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Third Degree AV Block

AV block 3rd degree, Complete heart block

PQ Interval

• Physiological PQ interval is (0.12 - 0.20s)
• PQ interval = P wave (approx. 0.08s) + PQ segment (approx. 0.10s)

1. Impulse originates in SA node
• When it travels to the atrial myocardium, it starts generating P wave
• 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 formed)
• Then it passes to the His bundle (no curve is formed)
4. Activation of the ventricular septum
• From the His bundle, the impulse through Purkinje fibers
• Begins to activate the ventricular septal myocardium
• Begins to form a Q wave

Backup Rhythm

• In AV block III, the ventricles should stop (absence of QRS complexes)
• Because impulses from the atria (P waves) are blocked at the AV junction
• On the ECG, we would see only P waves without QRS complexes (asystole)
• Protective mechanism is the activation of an ectopic focus below the block
• The ectopic focus starts generating impulses and results in:
• Junctional rhythm (focus is activated in the AV junction area)
• Ventricular rhythm (focus is activated in the ventricular area)

AV Block III Degree (QRS < 0.12s)

AV Block III Degree and Junctional Rhythm

• Laddergram shows the spread of impulses through the conduction system
• A - Atria, AV - AV junction, V - Ventricles
• SA node generates impulses regularly
• Heart rate of P waves (SA node) is 75/min
• All impulses from the SA node (P waves) are blocked suprahisally
• Below the AV block in the AV junction, an ectopic focus has been activated, leading to a junctional rhythm
• Frequency: 55/min
• Narrow QRS complexes (< 0.12s), as impulses spread to the ventricles through the conduction system
• AV dissociation - P waves are independent of QRS complexes
• PQ interval changes

AV Block II Degree

AV Block II Degree - Mobitz II (High Degree)

• Mobitz II means that the AV junction area intermittently conducts impulses to the ventricles
• The patient has a pre-existing left bundle branch block
• There is intermittent block of the right bundle branch
• Every 3rd impulse from the SA node passes through the right bundle branch and activates the ventricles
• Ventricular conduction is 3:1
• No AV dissociation, as there is a certain relationship between the P waves and QRS complexes
• PQ interval is the same at 0.18s, indicating that the atria and ventricles are electrically connected

3rd Degree AV Block

• AV Dissociation - P waves are independent of QRS complexes
• Atrial frequency (P waves): 60/min.
• Ventricular frequency (QRS complexes): 27/min.
• PQ interval varies
• This is an infra-Hisian 3rd Degree AV Block
• Wide QRS complexes (>0.12s), as a ventricular rhythm is activated

3rd Degree AV Block

• AV Dissociation - P waves are independent of QRS complexes
• Atrial frequency (P waves): 75/min.
• Ventricular frequency (QRS complexes): 27/min.
• PQ interval varies
• This is an infra-Hisian 3rd Degree AV Block
• Wide QRS complexes (>0.12s), as a ventricular ectopic focus is activated

2nd Degree AV Block - Mobitz II (4:1)

• AV Dissociation is not present because there is a consistent relationship between P waves and QRS complexes
• PQ interval is constant at 0.2s, indicating that the atria and ventricles are electrically connected
• Conduction to the ventricles is 4:1
• Every 4th P wave is conducted to the ventricles, resulting in a QRS complex
• Impulses (P waves) are intermittently blocked in the His bundle
• This is an 2nd Degree AV Block (High-Grade) - Mobitz II (4:1)

AV Block III Degree

• AV Dissociation - P waves are independent of QRS complexes
• Atrial Frequency (P waves): 60/min.
• Ventricular Frequency (QRS complexes): 27/min.
• PQ Interval is changing
• This is an Infranodal AV Block III Degree
• Wide QRS Complexes (>0.12s), due to activation of ventricular rhythm

AV Block III Degree

• On the ECG, we only see a continuous lead II
• Atrial Frequency (P waves): 85/min.
• Ventricular Frequency (QRS complexes): 42/min.
• Atrial frequency is approximately 2x higher than the ventricular frequency
• Consider AV Block II Degree with 2:1 Conduction
• Shortened PQ interval (<0.12s) could indicate:
• LGL Syndrome
• Shortened AV Conduction
• However, the P wave gradually merges into the QRS complex
• PQ interval is changing
• This is a Suprahisian AV Block III Degree
• Narrow QRS complexes (<0.12s), due to activation of junctional rhythm
• This represents Isorhythmic AV Dissociation
• Atria (P waves) and ventricles (QRS complexes) are electrically isolated
• However, their frequencies are such that they mimic mutual electrical connection (1st line)

AV Block of the Third Degree

• AV Dissociation - P waves are independent of QRS complexes
• Atrial frequency (P waves): 70/min.
• Ventricular frequency (QRS complexes): 45/min.
• PQ interval is variable
• It is a supraventricular AV block of the third degree
• Narrow QRS complexes (<0.12s), as a junctional rhythm is activated