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Second Degree AV Block - High grade

High grade 2nd degree AV block

PQ Interval

• Physiological PQ interval is (0.12 - 0.2s)
• PQ interval = P wave (approx. 0.08s) + PQ segment (approx. 0.1s)

1. Impulse originates in the SA node
• When it passes to the atrial myocardium, it begins to generate the P wave
• Simultaneously, it spreads through the conduction system to the AV node
• The impulse in the conduction system does not create a curve
2. The impulse enters the AV node
• The impulse spreads from the SA node
• through the myocardium (P wave)
• and through the conduction system (internodal pathways) to the AV node
• At the time of atrial activation (peak of the P wave)
• It arrives through the conduction system to the AV node
3. Slowed (decremental) conduction in the AV node
• The impulse is delayed in the AV node for approx. 0.1s (no curve is created)
• Then it passes into the His bundle (no curve is created)
4. Activation of the ventricular septum
• From the His bundle, the impulse travels through the Purkinje fibers
• Begins to activate the myocardium of the ventricular septum
• Begins to generate the Q wave

AV Block II Degree - Mobitz I (Wenckebach)

• The defect is in the AV node
• Conduction in the AV node through the AV node gradually lengthens (Wenckebach phenomenon)
• Mobitz I has the Wenckebach phenomenon:
• The PQ interval gradually lengthens
• until the 5th P wave is blocked, then the cycle repeats
• QRS complexes are narrow (<0.12s)
• Conduction to the ventricles is (5:4), from 5 P waves, 4 QRS complexes are produced

AV Block II Degree, Mobitz II (QRS>0.12s)

• The defect is infrahisian (below the His bundle)
• The bundle branch or fascicle intermittently blocks impulses (P waves)
• There is a pre-existing bundle branch block (LBBB, RBBB)
• QRS complexes are wide (>0.12s)
• 75% of Mobitz II AV blocks have wide QRS complexes
• because of a pre-existing bundle branch (fascicular) block
• The PQ interval is constant (Wenckebach phenomenon is not present)
• Mobitz II is an intermittent block in the AV junction (without lengthening of the PQ interval)
• Conduction to the ventricles is (3:2)

ECG and High-Degree AV Block

• PP interval is regular (SA node generates impulses regularly)
• Conduction to the ventricles is 3:1 or higher (3:1, 4:1, 5:1...)
• PQ interval is constant
• This indicates that the AV node intermittently conducts the P wave to the ventricles (resulting in a QRS)
  
  

High-Degree AV Block

• Laddergram illustrates the propagation of the impulse through the conduction system
• A - Atria, AV - AV junction, V - Ventricles
• PP interval is regular
• PQ interval is constant (180ms)
• Conduction to the ventricles is 3:1
• This is a second-degree AV block, Mobitz II
• Wide QRS complexes (> 0.12s)
• The patient has a pre-existing left bundle branch block
• There is intermittent block of the right bundle branch

Second-Degree AV Block with 2:1 Conduction

• Conduction to the ventricles is 2:1
• Every second P wave is blocked (blue arrows)
• With 2:1 conduction, it is uncertain whether it is Mobitz I or Mobitz II?
• It is unclear whether the PQ interval is prolonged (Mobitz I) or if every 2nd P wave is blocked (Mobitz II)

High-Grade AV Block

• PP interval is constant
• PQ interval is constant at 0.2s
• Conduction to the ventricles is 4:1
• This indicates a High-Grade AV Block
• This is a Hisian second-degree AV block - Mobitz II

High-Grade AV Block (5:1)

• PP interval is constant
• PQ interval is constant 0.2s
• Conduction to the ventricles is 5:1
• Out of five P waves, one is conducted to the ventricles (blue arrow)
• This is a High-Grade AV Block
• This is a Infra-Hisian AV Block II Degree - Mobitz II
• QRS complexes are wide (>0.12s)
• And have the shape of RBBB — note the V1 lead