Second degree heart block
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Second degree heart block is a disease of the electrical conduction system of the heart.
If this term is used without a qualifier, it is usually assumed to be regarding disease of the atrioventricular node (AV node). However it is possible to have a second degree heart block involving the sinoatrial node (SA node) as well.
There are two distinct types of second degree heart block, called type 1 and type 2. The distinction is made between them because type 1 second degree heart block is considered a more benign entity than type 2 second degree heart block.
Type 1 Second degree heart block
Type 1 Second degree heart block, also known as Mobitz I heart block or Wenckebach phenomenon, is a disease of the AV node.
Mobitz I heart block is characterized by progressive prolongation of the PR interval on the electrocardiogram (EKG) on consecutive beats followed by a blocked beat (dropped QRS complex). After the dropped QRS complex, the PR interval resets and the cycle repeats.
One of the baseline assumptions when determining if an individual has Mobitz I heart block is that the sinus rhythm has to be regular. If the sinus rhythm is not regular, there could be alternative explanations as to why the P wave wasn't followed by a QRS complex.
Type 2 Second degree heart block
Type 2 Second degree heart block, also known as Mobitz II heart block is a disease of the distal conduction system. This is also known as Infrahisian block because the level of block is below the bundle of His.
Mobitz II heart block is characterized on a surface ECG by a fixed PR interval with a dropped QRS complex after a certain number of P waves. For instance, for every 4 P waves, there are only 3 QRS complexes. Since there is no lengthening of the PR interval, the PR interval does not reset after the dropped QRS complex.