EMmedonline

Electrical activity normally begins in the sinoatrial node and that a wave of depolarisation spreads outwards through the atrial muscle to AV node, and then down the His bundle and its branches to ventricles.

The conduction can be delayed or blocked at any point.

The time taken for the spread of depolarisation from the SA node to the ventricular muscle is shown by PR interval and is less than 120 - 200ms(3-5 small squares).

FIRST DEGREE HEART BLOCK

• Here each wave of depolarisation that originates in the SA node is conducted to ventricles, but there is delay somewhere along the conduction pathway, then the PR interval is prolonged . This is called first degree heart block.

Clinical significance

• It is found in normal hearts.

• Increased vagal tone, digoxin toxicity, acute inferior MI , myocarditis.

SECOND DEGREE HEART BLOCK

• Sometimes excitation completey fails to pass through the AV node or the bundle of HIS . When this occurs intermittently it leads to second degree heart block .

• It can be either Mobitz Type I (Wenckebach phenomenon) or Type II Heart block

Second Degree Mobitz Type I Heart Block

• Here there is progressive prolongation of the PR interval and then failure of conduction of an atrial beat, followed by a conducted beat with a shorter PR interval and then a repetition . This called as wenckebach phenomenon.

• With Mobitz type 1, the classic AV wenckebach pattern each stimulus from the atria has progressive difficulty traversing the AV node to the ventricles(NOde becomes increasingly refractory).

Mobitz Type I Block : Note the Progressive increase in PR interval finally resulting in a dropped beat.

Clinical Significance

• Often transient

• Acute inferior MI

• Digoxin toxicity

• Myocarditis

• After cardiac surgery

Mobitz Type I Block : Note the Progressive increase in PR interval finally resulting in a dropped beat.

Treatment

• Specific therapy is not necessary unless slow ventricular rates produce signs of hypoperfusion.

• Atropine 0.5mg IV ,may repeat every 5min until desired effect or until the total dose reaches 2.0 mg.

• Second line : Transcutaneous pacing

Second Degree Mobitz Type II

• Here most of the beats are conducted with a constant PR interval , but occasional there is an atrial contraction without a subsequent ventricular contraction. This is called mobitz type 2.

Mobitz type II  : Note the non conducted P waves following two conducted P waves (3:2).Note that PR interval is fixed here.

Clinical Significance

• Type II blocks imply structural damage to infranodal conducting system , and are usually permanent and may progress suddenly to complete heart block, esp in AMI.

Treatment

• Atropine is the first drug of choice.

• Transcutaneous/ Transvenous pacing in unstable patients.

THIRD DEGREE /COMPLETE AV BLOCK

• Here AV node fails completely, the atria and ventricles beat independently.

• An escape pacemaker paces the ventricle at a rate slower than the atrial rate.

• Third degree AV block can occur at nodal or infranodal levels.

Complete Heart Block : Note that PP and RR Interval is regular, but PR interval is varying indicating AV dissosciation.

Nodal v/s Infranodal

• Block at the level of AV node is often caused by reversible factors, and is assosciated with relatively stable escape rhythm.

• Whereas a infranodal block progress rapidly and is usually due to irreversible causes and associated with unstable heart blocks.

• When third degree AV block occurs at the AV node, a junctional escape pacemaker takes over with ventricular rate of 40 – 60 beats/min with narrow QRS complex.

• If third degree AV block occurs at infranodal level, the ventricles are driven by a ventricular escape rhythm characterized by slow (Hr<40bpm) regular wide QRS complex.

Complete Heart Block Nodal level. Note that QRS complex is narrow with HR>40bpm suggestive of a nodal block.

Clinical Significance

• Develops in 8% of acute inferior MI.

• Infranodal third degree heart block indicates a structural disease to the infranodal conducting system.

Treatment

• Transvenous pacing in unstable patients.

Updated on 5/9/2015

Reference

• Goldberger's Clinical Electrocardiography. 8^th Edition.