Lecture 18: Mechanisms of Bradyarrhythmias

Question 1

What are the two primary categories for bradyarrhythmia mechanisms?

Answer 1

1. Impulse formation (SA node disease)
2. Impulse transmission (AV or His-Purkinje system disease
   Can be spontaneous or drug-induced

Question 2

What causes bradycardia?

Answer 2

SA node is not firing fast enough

Atria and ventricles are not electrically linked

Question 3

What is an ECG characteristic of sinus node dysfunction bradyarrythmia?

Answer 3

No p waves or slower conducting p waves (lower than 60 bpm)

Question 4

What are the 3 categories of sinus node dysfunction?

Answer 4

1. sinus bradycardia
2. chronotropic incompetence
3. tachy-brady syndrome (sinus syndrome)

Question 5

What are the characteristics of sinus bradycardia?

Answer 5

Less than 60bpm as you calculate on ECG

Question 6

What is sinus arrhythmia?

Answer 6

When the P-P interval is different among beats

Not in a regular rhythm

Question 7

What is chronotropic incompetence?

Answer 7

Chronotropic = pertaining to heart rate

Inability of the heart to adjust its rate appropriately to meet metabolic demands

Question 8

What are the manifestations of chronotropic incompetence?

Answer 8

Fatigue or lightheadedness with exertion
Maximum sinus rate < 100bpm
Failure to reach 70% of maximum predicted HR with exercise (220-age)

Question 9

What is tachycardia-bradycardia Syndrome?

Answer 9

Defined by atrial fibrillation + sinus node dysfunction
Goes from AF to sinus node dysfunction
Abnormal sinus node recovery time

Question 10

What is the treatment of sick sinus syndrome?

Answer 10

Give pacemaker to treat sinus bradycardia

Give medication for atrial fibrillation

Question 11

What is SA exit block?

Answer 11

A characteristic of SA node dysfunction that leads to bradycardia
When the sinus node fires but does not spread on particular beats (you are missing beats in the ECG but that shit is evenly spaced)

Question 12

What are the types of AV block?

Answer 12

1. First degree
2. Second degree
   i. Mobitz type I
   ii. Mobitz type II
   3rd degree

Question 13

What is 1st degree AV “block”?

Answer 13

When every p wave conducts to the ventricle but just does so slowly
“block” is a misnomer
More like AV “delay” rather than block

Question 14

What are the key characteristics of first degree heart “block”?

Answer 14

PR interval >200ms
Not really blocked, very p wave conducts to ventricle
Usually conduction slowing at level of AV node

Question 15

What is 2nd degree AV block?

Answer 15

When only SOME of the p waves conduct to the ventricle

Consists of Mobitz type 1 and Mobitz type 2

Question 16

What are the key characteristics of Mobitz type 1?

Answer 16

Aka of Wenckebach
One type of second degree heart block
Block occurs at AV node
Benign

Question 17

What are the key characteristics of Mobitz Type II?

Answer 17

Conduction block occurs at the His-Purkinje system
Malignant
The other type of second degree heart block

Question 18

What is the difference between Mobitz Type I and Mobitz Type II?

Answer 18

Type I is block at level of AV node
Type II is a block at level of His-Purkinje system
Type I is benign while Type II is malignant
Mnemonic = being #1 is good but being second sucks

Question 19

What are the ECG characteristics of Mobitz Type 1?

Answer 19

Characterized first by missing QRS complex after a p-wave
- Grouped beating
Typically narrow QRS and decremental conduction
Prolonging PR interval until blocked beat
-PR interval gets longer, longer, longer, then drops
PR after pause shorter than PR before the block
Good prognosis

Question 20

How do you tell if PR intervals are changing for Type I second degree block?

Answer 20

Look at PR interval RIGHT BEFORE the block and compare it with the PR interval RIGHT AFTER the block
If PR interval before is longer, then you have Type I

Question 21

What is decremental conduction?

Answer 21

When there is impaired conduction in a portion of fiber because of progressively lessening response of the unexcited portion
Manifested by decreasing speed of conduction and decreased amplitude of AP up until the block

Question 22

What are the ECG characteristics of Mobitz 2?

Answer 22

Typically WIDE QRS
Unpredictable blocked beats
PR intervals are constant (same length and all or none)

Question 23

What are the ECG features of RIGHT bundle branch block?

Answer 23

Look at V1
Look for a “rabbit ear” appearance in V1 for RBBB
If it has a short left ear and a long right ear, you are good to go
Also, comparing aVL, I, V5 and V6, normal all have small or nonexistent S waves
In RBBB, aVL, I, V5 and V6 all have deep and long S waves

Question 24

What are the ECG features of LBBB?

Answer 24

Look at V1
Small R wave then a long and broad S wave
-broad S wave notes the slow conduction from right to left (going opposite V1 which is the direction of right heart
In aVL, I, V5 and V6, the QRS complex is wider

Question 25

Are RBBB and LBBB part of second degree type II block?

Answer 25

No they just can be seen in association with Type II block

Question 26

Why is Mobitze II dangerous?

Answer 26

Unpredictable timing of progression
Less likely to have a reliable escape rhythm
Can go from beating to flatline in a few minutes
Can occur with LBBB and RBBB

Question 27

What are the characteristics of 3rd degree heart block?

Answer 27

Complete heart block
None of the p waves conduct to the ventricle
-p wave and QRS complex are disjointed
Substitute slower pacemaker takes over
-AV dissociation
Rate of junctional escape rhythm depends on location

Question 28

What is an escape rhythm?

Answer 28

A heart rhythm initiated by lower centers when the SA node fails to initiate impulses, when its rhythmicity is depressed or when its impulses are completely blocked
Safety mechanism (if SA conduction isn’t kicking in, you still have other places in heart that can take over automaticity)

Question 29

What are the ECG features of 3rd degree heart block?

Answer 29

Complete heart block
No p waves conduct to the ventricle
So the p waves and QRS complexes are disjointed and out of sync

Question 30

What is vasovagal syncope?

Answer 30

Happens when you have slowing down of both your SA node and AV node
You have sinus bradycardia (slowing of p waves)
Also have complete heart block