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Third degree heart block, also known as complete heart block, is a disease of the electrical system of the heart, in which the impulse generated in the top half of the heart (typically the SA node in the right atrium) does not propagate to the left or right ventricles.
Due to the blocked rhythm, an accessory pacemaker below the level of the block typically will activate the ventricles. Since this accessory pacemaker activates independently of the impulse generated at the SA node, two independent rhythms can be noted on the electrocardiogram (EKG). One rhythm, generated from the SA node, will activate the atria and create the p wave on the EKG. The second rhythm, generated from the accessory pacemaker, will activate the ventricles and produce the QRS complex on the EKG.
The hallmark of complete heart block is no apparent relationship between p waves and QRS complexes on the EKG.
Etiology Many conditions can cause third degree heart block. Progressive degeneration of the electrical conduction system of the heart can lead to third degree heart block. This may be preceded by second degree heart block, particularly Mobitz II heart block.
An acute myocardial infarction (heart attack), may also lead to third degree heart block. This may be due to damage to the AV node, preventing impulse propogation distal to that portion of the conduction system, or it may be due to damage of the distal conduction system.
An inferior wall myocardial infarction may cause damage to the AV node, causing third degree heart block. In this case, the damage is usually transitory, and the AV node may recover. Studies have shown that third degree heart block in the setting of an inferior wall myocardial infarction typically resolves within 2 weeks.
An anterior wall myocardial infarction may damage the distal conduction system of the heart, causing third degree heart block. This is typically extensive, permanent damage to the conduction system, necessitating a permanent pacemaker to be placed.
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