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Second Degree AV Block - Mobitz I (Wenckebach)

2nd degree AV Block, Mobitz I (Wenckebach)

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 travels 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 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. Delayed (decremental) conduction in the AV node
• The impulse lingers in the AV node for about 0.1s (no curve is generated)
• Then it moves into the His bundle (no curve is generated)
4. Activation of the ventricular septum
• From the His bundle the impulse through Purkinje fibers
• Begins to activate the myocardial tissue of the ventricular septum
• Begins to form the Q wave

AV Block II Degree - Mobitz I (Wenckebach)

• Conduction of each subsequent impulse
• gradually lengthens through the AV node
• until the impulse is blocked in the AV node
• The cycle then repeats
• On the ECG, the PQ interval progressively lengthens
  • Until after the P wave, no QRS complex follows
  • The cycle then repeats
    
    
• In AV node dysfunction
• The Wenckebach phenomenon occurs in the AV node
• Conduction gradually lengthens until it is blocked - Mobitz I
• In infra-nodal dysfunction (His bundle)
• The Wenckebach phenomenon does not occur
• The "all or nothing" law applies
• The impulse is either conducted or blocked
• Blockage of all impulses is AV Block III Degree

Sinus Rhythm

• Laddergram illustrates the spread of the impulse through the conduction system
• A - Atria, AV - AV junction, V - Ventricles
• PQ interval: 0.16s
• With a normal AV junction (AV node), the PQ interval is 0.12 - 0.2s

Second-Degree AV Block (Mobitz I, Mobitz II)

• Woldemar Mobitz
• He was a Russian physician who worked as a cardiologist in Germany
• In 1924, he described second-degree AV block on an 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, in 1906, described this second-degree AV block based on pulse (without ECG)
  • Later, Mobitz described it in more detail and it is more commonly referred to as Mobitz II, rarely as Hay

Second-Degree AV Block (Mobitz II)

• 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)

Second-Degree AV Block - Mobitz I (Wenckebach)

• PP interval is constant
• PQ interval gradually lengthens
• Until the P wave is blocked
• The cycle then repeats
• Conduction to the ventricles is 5:4

Second-Degree AV Block - Mobitz I (Wenckebach) and Inferior Wall STEMI

• Inferior wall STEMI (ST-Elevation Myocardial Infarction)
• ST elevations in the inferior leads (II, III, aVF)
• Inferior infarcts often lead to ischemic damage to the AV node
• Resulting in AV block of various degrees
• Continuous inferior lead II has conduction to the ventricles (2:1)
• It is unclear whether the PQ interval prolongs because every 2nd P wave is blocked. It could be:
• Mobitz I
• Mobitz II
• 5th and 6th PQ intervals prolong until the P wave is blocked (hidden in the T wave)
• This represents Second-Degree AV Block - Mobitz I with variable conduction 2:1 and 3:2

Second-Degree AV Block - Mobitz I (Wenckebach)

• PP interval is constant (800ms)
• PQ interval progressively prolongs (160ms...320ms)
• Until the P wave is blocked
• The cycle then repeats
• Conduction to the ventricles is (6:5)

AV Block II. Degree - Mobitz I (Wenckebach)

• On the ECG, there is a continuous ECG record (in 2 lines)
• PP interval is constant (800ms)
• PQ interval prolongs irregularly
• Some PQ intervals remain the same and do not prolong
• P wave is blocked
• The cycle then repeats
• Conduction to the ventricles is irregular
• PQ interval before the blocked P wave is longer
• than PQ interval after the blocked P wave
• It is Atypical AV Block II. Degree - Mobitz I (Wenckebach)
• Because PQ interval prolongs irregularly

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)
• PQ interval after a blocked P wave IS shorter
• than PQ interval before the blocked P wave
• PQ interval after a blocked P wave (160ms) is shorter
• by 30ms compared to PQ before the blocked P wave
• It is AV Block II. Degree - Mobitz I (Wenckebach)

AV Block II. Degree - Mobitz I (Wenckebach)

• PP interval is constant
• PQ interval is constant
• However, PQ interval after a blocked P wave is shorter
• Without a laddergram, it is harder to evaluate
• It is AV Block II. Degree - Mobitz I (Wenckebach)
• If the PQ interval after the blocked P wave were the same
• as before the blocked P wave, it would be classified as Mobitz II