In this type of second degree heart block there is intermittent failure of the AVN to conduct the atrial depolarisation to the ventricles. Generally the level of block is below the AVN and almost always occurs in patients with bundle branch disease.
By definition the PR intervals in all conducted depolarisations are constant. In this form of second degree block there is frequently prolongation of the QRS complex duration. Often junctional automaticity is impaired.
This type of heart block does not improve after exercise.
In patients with myocardial infarction this type of heart block may herald complete, or third degree, heart block.
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