Home /

Second Degree AV block - Conduction 2:1

2:1 2nd degree AV block, Fixed-Ratio 2:1 AV block

PQ Interval

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

1. An impulse originates in the SA node
• As it passes to the atrial myocardium, the P wave begins to form
• Simultaneously, it spreads through the conduction system towards the AV node
• The impulse in the conduction system does not create a wave
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 reaches the AV node through the conduction system
3. Slowed (decremental) conduction of the AV node
• The impulse delays in the AV node for approx. 0.1s (no wave is formed)
• Then it passes into the His bundle (no wave is formed)
4. Activation of the ventricular septum
• From the His bundle, the impulse travels through the Purkinje fibers
• It begins to activate the myocardium of the ventricular septum
• The Q wave starts to form

AV Block II Degree - Mobitz I (Wenckebach)

• Mobitz I is a nodal disorder (disorder in the AV node)
• Impulse conduction through the AV node gradually prolongs until the impulse is blocked (Wenckebach phenomenon)
• The ventricles are activated through the conduction system (His bundle, bundle branches...)
• Therefore, the QRS complexes are narrow (<0.12s)
• Unless aberrant conduction is present

AV Block II Degree - Mobitz I (Wenckebach)

• Laddergram illustrates the spread of the impulse through the conduction system
• A - Atria, AV - AV junction, V - Ventricles
• Mobitz I is a disorder of the AV node (which has the Wenckebach phenomenon)
• PP interval is constant (810ms) - sinus rhythm
• QRS complexes are narrow (<0.12s)
• PQ interval gradually prolongs until the P wave is blocked (no QRS complex follows the P wave)
• 1st PQ (190ms)
• 2nd PQ (290ms)
• 3rd P wave is blocked
• The cycle then repeats
• Conduction to the ventricles is (3:2)
• Of 3 P waves, 2 P waves are conducted to the ventricles (2 QRS complexes are produced), the cycle then repeats

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

• 75% of Mobitz II AV blocks have a wide QRS complex (>0.12s)
• Because it is an infrahisian block (below the His bundle)

AV Block II Degree - Mobitz II

• This is an infrahisian AV block II degree - Mobitz II
• 75% of Mobitz II AV blocks have a wide QRS complex (>0.12s)
• Mobitz II can block impulses infrahisially: at the level of the bundle branches or fascicles
• The likelihood of having an intermittent block at
• 2 sites (2 bundle branches)
• 3 sites (2 fascicles and the right bundle branch)
• is minimal
• Patients have a wide QRS complex because they have a pre-existing:
• Bundle branch block (LBBB, or RBBB)
• Bifascicular block (RBBB + one fascicle)
• In sinus rhythm, impulses pass to the ventricles through only one intact bundle branch (fascicle)
• On the ECG, there is a wide QRS complex
• Intermittent blockage of impulses occurs in the unblocked bundle branch (fascicle)
• Which we see on the ECG as blocked P waves (no QRS complex follows)
  
  
• PQ interval is constantly the same (180ms), this is the main difference from Mobitz I (Wenckebach)
• Every 3rd P wave is blocked below the His bundle
• No QRS complex follows it
• Conduction to the ventricles is (3:2)
• Of 3 P waves, 2 P waves are conducted to the ventricles (2 QRS complexes are produced), the cycle then repeats

Differential Diagnosis - AV Block II Degree (2:1)

• Have the patient undergo a long ECG recording
• On a long ECG recording, another conduction to the ventricles other than (2:1) will sooner or later appear
  
  
• AV Block II Degree - Mobitz I (Nodal disorder)
• Forms narrow QRS complexes because the impulse passes to the ventricles through the AV junction and through the conduction system
• Wide QRS complexes can occur with aberrant conduction
• In the AV node, there is a Wenckebach phenomenon (PQ interval gradually lengthens until the P wave is not conducted)
  
  
• AV Block II Degree - Mobitz II (Infranodal disorder)
• 75% of Mobitz II forms wide QRS (Infrahisian disorder)
• The patient already has a pre-existing bundle branch block (LBBB, RBBB), or a Bifascicular block
• Therefore, the QRS complexes are wide
• 25% of Mobitz II forms narrow QRS (Hisian disorder)
• The conduction system, except for the His bundle, is intact
  
  
• PQ interval in AV block II degree (2:1)
• Mobitz I has a prolonged PQ interval (almost always, but not a rule)
• Mobitz II has a non-prolonged PQ interval
  
  
• Administration of atropine in AV block II degree (2:1)
• Atropine is a parasympatholytic, it increases the frequency of the SA node and accelerates conduction through the AV node
• Mobitz I improves
  • It is a disorder of the AV node and atropine acts on the AV node
  • Conduction through the AV node increases, Wenckebach phenomenon subsides
  • A block (2:1) becomes a milder block (3:2, 4:3, 5:4)
• Mobitz II worsens
• It is an infranodal disorder, not an AV node disorder, and atropine acts on the AV node
• Mobitz II (2:1) may not respond to atropine, the conduction to the ventricles remains (2:1)
• After atropine, the frequency of the SA node increases and the conduction system below the AV junction is frequency overloaded
• With frequency, the refractory period (decremental conduction) lengthens
• and the AV block may worsen (3:1, 4:1, 5:1), until a complete AV block (III degree) develops
  
  
• Carotid sinus massage in AV block II degree (2:1)
• Carotid sinus massage blocks the AV node (slows conduction through the AV node) and reduces the frequency of the SA node
• Mobitz I worsens
• It is a disorder of the AV node and carotid sinus massage blocks the AV node
• A block (2:1) becomes a worse block (3:1, 4:1, 5:1)
• Mobitz II improves
• It is an infranodal disorder, not an AV node disorder, and carotid sinus massage blocks the AV node
• The frequency of the SA node decreases and conduction through the AV node lengthens
• The conduction system below the AV node (where the Mobitz II disorder is)
• is less burdened due to the lower frequency
• Therefore, Mobitz II may improve, a block (2:1) becomes (3:2, 4:3, 5:4)

AV Block II Degree with Conduction (2:1)

• Every second P wave is blocked (blue arrows)
• Narrow QRS complexes (< 0.12s)
• which suggests Mobitz I, very rarely Mobitz II
• Non-prolonged PQ interval (< 0.2s)
• suggests more likely Mobitz II
• Most likely it is AV Block II Degree - Mobitz I (narrow QRS complexes)

AV Block II Degree with Conduction (2:1)

• Wide QRS complexes (> 0.12s), suggests Mobitz II
• Non-prolonged PQ interval (< 0.2s), more indicative of Mobitz II
• According to the laddergram, it is Hisian AV Block II Degree - Mobitz II
• QRS complexes exhibit the pattern of Right Tawar's Branch Block (RTBB)

AV Block II Degree with Conduction (2:1)

• Narrow QRS complexes (< 0.12s)
• Suggestive of Mobitz I, very rarely Mobitz II
• Non-prolonged PQ interval (< 0.2s)
• More indicative of Mobitz II
• Most likely AV Block II Degree - Mobitz I (narrow QRS complexes)