

As heart rate increases, the dominant pacemaker within the SA node shifts cranially

Question 7

What is chronotropic incompetence?

Answer 7

Inadequate cardiac response to exercise

Maximum HR <80% of predicted max HR with exercise (220 - age)

Question 8

On an ECG you see progressive shortening of the PP interval until one sinus pause is an exact multiple of the base PP interval

What is causing this?

Answer 8

2nd degree Type II AV block

Question 9

Describe the ECG manifestations of sinus bradycardia

Answer 9

• Normal P-wave axis
• Every P-wave is followed by a QRS complex
• HR <60 BPM

Question 10

What causes 1st degree AV block?

Answer 10

AV block = 1st degree heart block

May be caused by…

• Increased parasympathetic (vagal) tone
• AV node scarring

Question 11

Describe the ECG manifestations of bradycardia-tachycardia syndrome

Answer 11

Alternating blocks of atrial bradycardia and tachycardia

Question 12

What are the two major causes of intrinsic SA node dysfunction?

Answer 12

Disorders of impulse initiation (phase 4 depolarization)

Failure of conduction out of the SA node (Heart block)

Question 13

Define bradycardia

Answer 13

Resting HR <50 BPM

Question 14

How does catecholamine stimulation affect the rate of phase 4 depolarization in pacemaker cells?

Answer 14

Catecholamine stimulation increases the rate of phase 4 depolarization

-> increases rate of upstroke -> increaes heart rate

(Ex: Exercise)

Question 15

How is intrinsic SA node dysfunction treated?

Answer 15

Pacemaker

Question 16

Which heart blocks can be classified as “AV delay”

Answer 16

1st degree AV block

2nd degree type I AV block

Question 17

What causes high-grade (3rd degree) AV block?

Answer 17

aka Complete Heart Block

• Occurs at the level of the His-Purkinje system
• There is no conduction from the atria to the ventricles
  
  • QRS complexes come from subsidary or “escape” pacemakers
    
    • Junctional (40-60 BPM) engages His-Purkinje
    • Ventricular (20-40 BPM) spreads through gap junctions, causing a wider QRS complex

Question 18

Define SA node dysfunction

Answer 18

Resting HR <40 BPM

OR

Pauses of >3 seconds between QRS complexes

Note: resting HR <40 may be normal in sleeping athletes

Question 19

List 5 major causes of extrinsic SA node dysfunction

Answer 19

Medications

Electrolyte abnormalities

Severe hypotension

Severe hypoxemia

Hypothyroidism

Question 20

A patient with high-grade AV block has normal-width QRS complexes on an ECG.

What is responsible for these QRS complexes?

What would you expect this patient’s heart rate to be?

Answer 20

A junctional escape rhythm is responsible for these QRS complexes. Depolarization engages the His-Purkinje system. This is fast, causing a normalQRS complex

You would expect heart rate to be 40-60 BPM

Question 21

Describe the ECG manifestations of SA exit block

Answer 21

• Normal P-wave axis
• Progressive shortening of the PP interval until…
  
  • One P-wave fails to conduct​(2nd degree Type I block)
  • The sinus pause is an exact multiple of the base PP interval (2nd degree Type II block)

Question 22

In a complete heart block, there is no conduction from the atria to the ventricles.

Where do QRS complexes come from?

Answer 22

In complete heart block, QRS complexes come from subsidary or “escape” pacemakers

• Junctional escape rhythm
  
  • 40-60 BPM
  • Engages His-Purkinje
  • Normal width of QRS complex
• Ventricular escape rhythm
  
  • 20-40 BPM
  • Spreads through gap junctions
  • Wider QRS complex due to slow conduction

Question 23

How does vagal stimulation affect the rate of phase 4 depolarization in pacemaker cells?

Answer 23

Vagal stimulation slows the rate of phase 4 depolarizaiton

-> decreases rate of upstroke -> decreases HR

Question 24

List 4 major causes of heart block (failure to conduct signals out of the SA node)

Answer 24

Age-associated fibrosis

COronary arteriosclerosis

Fibrosis related to hypertension or diabetes

Any cardiovascular disease

Question 25

A patient with high-grade AV block has wide QRS complexes on an ECG.

What is responsible for these QRS complexes?

What would you expect this patient’s heart rate to be?

Answer 25

A ventricular escape rhythm is responsible for these QRS complexes. Depolarization spreads through gap junctions. This is slower, causing a wider QRS complex

You would expect heart rate to be 20-40 BPM, due to slower conduction

Question 26

Which AV blocks require implantation a pacemaker?

Why?

Answer 26

2nd degree type II and 3rd degree

These blocks occur at the level of the His-Purkinje system

Question 27

On an ECG you see progressive shortening of the PP interval until one P-wave fails to conduct

What is causing this?

Answer 27

2nd degree Type I AV block

Question 28

Describe the ECG manifestations of sinus arrest

Answer 28

• Normal P-wave axis
• Every P-wave followed by a QRS complex
• Pauses of >3 seconds without atrial activity (No p-waves)

Question 29

What medications might cause extrinsic SA node dysfunction?

Answer 29

Beta blockers

Ca2+ channel blockers

Digoxin

Anti-arrhythmic drugs

Question 30

How is extrinsic SA node dysfunction treated?

Answer 30

Remove the source

(ex: discontinue medication, fix electrolyte abormality, treat hypotension, hypoxemia, hypothyroidism)