• Atrioventricular (AV) Node
• PQ Interval
• AV Block of the Second Degree - Mobitz II
• ECG and AV Block of the Second Degree - Mobitz II
• AV Block II Degree (Mobitz I, Mobitz II)
• Narrow QRS Complex (<0.12s)
• Wide QRS Complex (>0.12s)
• Ventricular Conduction

Atrioventricular (AV) Node

• In sinus rhythm, impulses are generated regularly (approx. 60/min.) in the SA node
• Each impulse travels through the atria (P wave) to the AV node
• In the AV node, the impulse slows down by approximately 0.1s
• The atria have time to pump blood into the ventricles during this delay
• Then the impulse continues to the ventricles (QRS complex)

AV Block of the Second Degree - Mobitz II

• It is an intermittent block of impulses, always infranodal (below the AV node)
• Impulses can be blocked infranodally in 3 locations:
• His bundle (in the His bundle)
• At the beginning of the Tawara branches
• In the right Tawara branch and in the fascicles
  
  

• In the case of AV node dysfunction, Wenckebach phenomenon occurs: AV Block of the Second Degree - Mobitz I - Wenckebach
• Conduction of impulses through the AV node gradually prolongs
• Until the impulse is blocked. Then the cycle repeats
  
  
• In the case of infranodal dysfunction (below the AV node), AV Block of the Second Degree - Mobitz II occurs
• The principle of "all or nothing" applies, the impulse is either conducted or blocked
• Blockage of all impulses is AV Block of the Third Degree (absolute isolation of the atria from the ventricles)
  
  
• AV Block of the Second Degree - Mobitz II
• Is more dangerous than Mobitz I
• It can progress to AV Block of the Third Degree (absolute isolation of the atria from the ventricles)
• Patients with Mobitz II are indicated for the implantation of a pacemaker
• Mobitz I does not require a pacemaker

Sinus Rhythm

• Laddergram illustrates the propagation of impulses through the conduction system
• A - Atria, AV - AV junction, V - Ventricles
• PQ interval: 0.16s
• In an intact AV junction (AV node), the PQ interval is 0.12 - 0.20s
• A P wave is followed by a QRS complex

AV Block II Degree (Mobitz I, Mobitz II)

• Woldemar Mobitz
• He was a Russian physician who worked as a cardiologist in Germany
• In 1924, he described AV Block II Degree on ECG and divided it into 2 types (Mobitz I, II)
  
  
• Mobitz I (Wenckebach)
• It is often referred to as Wenckebach
• Because the Wenckebach phenomenon occurs in the AV node
• Mobitz II (Hay)
  • John Hay was an English physician who described this AV block II degree in 1906 based on pulse (without ECG)
  • Later, Mobitz described it in more detail; it is more commonly referred to as Mobitz II, rarely as Hay

AV Block II Degree (Mobitz II)

• The disorder is infranodal (below the AV node)
• PQ interval remains constant (Wenckebach phenomenon is not present)
• AV Block II Degree, Mobitz II
• Any P waves are intermittently blocked (without prolongation of the PQ interval)
• The infranodal area of the AV junction intermittently blocks impulses
• Conduction to the ventricles is (4:3)

Wide QRS Complex (>0.12s)

Wide QRS Complex (>0.12s)

• This is a infra-Hisian AV block II degree - Mobitz II
• 75% of Mobitz II AV blocks have a wide QRS complex (>0.12s)
• Mobitz II can block impulses infra-Hisianly: at the level of Tawara's branches or fascicles
• The likelihood of intermittent blockage at once at
• 2 locations (2 Tawara's branches)
• 3 locations (2 fascicles and the right Tawara's branch)
• is minimal
• Patients have a wide QRS complex because they have a pre-existing:
• Branch block (left bundle branch block or right bundle branch block)
• Bifascicular block (right bundle branch block + one fascicle)
• In sinus rhythm, impulses reach the ventricles only through one intact Tawara's branch (Fascicle)
• On the ECG, there is a wide QRS complex
• Intermittently there is blockage of impulses in the unblocked Tawara's branch (Fascicle)
• This appears on the ECG as blocked P waves (no QRS complex follows them)

AV Block II Degree - Mobitz II (3:1)

• The PP interval remains unchanged
• After every 3rd P wave there is a narrow QRS complex
• The ventricular conduction is 3:1
• Narrow QRS complexes (<0.12s)
• This is a His bundle AV block II degree - Mobitz II
• There is also AV block I degree
• PQ interval is prolonged (> 0.2s)

AV Block II Degree - Mobitz II (3:2 and 2:1)

• The PP interval remains unchanged
• Every 3rd or every 2nd P wave is blocked (some P waves are hidden within the T wave)
• There is a variable conduction to the ventricles 3:2 and 2:1
• Narrow QRS complexes (<0.12s)
• This indicates a hisian AV block II Degree - Mobitz II

AV Block II Degree with Conduction (3:2)

• The PP interval remains unchanged
• Every 3rd P wave is blocked
• Conduction to the ventricles is 3:2
• Wide QRS complexes (>0.12s)
• Indicates an infrahisian AV Block II Degree - Mobitz II
• Patient has a preexisting left bundle branch block (deep S in V1)
• And intermittently blocks the right bundle branch

AV Block II Degree - Mobitz II

• Laddergram illustrates the spread of impulses through the conduction system
• A - Atria, AV - AV junction, V - Ventricles
• PP interval is constant
• PQ interval is constant
• Every 3rd or every 2nd P wave is blocked
• There is a variable conduction to the ventricles of 3:2 and 2:1
• QRS complexes are wide (>0.12s) and have the appearance of right bundle branch block (RBBB)
• Thus, the impulse passes to the ventricles only through the left bundle branch
• In the left bundle branch, impulses are intermittently blocked
• As seen by the fact that no QRS complex follows the P wave
• AV Block II Degree - Mobitz II occurs because
• impulses are intermittently blocked in the left bundle branch

AV Block II Degree - Mobitz I (Wenckebach)

• PP interval is constant (800ms)
• PQ interval is constant (190ms), except for the PQ interval after a blocked P wave (160ms)
• The PQ interval after a blocked P wave is shorter than the PQ before the blocked P wave
• The PQ interval after the blocked P wave (160ms) is shorter by 30ms
• compared to the PQ before the blocked P wave
• This is AV Block II Degree - Mobitz I (Wenckebach)

AV Block II Degree - Mobitz II

• The ECG shows bifascicular block
• Left anterior fascicular block
• Right Tawar bundle branch block
• The ventricles are activated only through the left posterior fascicle
• This is AV Block II Degree - Mobitz II
• Intermittent blockage of impulses (in the left posterior fascicle)
• PP interval is constant
• PQ interval is constant
• Conduction to the ventricles is (3:2) and 1x (2:1)

Alternating Tawar Bundle Branch Block

• At the beginning of the ECG there is a left Tawar bundle branch block (LBBB)
• After a atrial extrasystole
• there is a change to a right Tawar bundle branch block (RBBB)
• This indicates a severe damage to the conduction system
• Both Tawar bundle branches are damaged
• PQ interval with LBBB is longer than the PQ interval with RBBB
• This suggests that the right Tawar bundle branch is more damaged
• Because it takes longer to activate the ventricles
• Such a patient may develop
• AV block II degree - Mobitz II
• AV block III degree
• A patient with such an ECG is indicated for the implantation of a pacemaker