Heart Blocks

Heart block refers to a pathological delay in AV conduction, either at the AV node or beyond. Signs of heart block lie in the PR interval and P to QRS relationship.

 

First Degree Heart Block

In first degree heart block the cardiac rhythm is sinus in origin but the time from the initial depolarization of the atria to the initial depolarization of the ventricles is abnormally delayed. This pathologic delay is reflected in a PR interval longer than its upper limit of 200 ms. Nevertheless, each P wave of atrial contraction is followed by a QRS complex of ventricular contraction.

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Second Degree Heart Block

When transmission of the depolarizing impulse from the sinus node through the AV conduction system of the heart is interrupted intermittently, P wave of atrial contraction is no longer followed by a QRS complex of ventricular contraction in the interrupted beat. This is second degree heart block.

There are 2 types of second degree heart block: Mobitz type I & Mobitz type II. In Mobitz type I block there is progressive prolongation of the PR interval, indicating increasing delay in AV conduction, before it fails altogether.

When failure in AV conduction occurs, the P wave of atrial contraction is not followed by a QRS complex. After this missed ventricular beat, the PR interval returns to its shorter duration and the cycle of progressive PR prolongation and missed ventricular beat repeats itself.

In Mobitz Type II block, a non-conducted P wave not followed by a QRS complex occurs suddenly without progressive prolongation of the PR interval. That is, the PR interval, which can be normal or prolonged, is constant before the non-conducted beat materializes.

Mobitz type II second degree heart block indicates more serious disease of the conduction system in regions below the AV node and can progress to total failure of AV conduction (third degree heart block) without warning.

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Third Degree Heart Block

In third degree (complete) heart block, all the SA node impulses are blocked and not conducted to the ventricles. In the absence of an alternative pacemaker, ventricular contraction comes to a standstill and the patient dies. But most probably an ectopic pacemaker below the block takes over ventricular pacing and the patient survives. Since the SA node and the ectopic pacemaker pace the atria and ventricles independently, the P waves bear no relationship to the QRS complexes.

Two types of QRS complexes can be seen in third degree heart block:

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If the block is high in the AV node and the ventricular pacemaker is located lower in the AV junction, the QRS complex is normal in width because ventricular activation is via the bundle branches.

 

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If the block is low in the AV junction, the ventricles are paced by an idioventricular pacemaker and the QRS complexes will be widerthan normal because the ventricles are no longer activated via the bundle branches.

 

Bundle Branch Block

This topic is covered in a later section.

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