2. Atrioventricular block (heart block)
An AV block exists if the atrial impulse is conducted with delay or is not conducted
at all to the ventricle when the AV junction is not physiologically refractory
During AV block, block can occur in AV node, His bundle, or bundle branches
Disturbance of impulse conduction that can be permanent or transient
2Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine, 10th Edition, Chpater 37, Page no 792-95
3. Classified by severity into three categories
First-degree heart block, conduction is prolonged but all impulses are conducted
Second-degree heart block in two forms, Mobitz type I (Wenckebach) and type II
Type I heart block is characterized by progressive lengthening of the conduction
time until an impulse is not conducted
Type II heart block denotes an occasional or repetitive sudden block of conduction
of an impulse, without prior measurable lengthening of conduction time.
When no impulses are conducted, complete or third-degree block
Advanced or highgrade heart block,indicate blockage of two or more consecutive
impulses
3Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine, 10th Edition, Chpater 37, Page no 792-95
4. First-Degree Atrioventricular Block
Every atrial impulse is conducted to the ventricles and a regular ventricular rate
PR interval exceeds 0.20 second in adults.
PR intervals can exceed the P-P interval, known as skipped P waves
Result from a conduction delay in the AV node (A-H interval), in the His-Purkinje
system (H-V interval), or at both sites
Equally delayed conduction over both bundle branches
Intra-atrial conduction delay can result in PR prolongation
4Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine, 10th Edition, Chpater 37, Page no 792-95
5. First-Degree Atrioventricular Block
QRS complex on the scalar ECG is normal, the AV delay in the AV node
QRS complex shows a bundle branch block pattern, the conduction delay within the AV node
or the His-Purkinje system
Acceleration of the atrial rate or enhancement of vagal tone by carotid massage cause first-
degree AV nodal block to progress to type I second-degree AV block
5Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine, 10th Edition, Chpater 37, Page no 792-95
6. Second-Degree Atrioventricular Block
Blocking of some atrial impulses conducted to the ventricle
Nonconducted P wave can be intermittent or frequent, occur at regular or irregular intervals,
and be preceded by fixed or lengthening PR intervals
Type I second-degree AV block is characterized by progressive PR prolongation culminating
in a nonconducted P wave whereas in type II second-degree AV block, the PR interval
remains constant before the blocked P wave
6Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine, 10th Edition, Chpater 37, Page no 792-95
8. Second-Degree Atrioventricular Block
AV block is intermittent and repetitive and can block several P waves in a row
Mobitz type I and Mobitz type II are applied to the two types of block
Wenckebach block refers to type I block only
Type I conduction disturbance can be difficultm to recognize.
During a typical type I block, the increment in conduction time is greatest in the second beat
of the Wenckebach group, and the absolute increase in conduction time decreases
progressively over subsequent beats.
8Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine, 10th Edition, Chpater 37, Page no 792-95
10. Characteristics of classic Wenckebach group beats
Interval between successive beats progressively decreases, although conduction time
increases (but by a decreasing function)
Duration of the pause produced by the nonconducted impulse is less than twice the interval
preceding the blocked impulse (which is usually the shortest interval)
Cycle that follows nonconducted beat (beginning the Wenckebach group) is longer than cycle
preceding the blocked impulse
Typical grouping occurs in < 50% of patients with a type I Wenckebach AV block
10Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine, 10th Edition, Chpater 37, Page no 792-95
11. Differentiation of Type I from Type II
Atrioventricular Block
A 2:1 AV block can be a form of type I or type II AV block
If QRS is normal, block likely type I and in AV node, and search for transition of the 2:1
block to a 3:2 block, during which the PR interval lengthens in the second cardiac cycle
If bundle branch block is present, block can be in AV node or His- Purkinje
11Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine, 10th Edition, Chpater 37, Page no 792-95
12. Differentiation of Type I from Type II
Atrioventricular Block
First-degree and type I second-degree AV block can occur in normal healthy children, and a
Wenckebach AV block normal in athletes due to increase in resting vagal tone
In patients who have chronic second-degree AV nodal block (proximal to the His bundle)
without structural heart disease, the course is relatively benig, whereas in those with structural
heart disease, the prognosis is poor.
12Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine, 10th Edition, Chpater 37, Page no 792-95
13. Third-Degree (Complete) Atrioventricular Block
No atrial activity is conducted to the ventricles
Atrial pacemaker can be sinus or ectopic (tachycardia, flutter, or fibrillation) or can
result from AV junctional focus above the block with retrograde atrial conduction
Ventricular focus is located just below the region of the block, which can be above
or below the His bundle bifurcation.
Ventricular pacemaker closer to the His bundle stable and faster escape rate than
can those located more distally in the conduction system.
13Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine, 10th Edition, Chpater 37, Page no 792-95
15. Third-Degree (Complete) Atrioventricular Block
Result from a block at AV node, bundle of His, Purkinje
If block proximal to the His bundle, there will be normal QRS complexes at 40 to
60 beats/minute
Intrahisian block poor prognosis
Acquired complete AV block occurs most commonly distal to the bundle of His
because of trifascicular conduction disturbance.
QRS complex is abnormal, and ventricular rate < 40 beats/minute
15Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine, 10th Edition, Chpater 37, Page no 792-95
16. Third-Degree (Complete) Atrioventricular Block
Paroxysmal AV block caused by vagotonic reflexes
Surgery, electrolyte disturbances, myoendocarditis, tumors, Chagas disease,
rheumatoid nodules, calcific aortic stenosis, myxedema, polymyositis, infiltrative
processes (e.g., amyloidosis, sarcoidosis, scleroderma) can produce AV block
Rapid rates sometimes followed by block (called tachycardia-dependent AV
block), which is thought to be due to a phase 3 block (block caused by incomplete
action potential recovery), postrepolarization refractoriness, and concealed
conduction in the AV node
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17. Third-Degree (Complete) Atrioventricular Block
Pause-dependent paroxysmal AV block results in AV block after a pause or during
relative bradycardia
Referred to as a phase 4 block because it is thought that spontaneous
depolarizations during the resting phase of the action potential result in an inability
to depolarize,
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18. Third-Degree (Complete) Atrioventricular Block
In children AV block is congenital
AV block can be an isolated finding or be associated with other lesions
Neonatal autoimmune disease, account for most cases of heart block
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19. Third-Degree (Complete) Atrioventricular Block
Children are asymptomatic, some children, symptoms requiring pacemake
Adams-Stokes attacks can occur
Heart rate at rest 50 beats/minute or less correlates with the incidence of syncope
Prolonged recovery times of escape foci after rapid pacing , slow heart rates on 24-hour
electrocardiographic recordings, and the occurrence of paroxysmal tachycardias may be
factors predisposing to the development of symptoms
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20. Clinical Features
First-degree AV block, long a to c wave interval in the jugular venous pulse
Diminished first heart sound
In type I second-degree AV block, the heart rate may increase imperceptibly with gradually
diminishing intensity of the first heart sound; widening of the a to c interval, terminated by a
pause; and an a wave not followed by a v wave.
Intermittent ventricular pauses and a waves in the neck not followed by v waves characterize
type II AV block. First heart sound maintains a constant intensity
20Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine, 10th Edition, Chpater 37, Page no 792-95
21. Clinical Features
In complete AV block, the findings are the same as those in AV dissociation
Significant clinical manifestations of first- and second-degree AV block usually consist of
palpitations or subjective feelings of the heart “missing a beat.”
Persistent 2:1 AV block can produce symptoms of chronic bradycardia
Complete AV block can be accompanied by signs and symptoms of reduced cardiac output,
syncope or presyncope, angina, or palpitations from ventricular tachyarrhythmias
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22. Management
Holter or external loop recorders can be useful
Longer periods of recording require an implantable loop recorder
In patients with presyncope or syncope, one should suspect intermittent infra-His block in
those with bundle branch block or an intraventricular conduction defect.
An EP study to thoroughly evaluate AV conduction (including infusion of isoproterenol
and/or procainamide) may be warranted to make the diagnosis, particularly in those with
severe symptoms
22Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine, 10th Edition, Chpater 37, Page no 792-95
23. Management
Drugs cannot be relied on to increase the heart rate
Temporary or permanent pacemaker for symptomatic bradyarrhythmias.
For short-term therapy, atropine are useful
Isoproterenol can be used transiently
Symptomatic AV block or high-grade AV block (e.g., infrahisian, type II AV block, third-
degree heart block not caused by congenital AV block), permanent pacemaker placement is
the treatment of choice.
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