Lecture 18: Mechanisms of Bradyarrhythmias Flashcards
What are the two primary categories for bradyarrhythmia mechanisms?
- Impulse formation (SA node disease)
- Impulse transmission (AV or His-Purkinje system disease
Can be spontaneous or drug-induced
What causes bradycardia?
SA node is not firing fast enough
Atria and ventricles are not electrically linked
What is an ECG characteristic of sinus node dysfunction bradyarrythmia?
No p waves or slower conducting p waves (lower than 60 bpm)
What are the 3 categories of sinus node dysfunction?
- sinus bradycardia
- chronotropic incompetence
- tachy-brady syndrome (sinus syndrome)
What are the characteristics of sinus bradycardia?
Less than 60bpm as you calculate on ECG
What is sinus arrhythmia?
When the P-P interval is different among beats
Not in a regular rhythm
What is chronotropic incompetence?
Chronotropic = pertaining to heart rate
Inability of the heart to adjust its rate appropriately to meet metabolic demands
What are the manifestations of chronotropic incompetence?
Fatigue or lightheadedness with exertion
Maximum sinus rate < 100bpm
Failure to reach 70% of maximum predicted HR with exercise (220-age)
What is tachycardia-bradycardia Syndrome?
Defined by atrial fibrillation + sinus node dysfunction
Goes from AF to sinus node dysfunction
Abnormal sinus node recovery time
What is the treatment of sick sinus syndrome?
Give pacemaker to treat sinus bradycardia
Give medication for atrial fibrillation
What is SA exit block?
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)
What are the types of AV block?
- First degree
- Second degree
i. Mobitz type I
ii. Mobitz type II
3rd degree
What is 1st degree AV “block”?
When every p wave conducts to the ventricle but just does so slowly
“block” is a misnomer
More like AV “delay” rather than block
What are the key characteristics of first degree heart “block”?
PR interval >200ms
Not really blocked, very p wave conducts to ventricle
Usually conduction slowing at level of AV node
What is 2nd degree AV block?
When only SOME of the p waves conduct to the ventricle
Consists of Mobitz type 1 and Mobitz type 2
What are the key characteristics of Mobitz type 1?
Aka of Wenckebach
One type of second degree heart block
Block occurs at AV node
Benign
What are the key characteristics of Mobitz Type II?
Conduction block occurs at the His-Purkinje system
Malignant
The other type of second degree heart block
What is the difference between Mobitz Type I and Mobitz Type II?
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
What are the ECG characteristics of Mobitz Type 1?
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
How do you tell if PR intervals are changing for Type I second degree block?
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
What is decremental conduction?
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
What are the ECG characteristics of Mobitz 2?
Typically WIDE QRS
Unpredictable blocked beats
PR intervals are constant (same length and all or none)
What are the ECG features of RIGHT bundle branch block?
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
What are the ECG features of LBBB?
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
Are RBBB and LBBB part of second degree type II block?
No they just can be seen in association with Type II block
Why is Mobitze II dangerous?
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
What are the characteristics of 3rd degree heart block?
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
What is an escape rhythm?
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)
What are the ECG features of 3rd degree heart block?
Complete heart block
No p waves conduct to the ventricle
So the p waves and QRS complexes are disjointed and out of sync
What is vasovagal syncope?
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
