First degree AV block,Second degree AV block (Mobitz type I,Mobitz type Il),Third degree AV block

AV CONDUCTION DISORDERS

When AV nodal block occurs the His bundle can generate a heart rate of 40 – 60 beats per minute with normal QRS complexes.
If the distal His Purkinje system takes over then the heart rate generated is 25 – 45 beats per minute with wide QRS complexes.
This rhythm is unstable and can degenerate into VT,-VF or cardiac standstill.

Etiology

· Myocardial infarction
· Digitalis
· CCB
· Coronary spasm
· Beta blockers
· Myocarditis
· Infectious mononucleiosis
· Sarcoidosis
· Rheumatic fever
· Lyme disease
· Mesotheliomas.
Lev’s disease (calcification and sclerosis of conduction system). Lenegre’s disease (sclera degenerative disease).

First degree AV block

AV conduction is prolonged.
It is recognized on the electrocardiogram by a prolonged P-R interval.
PR interval is more than 0.20 seconds.
A heart block in which the conduction of impulses through the atrioventricular node is delayed but all atrial beats are followed by ventricular beats.

Second degree AV block

There is intermittent AV block and some atrial impulses do not conduct to ventricles.
A form of atrioventricular block in which only some atrial impulses are conducted to the ventricles. Two variants exist: Mobitz I (Wenckebach) and Mobitz II.

Mobitz type I – second degree A V block

In Mobitz I, the PR intervals become progressively longer until a QRS complex is dropped.
Because of the dropped beats, the QRS complexes appear to be clustered (a phenomenon called “grouped beating”) on the electrocardiogram.
This is also called AV Wenckebach block.
There is progressive increase of PR interv I till there is a blocking of an atrial impulse resulting in absence of QRST following the P wave ,
The block is in the AV node and so the QRS is normal.
It is seen in inferior wall MI, drug intake like digitalis, beta blockers, and CCBs.
It does not need aggressive treatment.

Mobitz type Il – second degree A V block

The PR intervals are constant and suddenly there is failure of conduction due to disease of His Purkinjy system.
In Mobitz II, PR intervals have a constant length, but QRS complexes are dropped periodically, usually every second, third, or fourth beat
The QRS is wide.
After a PQRST sequence suddenly there is a P wave which does not conduct to the ventricles and so there is no QRST following the P.
Pacemaker implantation is usually necessary. ~It usually occurs with anterior wall infarction.
Treatment is a cardiac pacemaker implantation.

Third degree AV block —

No atrial impulse propagates to the ventricles in third degree AV block.
The atria and ventricles beat independently at different rates.
The atria beats at around 72 beats per minute whereas the ventricles beat at rate of 40-50 per minute.
In congenital AV block the heart rate increases with exercise.
The QRS complexes representing ventricular beats which come at 40-55 beats per minute, increase with atropine or exercise, originate in the AV node.
This may not be very serious and pacemaker may not be required.
If the QRS is wide and the rate is less than 40 beats per minute, the block is in or distal to His bundle and treatment is pacemaker implantation.
It is recognized in the ECG by P waves and QRS coming at different times, having no relation with each other.
The P waves comes at around 72 beats per minute and the QRS comes at about 40-50 beats per minute.
So there are more Ps than QRS and the ventricular rate is slow.

Treatment of AV block