AV Blocks Lecture

Question 1

AV Block

Answer 1

A block in the cardiac conduction system that causes a disruption of atrial‐to‐ ventricular electrical conduction

Question 2

First degree AV Block

Answer 2

prolongs AV node conduction
P wave precedes QRS complex
PR interval more than 0.2 sec (5 boxes or 1 large square)

Question 3

etiology of 1st degree AV Block

Answer 3

Normal variant

• Presence of atherosclerosis, HTN, DM enhances chances
• Degeneration of conduction system/fibrosis congenital heart disease
• CAD - ischemia
• Drugs - BB, CCB, digitalis, antiarrhythmias (Class 1&3)
• Endocrine - hypothyroid, hyperthyroid, adrenal insufficiency
• Inflammatory - RF, SLE, MCTD, myocarditis
• Infiltrative - Amyloidosis, sarcoid, hemochromatosis
• Valvular calcification - mitral, aortic

Question 4

2nd degree AV Block - Mobitz I (Wencke Bach)

Answer 4

• progressive PR-interval prolongation prior to dropped QRS due to earlier arrival in relative refractory period of AV conduction
• Impairment of AV conduction (AV node)
• Transient
• “Grouped beats”
• Seen in inferior AMI
• Level of block is at level of AV node
• Narrow QRS complex

Question 5

Etiology of Mobitz I

Answer 5

Causes of 1st degree AV block
Digitalis toxicity
Ischemic events (MI-inferior)
Myocarditis

Question 6

Etiology of 2nd degree AV Block - Mobitz II

Answer 6

Ischemic heart disease
Acute ANTERIOR MI
Degeneration of conduction system

Question 7

2nd degree AV Block - Mobitz II

Answer 7

PR interval uniform
Dropped Beat (QRS) - P wave fails to conduct

Block occurs at the level of:
Bundle of HIS
Both bundle branches
Fascicular branches

Progressive/irreversible

May be seen with ANTERIOR AMI because block is distal to AV node - worse prognosis

Question 8

3rd Degree Heart Block (Complete Heart Block)

Answer 8

P waves never related to QRS complexes

2 independent rhythms
-AV Dissociation - no P waves conduct to the ventricle

Can occur above or below AV node

Above: Junctional rhythm - narrow QRS (rate 40-55)
Below: Ventricular pacemaker - wide QRS (rate 20-40)

Question 9

Etiology of 3rd degree Heart block

Answer 9

Ischemic
Infiltrative diseases
Cardiac surgery:
-By-pass, vlave replacement
-myocarditis
-degenerative

Question 10

Treatment for 3rd degree AV block

Answer 10

pacemaker

Question 11

What to check on every ECG

Answer 11

PR Interval:

• increased consistently in 1st degree AV block
• Progressively increases in each series of cycles with Wenckebach
• Totally variable in 3rd degree AV block
• Decreased in WPW and LGL syndromes

P without QRS response:

• Wenckebach and Mobitz 2 AV blocks
• 3rd degree AV block - independent