AV Conduction Disturbances

SA (Sinoatrial) node is the pacemaker of human heart.AV (Atrio-ventricular) node is the conduction junction between Atria and ventricles.It transmits the impulses originating in the SA node (present on right atrium) to the ventricles.Any conductional defects in the AV node may cause ventricular tachyarrhythmias and ventricular failure.
In any suspected case of conduction disturbances, the physician must assess

• The site of block
• The risk of turning into a complete block
• The probability that a subsidiary escape rhythm develops distal to the block.

If the escape rhythm pace maker is in the HIS bundle, the ectopic impulse is stable and the rate is usually 40 - 60 beats/min.QRS complex is normal in duration.

If ectopic rhythm is present in HIS-Purkenjee system, ectopic rhythm is unstable and rate is usually 25 - 45 beats/mins.QRS complex id prolonged.

Etiology :
Seen in following conditions-
Chronic athlets have hypervagal stimulation.Mobitz type two AV conduction block is mostly seen in them.
Drugs-Digitalis toxicity,Over dosage of Calcium channel blockers, Beta blockers.
Infections-Viral myocarditits, Infectious mononucleosis, Sarcaidosis, Rheumatic heart disease.
Amyloidosis, SLE,Kyme disease.
Myocardial infarction, Coronary spasms.

Levis Disease - Calcification of myocardial skeleton.Usually involve aortic and mitral valves.
Lenegre's Disease - Primary sclerodegenerative disease of conducting tissue.No involvement of myocardium.

Hypertension and Aortic.mitral valve stenosis accelerate the sclerodegenerative process of conducting system and even have a direct sclerosis and fibrosis of conducting system.

First degree Block : (Prolonged PR Interval > 0.2 sec)
PR interval depends on Atrial, AV nodal, HIS purkinjee system activation.
Normal QRS duration with prolonged PR interval - Seen in AV Nodal block
Normal PR interval with prolonged QRS interval - seen in His-purkinjee block
If both QRS and PR intervals are prolonged - Block can be present anywhere in the three sites.

Second degree Block :
Some impulses fail to reach ventricles.
It is of two types - Mobitz type 1 (Winkebach phenomenon) and Mobitz type 2

• Mobitz type 1 - Progressive Prolongation of PR interval followed by a missed P wave.The pause is less than two normal sinus intervals.The block is always localised to AV node with normal QRS interval.It is seen in inferior wall infarction or drug intoxication with digitalis, Beta blockers and calcium channel blockers.Also seen in normal people with high vagal tones ( athletes).It never progresses to complete block.Even if it progresses, it is stable and pacemaker implantation or aggressive therapy is rarely indicated.
• Mobitz type 2 - Sudden loss of P wave with no prior changes in PR interval.It is a HIS-Purkinje system.If QRS interval is normal, the block is intra HIS block.It is seen in atrio-septal infarction, sclerodegenerative disorders of fibrous skeleton of heart.It progresses to complete heart block with unstable ectopic pacemakers.Therefore, pacemaker implantation becomes mandatory.

In high degree AV blocks, there are two or more consecutively blocked P waves.But intermittent conduction is present.

Third degree Block :
In this case, there is no conduction from atria to the ventricles.The rate is usually 40 to 55 beats/min.In case of Congenital AV block, the block is limited to AV node.The rate increases with Atropine or exercise.But if the escape pacemaker is in the HIS bundle, the escape pacemaker is less responsive to these perturbations.
Some people with Infra HIS bundle block are capable of 'retrograde conduction'(reverse conduction).In these people, a dual chamber pacemaker is needed because a normal pacemaker may cause 'Pacemaker syndrome' in these patients.

AV Block Summary -

• QRS normal, PR interval prolonged - AV nodal Block (PR > 0.3 secs)
• QRS prolonged, PR interal normal - HIS bundle block or HIS-Purkinje system block.
• Atropine and exercise - Improves conduction - AV Block
• Atropine and exercise - worsens conduction - HIS or HIS purkinje block.
• Carotid sinus pressure - worsens conduction - AV block
• Carotid Sinus pressure - Improves conduction - HIS and HIS purkinjee block.
• Retrograde conduction is present in HIS and HIS Purkinje system blocks.